Development of the Abdominal Organs

Review of Early Embryologic Development

Blastocyst Blastocyst A post-morula preimplantation mammalian embryo that develops from a 32-cell stage into a fluid-filled hollow ball of over a hundred cells. A blastocyst has two distinctive tissues. The outer layer of trophoblasts gives rise to extra-embryonic tissues. The inner cell mass gives rise to the embryonic disc and eventual embryo proper. Fertilization and First Week and bilaminar disc Bilaminar disc Embryoblast and Trophoblast Development

The morula Morula An early embryo that is a compact mass of about 16 blastomeres. It resembles a cluster of mulberries with two types of cells, outer cells and inner cells. Morula is the stage before blastula in non-mammalian animals or a blastocyst in mammals. Fertilization and First Week (ball of cells) undergoes a process called blastulation, in which a cavity begins to form. The cells then begin differentiating into outer and inner cell masses.

• Outer cell mass Mass Three-dimensional lesion that occupies a space within the breast Imaging of the Breast → trophoblast Trophoblast Cells lining the outside of the blastocyst. After binding to the endometrium, trophoblasts develop into two distinct layers, an inner layer of mononuclear cytotrophoblasts and an outer layer of continuous multinuclear cytoplasm, the syncytiotrophoblasts, which form the early fetal-maternal interface (placenta). Fertilization and First Week → placenta Placenta A highly vascularized mammalian fetal-maternal organ and major site of transport of oxygen, nutrients, and fetal waste products. It includes a fetal portion (chorionic villi) derived from trophoblasts and a maternal portion (decidua) derived from the uterine endometrium. The placenta produces an array of steroid, protein and peptide hormones (placental hormones). Placenta, Umbilical Cord, and Amniotic Cavity and membranes
• Inner cell mass Mass Three-dimensional lesion that occupies a space within the breast Imaging of the Breast → embryoblast Embryoblast Embryoblast and Trophoblast Development → bilaminar disc Bilaminar disc Embryoblast and Trophoblast Development:
  • Epiblast Epiblast Embryoblast and Trophoblast Development
  • Hypoblast Hypoblast Embryoblast and Trophoblast Development
• Amniotic sac: a cavity of fluid that develops “above” the epiblast Epiblast Embryoblast and Trophoblast Development (between epiblast Epiblast Embryoblast and Trophoblast Development and trophoblast Trophoblast Cells lining the outside of the blastocyst. After binding to the endometrium, trophoblasts develop into two distinct layers, an inner layer of mononuclear cytotrophoblasts and an outer layer of continuous multinuclear cytoplasm, the syncytiotrophoblasts, which form the early fetal-maternal interface (placenta). Fertilization and First Week)
• Primitive yolk sac Yolk Sac The first of four extra-embryonic membranes to form during embryogenesis. In reptiles and birds, it arises from endoderm and mesoderm to incorporate the egg yolk into the digestive tract for nourishing the embryo. In placental mammals, its nutritional function is vestigial; however, it is the source of intestinal mucosa; blood cells; and germ cells. It is sometimes called the vitelline sac, which should not be confused with the vitelline membrane of the egg. Embryoblast and Trophoblast Development:
  • A cavity that forms “below” the hypoblast Hypoblast Embryoblast and Trophoblast Development 
  • Important in GI development

Trilaminar disc Trilaminar disc Gastrulation and Neurulation

The bilaminar disk undergoes a process called gastrulation Gastrulation Both gastrulation and neurulation are critical events that occur during the 3rd week of embryonic development. Gastrulation is the process by which the bilaminar disc differentiates into a trilaminar disc, made up of the 3 primary germ layers: the ectoderm, mesoderm, and endoderm. Gastrulation and Neurulation to form the trilaminar disc Trilaminar disc Gastrulation and Neurulation. There are 3 layers of the trilaminar disc Trilaminar disc Gastrulation and Neurulation:

• Ectoderm Ectoderm The outer of the three germ layers of an embryo. Gastrulation and Neurulation (continuous with the amnion Amnion The innermost membranous sac that surrounds and protects the developing embryo which is bathed in the amniotic fluid. Amnion cells are secretory epithelial cells and contribute to the amniotic fluid. Placenta, Umbilical Cord, and Amniotic Cavity)
• Mesoderm Mesoderm The middle germ layer of an embryo derived from three paired mesenchymal aggregates along the neural tube. Gastrulation and Neurulation:
  • Paraxial mesoderm Mesoderm The middle germ layer of an embryo derived from three paired mesenchymal aggregates along the neural tube. Gastrulation and Neurulation
  • Intermediate mesoderm Intermediate mesoderm Development of the Urogenital System
  • Lateral plate mesoderm Mesoderm The middle germ layer of an embryo derived from three paired mesenchymal aggregates along the neural tube. Gastrulation and Neurulation (LPM):
    • Somatic layer of the LPM
    • Splanchnic layer of the LPM
  • Extraembryonic mesoderm Extraembryonic mesoderm Embryoblast and Trophoblast Development:
    • Surrounds the amniotic cavity Amniotic cavity Embryoblast and Trophoblast Development → continuous with somatic LPM
    • Surrounds the yolk sac Yolk Sac The first of four extra-embryonic membranes to form during embryogenesis. In reptiles and birds, it arises from endoderm and mesoderm to incorporate the egg yolk into the digestive tract for nourishing the embryo. In placental mammals, its nutritional function is vestigial; however, it is the source of intestinal mucosa; blood cells; and germ cells. It is sometimes called the vitelline sac, which should not be confused with the vitelline membrane of the egg. Embryoblast and Trophoblast Development → continuous with the splanchnic LPM
• Endoderm Endoderm The inner of the three germ layers of an embryo. Gastrulation and Neurulation
  • Embryonic endoderm Endoderm The inner of the three germ layers of an embryo. Gastrulation and Neurulation (usually just called the endoderm Endoderm The inner of the three germ layers of an embryo. Gastrulation and Neurulation) → becomes the primitive gut tube
  • Extraembryonic endoderm Endoderm The inner of the three germ layers of an embryo. Gastrulation and Neurulation → lines the yolk sac Yolk Sac The first of four extra-embryonic membranes to form during embryogenesis. In reptiles and birds, it arises from endoderm and mesoderm to incorporate the egg yolk into the digestive tract for nourishing the embryo. In placental mammals, its nutritional function is vestigial; however, it is the source of intestinal mucosa; blood cells; and germ cells. It is sometimes called the vitelline sac, which should not be confused with the vitelline membrane of the egg. Embryoblast and Trophoblast Development
  • Secondary yolk sac Yolk Sac The first of four extra-embryonic membranes to form during embryogenesis. In reptiles and birds, it arises from endoderm and mesoderm to incorporate the egg yolk into the digestive tract for nourishing the embryo. In placental mammals, its nutritional function is vestigial; however, it is the source of intestinal mucosa; blood cells; and germ cells. It is sometimes called the vitelline sac, which should not be confused with the vitelline membrane of the egg. Embryoblast and Trophoblast Development: the cavity between embryonic and extraembryonic endoderm Endoderm The inner of the three germ layers of an embryo. Gastrulation and Neurulation
• Folding of the trilaminar disk (folds in 2 directions):
  • Lateral folding:
    • Creates a cylindrical structure surrounded by ectoderm Ectoderm The outer of the three germ layers of an embryo. Gastrulation and Neurulation and the amniotic cavity Amniotic cavity Embryoblast and Trophoblast Development
    • Creates the primitive gut tube internally from endoderm Endoderm The inner of the three germ layers of an embryo. Gastrulation and Neurulation
  • Cranial–caudal folding:
    • Creates a cranial end and a caudal end ( embryo Embryo The entity of a developing mammal, generally from the cleavage of a zygote to the end of embryonic differentiation of basic structures. For the human embryo, this represents the first two months of intrauterine development preceding the stages of the fetus. Fertilization and First Week becomes “bean-shaped”)

Forces the yolk sac Yolk Sac The first of four extra-embryonic membranes to form during embryogenesis. In reptiles and birds, it arises from endoderm and mesoderm to incorporate the egg yolk into the digestive tract for nourishing the embryo. In placental mammals, its nutritional function is vestigial; however, it is the source of intestinal mucosa; blood cells; and germ cells. It is sometimes called the vitelline sac, which should not be confused with the vitelline membrane of the egg. Embryoblast and Trophoblast Development farther from the body.→ The elongating stalk connecting the yolk sac Yolk Sac The first of four extra-embryonic membranes to form during embryogenesis. In reptiles and birds, it arises from endoderm and mesoderm to incorporate the egg yolk into the digestive tract for nourishing the embryo. In placental mammals, its nutritional function is vestigial; however, it is the source of intestinal mucosa; blood cells; and germ cells. It is sometimes called the vitelline sac, which should not be confused with the vitelline membrane of the egg. Embryoblast and Trophoblast Development to the gut tube is the vitelline duct Vitelline duct The narrow tube connecting the yolk sac with the midgut of the embryo; persistence of all or part of it in post-fetal life produces abnormalities, of which the commonest is meckel diverticulum. Meckel’s Diverticulum.

Obliteration of the vitelline duct Vitelline duct The narrow tube connecting the yolk sac with the midgut of the embryo; persistence of all or part of it in post-fetal life produces abnormalities, of which the commonest is meckel diverticulum. Meckel’s Diverticulum and yolk sac Yolk Sac The first of four extra-embryonic membranes to form during embryogenesis. In reptiles and birds, it arises from endoderm and mesoderm to incorporate the egg yolk into the digestive tract for nourishing the embryo. In placental mammals, its nutritional function is vestigial; however, it is the source of intestinal mucosa; blood cells; and germ cells. It is sometimes called the vitelline sac, which should not be confused with the vitelline membrane of the egg. Embryoblast and Trophoblast Development

• As the embryo Embryo The entity of a developing mammal, generally from the cleavage of a zygote to the end of embryonic differentiation of basic structures. For the human embryo, this represents the first two months of intrauterine development preceding the stages of the fetus. Fertilization and First Week folds, the yolk sac Yolk Sac The first of four extra-embryonic membranes to form during embryogenesis. In reptiles and birds, it arises from endoderm and mesoderm to incorporate the egg yolk into the digestive tract for nourishing the embryo. In placental mammals, its nutritional function is vestigial; however, it is the source of intestinal mucosa; blood cells; and germ cells. It is sometimes called the vitelline sac, which should not be confused with the vitelline membrane of the egg. Embryoblast and Trophoblast Development is pushed farther and farther away from the body.
• Vitelline duct Vitelline duct The narrow tube connecting the yolk sac with the midgut of the embryo; persistence of all or part of it in post-fetal life produces abnormalities, of which the commonest is meckel diverticulum. Meckel’s Diverticulum: elongating stalk that connects the midgut to the yolk sac Yolk Sac The first of four extra-embryonic membranes to form during embryogenesis. In reptiles and birds, it arises from endoderm and mesoderm to incorporate the egg yolk into the digestive tract for nourishing the embryo. In placental mammals, its nutritional function is vestigial; however, it is the source of intestinal mucosa; blood cells; and germ cells. It is sometimes called the vitelline sac, which should not be confused with the vitelline membrane of the egg. Embryoblast and Trophoblast Development as it is pushed away
• Growth of the amniotic cavity Amniotic cavity Embryoblast and Trophoblast Development around weeks 4–6:
  • The amniotic cavity Amniotic cavity Embryoblast and Trophoblast Development starts on the dorsal side of the embryo Embryo The entity of a developing mammal, generally from the cleavage of a zygote to the end of embryonic differentiation of basic structures. For the human embryo, this represents the first two months of intrauterine development preceding the stages of the fetus. Fertilization and First Week
    • Grows and extends around the cranial and caudal ends of the embryo Embryo The entity of a developing mammal, generally from the cleavage of a zygote to the end of embryonic differentiation of basic structures. For the human embryo, this represents the first two months of intrauterine development preceding the stages of the fetus. Fertilization and First Week 
    • Edges of the growing cavity meet on the ventral side at the abdomen → pinch off the yolk sac Yolk Sac The first of four extra-embryonic membranes to form during embryogenesis. In reptiles and birds, it arises from endoderm and mesoderm to incorporate the egg yolk into the digestive tract for nourishing the embryo. In placental mammals, its nutritional function is vestigial; however, it is the source of intestinal mucosa; blood cells; and germ cells. It is sometimes called the vitelline sac, which should not be confused with the vitelline membrane of the egg. Embryoblast and Trophoblast Development/obliterate the vitelline duct Vitelline duct The narrow tube connecting the yolk sac with the midgut of the embryo; persistence of all or part of it in post-fetal life produces abnormalities, of which the commonest is meckel diverticulum. Meckel’s Diverticulum
• Clinical relevance: Failure of the vitelline duct Vitelline duct The narrow tube connecting the yolk sac with the midgut of the embryo; persistence of all or part of it in post-fetal life produces abnormalities, of which the commonest is meckel diverticulum. Meckel’s Diverticulum to obliterate results in Meckel diverticulum Diverticulum A pouch or sac opening from the colon. Diverticular Disease:
  • A true diverticulum Diverticulum A pouch or sac opening from the colon. Diverticular Disease (contains all layers of the bowel wall) 
  • Arises from the antimesenteric surface of the middle-to-distal ileum Ileum The distal and narrowest portion of the small intestine, between the jejunum and the ileocecal valve of the large intestine. Small Intestine: Anatomy, anchoring it to the anterior abdominal wall Abdominal wall The outer margins of the abdomen, extending from the osteocartilaginous thoracic cage to the pelvis. Though its major part is muscular, the abdominal wall consists of at least seven layers: the skin, subcutaneous fat, deep fascia; abdominal muscles, transversalis fascia, extraperitoneal fat, and the parietal peritoneum. Surgical Anatomy of the Abdomen.
  • May lead to fistula Fistula Abnormal communication most commonly seen between two internal organs, or between an internal organ and the surface of the body. Anal Fistula, volvulus Volvulus A volvulus is the twisting or axial rotation of a portion of the bowel around its mesentery. The most common site of volvulus in adults is the colon; most frequently the sigmoid volvulus. Patients typically present with symptoms of bowel obstruction such as abdominal pain, distension, vomiting, and constipation/obstipation. Volvulus, small bowel obstruction Bowel obstruction Any impairment, arrest, or reversal of the normal flow of intestinal contents toward the anal canal. Ascaris/Ascariasis, and/or necrosis Necrosis The death of cells in an organ or tissue due to disease, injury or failure of the blood supply. Ischemic Cell Damage

Overview of Gut Tube Differentiation

Structures derived from the gut tube

The primitive gut tube is formed from endoderm Endoderm The inner of the three germ layers of an embryo. Gastrulation and Neurulation at the completion of lateral folding. The gut tube can initially be divided into 3 areas: foregut, midgut, and hindgut.

• Foregut: abdominal derivatives supplied by celiac artery
  • Esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus: Anatomy 
  • Stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach: Anatomy
  • Proximal duodenum Duodenum The shortest and widest portion of the small intestine adjacent to the pylorus of the stomach. It is named for having the length equal to about the width of 12 fingers. Small Intestine: Anatomy (above the ampulla of Vater)
  • Liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy
  • Gallbladder Gallbladder The gallbladder is a pear-shaped sac, located directly beneath the liver, that sits on top of the superior part of the duodenum. The primary functions of the gallbladder include concentrating and storing up to 50 mL of bile. Gallbladder and Biliary Tract: Anatomy and bile Bile An emulsifying agent produced in the liver and secreted into the duodenum. Its composition includes bile acids and salts; cholesterol; and electrolytes. It aids digestion of fats in the duodenum. Gallbladder and Biliary Tract: Anatomy ducts
  • Pancreas Pancreas The pancreas lies mostly posterior to the stomach and extends across the posterior abdominal wall from the duodenum on the right to the spleen on the left. This organ has both exocrine and endocrine tissue. Pancreas: Anatomy
  • Structures derived from the foregut above the abdomen:
    • Primordial pharynx Pharynx The pharynx is a component of the digestive system that lies posterior to the nasal cavity, oral cavity, and larynx. The pharynx can be divided into the oropharynx, nasopharynx, and laryngopharynx. Pharyngeal muscles play an integral role in vital processes such as breathing, swallowing, and speaking. Pharynx: Anatomy and its derivatives
    • Cervical and thoracic esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus: Anatomy
    • Respiratory tract
• Midgut: derivatives supplied by the superior mesenteric artery Superior mesenteric artery A large vessel supplying the whole length of the small intestine except the superior part of the duodenum. It also supplies the cecum and the ascending part of the colon and about half the transverse part of the colon. It arises from the anterior surface of the aorta below the celiac artery at the level of the first lumbar vertebra. Small Intestine: Anatomy (SMA)
  • Distal duodenum Duodenum The shortest and widest portion of the small intestine adjacent to the pylorus of the stomach. It is named for having the length equal to about the width of 12 fingers. Small Intestine: Anatomy (below the ampulla of Vater)
  • Jejunum Jejunum The middle portion of the small intestine, between duodenum and ileum. It represents about 2/5 of the remaining portion of the small intestine below duodenum. Small Intestine: Anatomy
  • Ileum Ileum The distal and narrowest portion of the small intestine, between the jejunum and the ileocecal valve of the large intestine. Small Intestine: Anatomy
  • Cecum Cecum The blind sac or outpouching area of the large intestine that is below the entrance of the small intestine. It has a worm-like extension, the vermiform appendix. Colon, Cecum, and Appendix: Anatomy
  • Appendix Appendix A worm-like blind tube extension from the cecum. Colon, Cecum, and Appendix: Anatomy
  • Ascending colon Ascending colon The segment of large intestine between the cecum and the transverse colon. It passes cephalad from the cecum to the caudal surface of the right lobe of the liver where it bends sharply to the left, forming the right colic flexure. Colon, Cecum, and Appendix: Anatomy
  • Proximal ⅔ of the transverse colon Transverse colon The segment of large intestine between ascending colon and descending colon. It passes from the right colic flexure across the abdomen, then turns sharply at the left colonic flexure into the descending colon. Colon, Cecum, and Appendix: Anatomy
• Hindgut: derivatives supplied by the inferior mesenteric artery Inferior mesenteric artery The artery supplying nearly all the left half of the transverse colon, the whole of the descending colon, the sigmoid colon, and the greater part of the rectum. It is smaller than the superior mesenteric artery and arises from the aorta above its bifurcation into the common iliac arteries. Small Intestine: Anatomy (IMA)
  • Distal ⅓ of the transverse colon Transverse colon The segment of large intestine between ascending colon and descending colon. It passes from the right colic flexure across the abdomen, then turns sharply at the left colonic flexure into the descending colon. Colon, Cecum, and Appendix: Anatomy
  • Descending colon Descending colon The segment of large intestine between transverse colon and the sigmoid colon. Colon, Cecum, and Appendix: Anatomy
  • Sigmoid Sigmoid A segment of the colon between the rectum and the descending colon. Volvulus colon Colon The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix: Anatomy
  • Rectum Rectum The rectum and anal canal are the most terminal parts of the lower GI tract/large intestine that form a functional unit and control defecation. Fecal continence is maintained by several important anatomic structures including rectal folds, anal valves, the sling-like puborectalis muscle, and internal and external anal sphincters. Rectum and Anal Canal: Anatomy
  • Superior portion of the anal canal

Structures derived from endoderm Endoderm The inner of the three germ layers of an embryo. Gastrulation and Neurulation

Key structures derived from endoderm Endoderm The inner of the three germ layers of an embryo. Gastrulation and Neurulation related to development of the abdominal organs:

• Epithelial lining of the entire GI tract
• Mucosal and submucosal glands

Structures derived from mesoderm Mesoderm The middle germ layer of an embryo derived from three paired mesenchymal aggregates along the neural tube. Gastrulation and Neurulation

Key structures derived from mesoderm Mesoderm The middle germ layer of an embryo derived from three paired mesenchymal aggregates along the neural tube. Gastrulation and Neurulation related to development of the abdominal organs: 

Splanchnic layer of the LPM: 

• Alimentary canal wall (i.e., intestinal walls)
  • Muscular tissue (e.g., circular and longitudinal layers, sphincters)
  • Connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue: Histology
• Vasculature
• Visceral peritoneum Visceral peritoneum Peritoneum: Anatomy
• Mesenteries:
  • Dorsal and ventral mesogastrium → greater and lesser omentum Omentum Peritoneum: Anatomy, falciform ligament Falciform Ligament Liver: Anatomy
  • Mesentery Mesentery A layer of the peritoneum which attaches the abdominal viscera to the abdominal wall and conveys their blood vessels and nerves. Peritoneum: Anatomy of the small intestine Small intestine The small intestine is the longest part of the GI tract, extending from the pyloric orifice of the stomach to the ileocecal junction. The small intestine is the major organ responsible for chemical digestion and absorption of nutrients. It is divided into 3 segments: the duodenum, the jejunum, and the ileum. Small Intestine: Anatomy
  • Transverse and sigmoid Sigmoid A segment of the colon between the rectum and the descending colon. Volvulus mesocolon
  • Numerous named abdominal ligaments

Somatic layer of the LPM: parietal Parietal One of a pair of irregularly shaped quadrilateral bones situated between the frontal bone and occipital bone, which together form the sides of the cranium. Skull: Anatomy peritoneum Peritoneum The peritoneum is a serous membrane lining the abdominopelvic cavity. This lining is formed by connective tissue and originates from the mesoderm. The membrane lines both the abdominal walls (as parietal peritoneum) and all of the visceral organs (as visceral peritoneum). Peritoneum: Anatomy

Structures derived from ectoderm Ectoderm The outer of the three germ layers of an embryo. Gastrulation and Neurulation

Key structures derived from ectoderm Ectoderm The outer of the three germ layers of an embryo. Gastrulation and Neurulation related to development of the abdominal organs: 

• Neural crest cells Neural crest cells Gastrulation and Neurulation: enteric nerve plexus (i.e., myenteric and submucosal plexuses)
• Nerves: innervate the abdominal organs
• Membranes:
  • At the cranial and caudal ends, the ectoderm Ectoderm The outer of the three germ layers of an embryo. Gastrulation and Neurulation and endoderm Endoderm The inner of the three germ layers of an embryo. Gastrulation and Neurulation meet (with no mesoderm Mesoderm The middle germ layer of an embryo derived from three paired mesenchymal aggregates along the neural tube. Gastrulation and Neurulation between) forming membranes that will ultimately rupture, becoming the openings of the GI tract
  • Cranial end: oropharyngeal membrane → mouth
  • Caudal end: cloacal membrane → anus
• Epithelium Epithelium The epithelium is a complex of specialized cellular organizations arranged into sheets and lining cavities and covering the surfaces of the body. The cells exhibit polarity, having an apical and a basal pole. Structures important for the epithelial integrity and function involve the basement membrane, the semipermeable sheet on which the cells rest, and interdigitations, as well as cellular junctions. Surface Epithelium: Histology: lines the distal anal canal

Foregut Derivatives, Omenta, and Spleen

Esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus: Anatomy

Separation from the respiratory system:

• Lung bud Lung bud Development of the Respiratory System ( respiratory diverticulum Respiratory diverticulum Development of the Respiratory System):
  • Buds off of the ventral side of the foregut around week 4
  • Simultaneously grows out (ventrally) and down (caudally)
  • Includes both endoderm Endoderm The inner of the three germ layers of an embryo. Gastrulation and Neurulation and surrounding splanchnic LPM
• Tracheoesophageal groove Tracheoesophageal groove Development of the Respiratory System (or ridge):
  • As the lung bud Lung bud Development of the Respiratory System grows out and down, the tracheoesophageal groove Tracheoesophageal groove Development of the Respiratory System appears as lateral indentations between the new lung bud Lung bud Development of the Respiratory System and the foregut.
  • The grooves/ridges move in medially, pinching off the lung bud Lung bud Development of the Respiratory System, and forming the tracheoesophageal septum.
• The tracheoesophageal septum creates 2 separate tubes:
  • Esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus: Anatomy (posteriorly, from the original foregut)
  • Trachea Trachea The trachea is a tubular structure that forms part of the lower respiratory tract. The trachea is continuous superiorly with the larynx and inferiorly becomes the bronchial tree within the lungs. The trachea consists of a support frame of semicircular, or C-shaped, rings made out of hyaline cartilage and reinforced by collagenous connective tissue. Trachea: Anatomy (anteriorly, from the lung bud Lung bud Development of the Respiratory System)
• Clinical relevance:
  • Tracheoesophageal fistula Fistula Abnormal communication most commonly seen between two internal organs, or between an internal organ and the surface of the body. Anal Fistula (TEF): occurs when the tracheoesophageal grooves fail to fully close in the midline 
  • Esophageal atresia Atresia Hypoplastic Left Heart Syndrome (HLHS): portions of the esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus: Anatomy fail to form; often coexists with TEF

Growth and descent:

• Esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus: Anatomy lengthens and descends together with the heart and lungs Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs: Anatomy
• Completes development by 7th week

Stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach: Anatomy

• Development begins around the 4th week
• Fusiform dilation of the foregut: site of primordial stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach: Anatomy
• The lengthening esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus: Anatomy places the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach: Anatomy below the diaphragm Diaphragm The diaphragm is a large, dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. The diaphragm consists of muscle fibers and a large central tendon, which is divided into right and left parts. As the primary muscle of inspiration, the diaphragm contributes 75% of the total inspiratory muscle force. Diaphragm: Anatomy
• Posterior wall grows more quickly than anterior wall → develops a C shape
  • Posterior wall → greater curvature Greater curvature Stomach: Anatomy
  • Anterior wall → lesser curvature Lesser curvature Stomach: Anatomy
• 90-degree rotation Rotation Motion of an object in which either one or more points on a line are fixed. It is also the motion of a particle about a fixed point. X-rays clockwise around longitudinal axis:
  • Greater curvature Greater curvature Stomach: Anatomy (dorsal):
    • Rotates anteriorly and to the left
    • Left vagus nerve Vagus nerve The 10th cranial nerve. The vagus is a mixed nerve which contains somatic afferents (from skin in back of the ear and the external auditory meatus), visceral afferents (from the pharynx, larynx, thorax, and abdomen), parasympathetic efferents (to the thorax and abdomen), and efferents to striated muscle (of the larynx and pharynx). Pharynx: Anatomy innervates anterior wall
  • Lesser curvature Lesser curvature Stomach: Anatomy (ventral):
    • Rotates posteriorly and to the right
    • Right vagus nerve Vagus nerve The 10th cranial nerve. The vagus is a mixed nerve which contains somatic afferents (from skin in back of the ear and the external auditory meatus), visceral afferents (from the pharynx, larynx, thorax, and abdomen), parasympathetic efferents (to the thorax and abdomen), and efferents to striated muscle (of the larynx and pharynx). Pharynx: Anatomy innervates posterior wall
• Smaller rotation Rotation Motion of an object in which either one or more points on a line are fixed. It is also the motion of a particle about a fixed point. X-rays clockwise around the anteroposterior axis:
  • Cardiac portion of the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach: Anatomy moves down and to the left
  • Pyloric portion moves up and to the right

Omenta

The greater and lesser omenta are formed from the dorsal and ventral mesogastrium (of mesodermal origin). As they rotate with the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach: Anatomy, they create the greater and lesser sacs.

Dorsal mesogastrium:

• Suspends stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach: Anatomy from the dorsal wall
• Originally located in the median plane
• Moved to the left during 90-degree longitudinal rotation Rotation Motion of an object in which either one or more points on a line are fixed. It is also the motion of a particle about a fixed point. X-rays
• Creates a space posterior to the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach: Anatomy called the lesser sac (i.e., omental bursa)
• Forms several important structures:
  • After rotation Rotation Motion of an object in which either one or more points on a line are fixed. It is also the motion of a particle about a fixed point. X-rays of the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach: Anatomy, bulges down to form the greater omentum Omentum Peritoneum: Anatomy
  • Gastrosplenic ligament Gastrosplenic ligament Spleen: Anatomy: connects the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach: Anatomy and spleen Spleen The spleen is the largest lymphoid organ in the body, located in the LUQ of the abdomen, superior to the left kidney and posterior to the stomach at the level of the 9th-11th ribs just below the diaphragm. The spleen is highly vascular and acts as an important blood filter, cleansing the blood of pathogens and damaged erythrocytes. Spleen: Anatomy
  • Splenorenal ligament Splenorenal ligament Spleen: Anatomy: connects the spleen Spleen The spleen is the largest lymphoid organ in the body, located in the LUQ of the abdomen, superior to the left kidney and posterior to the stomach at the level of the 9th-11th ribs just below the diaphragm. The spleen is highly vascular and acts as an important blood filter, cleansing the blood of pathogens and damaged erythrocytes. Spleen: Anatomy to the posterior abdominal wall Abdominal wall The outer margins of the abdomen, extending from the osteocartilaginous thoracic cage to the pelvis. Though its major part is muscular, the abdominal wall consists of at least seven layers: the skin, subcutaneous fat, deep fascia; abdominal muscles, transversalis fascia, extraperitoneal fat, and the parietal peritoneum. Surgical Anatomy of the Abdomen (near the kidney)
• Anterior to the greater omentum Omentum Peritoneum: Anatomy is the greater sac
• Structures developing within the dorsal mesogastrium:
  • Spleen Spleen The spleen is the largest lymphoid organ in the body, located in the LUQ of the abdomen, superior to the left kidney and posterior to the stomach at the level of the 9th-11th ribs just below the diaphragm. The spleen is highly vascular and acts as an important blood filter, cleansing the blood of pathogens and damaged erythrocytes. Spleen: Anatomy
  • Pancreas Pancreas The pancreas lies mostly posterior to the stomach and extends across the posterior abdominal wall from the duodenum on the right to the spleen on the left. This organ has both exocrine and endocrine tissue. Pancreas: Anatomy

Ventral mesogastrium:

• Attached to the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach: Anatomy, duodenum Duodenum The shortest and widest portion of the small intestine adjacent to the pylorus of the stomach. It is named for having the length equal to about the width of 12 fingers. Small Intestine: Anatomy, liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy, and ventral abdominal wall Abdominal wall The outer margins of the abdomen, extending from the osteocartilaginous thoracic cage to the pelvis. Though its major part is muscular, the abdominal wall consists of at least seven layers: the skin, subcutaneous fat, deep fascia; abdominal muscles, transversalis fascia, extraperitoneal fat, and the parietal peritoneum. Surgical Anatomy of the Abdomen
• Originally located in the median plane
• Moved to the right during 90-degree longitudinal rotation Rotation Motion of an object in which either one or more points on a line are fixed. It is also the motion of a particle about a fixed point. X-rays 
• Closes off the lesser sac between the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach: Anatomy and liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy
• Forms 2 primary structures:
  • Lesser omentum Omentum Peritoneum: Anatomy: connects the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach: Anatomy and liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy
  • Falciform ligament Falciform Ligament Liver: Anatomy: connects the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy and the anterior abdominal wall Abdominal wall The outer margins of the abdomen, extending from the osteocartilaginous thoracic cage to the pelvis. Though its major part is muscular, the abdominal wall consists of at least seven layers: the skin, subcutaneous fat, deep fascia; abdominal muscles, transversalis fascia, extraperitoneal fat, and the parietal peritoneum. Surgical Anatomy of the Abdomen
• Structures developing within the ventral mesogastrium:
  • Liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy
  • Gallbladder Gallbladder The gallbladder is a pear-shaped sac, located directly beneath the liver, that sits on top of the superior part of the duodenum. The primary functions of the gallbladder include concentrating and storing up to 50 mL of bile. Gallbladder and Biliary Tract: Anatomy
  • Bile Bile An emulsifying agent produced in the liver and secreted into the duodenum. Its composition includes bile acids and salts; cholesterol; and electrolytes. It aids digestion of fats in the duodenum. Gallbladder and Biliary Tract: Anatomy ducts

Duodenum Duodenum The shortest and widest portion of the small intestine adjacent to the pylorus of the stomach. It is named for having the length equal to about the width of 12 fingers. Small Intestine: Anatomy

• Development begins in the 4th week
• Above the ampulla of Vater: derived from caudal part of the foregut → supplied by the celiac artery
• Below the ampulla of Vater: derived from cranial part of the midgut → supplied by SMA
• Elongates and forms C-shaped loop
• Joins together with the developing pancreas Pancreas The pancreas lies mostly posterior to the stomach and extends across the posterior abdominal wall from the duodenum on the right to the spleen on the left. This organ has both exocrine and endocrine tissue. Pancreas: Anatomy attached to the posterior body wall
• Duodenal obliteration and recanalization:
  • Week 5: rapid cell proliferation in its walls leads to complete obstruction of the duodenal lumen
  • By the end of week 8:
    • Vacuoles form within the duodenum Duodenum The shortest and widest portion of the small intestine adjacent to the pylorus of the stomach. It is named for having the length equal to about the width of 12 fingers. Small Intestine: Anatomy 
    • Vacuoles grow and merge → lumen recanalization
  • Clinical relevance: failure of the lumen to fully recanalize results in either:
    • Duodenal stenosis Duodenal Stenosis Congenital Duodenal Obstruction: narrowing of the duodenum Duodenum The shortest and widest portion of the small intestine adjacent to the pylorus of the stomach. It is named for having the length equal to about the width of 12 fingers. Small Intestine: Anatomy
    • Duodenal atresia Atresia Hypoplastic Left Heart Syndrome (HLHS): a complete obstruction persists; presents with bilious emesis and a double-bubble sign on radiography

Liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy and biliary ducts

• Development begins in the 3rd‒4th week
• Appears from the caudal portion of the foregut as the hepatic diverticulum Diverticulum A pouch or sac opening from the colon. Diverticular Disease, which goes on to form the:
  • Liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy ( endoderm Endoderm The inner of the three germ layers of an embryo. Gastrulation and Neurulation differentiates into hepatocytes Hepatocytes The main structural component of the liver. They are specialized epithelial cells that are organized into interconnected plates called lobules. Liver: Anatomy)
  • Intrahepatic ducts
  • Extrahepatic portions of the hepatic ducts
• As the hepatic diverticulum Diverticulum A pouch or sac opening from the colon. Diverticular Disease enlarges, the connection to the duodenum Duodenum The shortest and widest portion of the small intestine adjacent to the pylorus of the stomach. It is named for having the length equal to about the width of 12 fingers. Small Intestine: Anatomy narrows → common bile Bile An emulsifying agent produced in the liver and secreted into the duodenum. Its composition includes bile acids and salts; cholesterol; and electrolytes. It aids digestion of fats in the duodenum. Gallbladder and Biliary Tract: Anatomy duct
• A smaller bud grows off the caudal side of the hepatic diverticulum Diverticulum A pouch or sac opening from the colon. Diverticular Disease:
  • Forms the gallbladder Gallbladder The gallbladder is a pear-shaped sac, located directly beneath the liver, that sits on top of the superior part of the duodenum. The primary functions of the gallbladder include concentrating and storing up to 50 mL of bile. Gallbladder and Biliary Tract: Anatomy
  • Connection between the caudal bud and the gallbladder Gallbladder The gallbladder is a pear-shaped sac, located directly beneath the liver, that sits on top of the superior part of the duodenum. The primary functions of the gallbladder include concentrating and storing up to 50 mL of bile. Gallbladder and Biliary Tract: Anatomy → cystic Cystic Fibrocystic Change duct 
• The hepatic diverticulum Diverticulum A pouch or sac opening from the colon. Diverticular Disease grows into the ventral mesogastrium.
• Weeks 5–9:
  • Hematopoietic stem cells Hematopoietic stem cells Progenitor cells from which all blood cells derived. They are found primarily in the bone marrow and also in small numbers in the peripheral blood. Bone Marrow: Composition and Hematopoiesis (mesodermal origin) migrate into the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy:
    • Hematopoiesis Hematopoiesis The development and formation of various types of blood cells. Hematopoiesis can take place in the bone marrow (medullary) or outside the bone marrow (extramedullary hematopoiesis). Bone Marrow: Composition and Hematopoiesis begins 
    • Liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy is the primary site of hematopoiesis Hematopoiesis The development and formation of various types of blood cells. Hematopoiesis can take place in the bone marrow (medullary) or outside the bone marrow (extramedullary hematopoiesis). Bone Marrow: Composition and Hematopoiesis up until about 28 weeks, when the primary job is taken over by the bone marrow Bone marrow The soft tissue filling the cavities of bones. Bone marrow exists in two types, yellow and red. Yellow marrow is found in the large cavities of large bones and consists mostly of fat cells and a few primitive blood cells. Red marrow is a hematopoietic tissue and is the site of production of erythrocytes and granular leukocytes. Bone marrow is made up of a framework of connective tissue containing branching fibers with the frame being filled with marrow cells. Bone Marrow: Composition and Hematopoiesis.
  • Synthesis Synthesis Polymerase Chain Reaction (PCR) of bile Bile An emulsifying agent produced in the liver and secreted into the duodenum. Its composition includes bile acids and salts; cholesterol; and electrolytes. It aids digestion of fats in the duodenum. Gallbladder and Biliary Tract: Anatomy acid 
• Week 12: beginning of bile Bile An emulsifying agent produced in the liver and secreted into the duodenum. Its composition includes bile acids and salts; cholesterol; and electrolytes. It aids digestion of fats in the duodenum. Gallbladder and Biliary Tract: Anatomy secretion Secretion Coagulation Studies by hepatocytes Hepatocytes The main structural component of the liver. They are specialized epithelial cells that are organized into interconnected plates called lobules. Liver: Anatomy

Pancreas Pancreas The pancreas lies mostly posterior to the stomach and extends across the posterior abdominal wall from the duodenum on the right to the spleen on the left. This organ has both exocrine and endocrine tissue. Pancreas: Anatomy

• Development begins in the 4th week
• Develops primarily from a dorsal pancreatic bud off the foregut:
  • Accessory pancreatic duct Accessory pancreatic duct Pancreas: Anatomy
  • Part of the pancreatic head
  • Body 
  • Tail of the pancreas Pancreas The pancreas lies mostly posterior to the stomach and extends across the posterior abdominal wall from the duodenum on the right to the spleen on the left. This organ has both exocrine and endocrine tissue. Pancreas: Anatomy
• A smaller ventral pancreatic bud contributes to the:
  • Pancreatic head
  • Uncinate process
  • Main pancreatic duct Main pancreatic duct Pancreas: Anatomy
• Both endocrine and exocrine cells arise from the pluripotent endoderm-derived cells of the pancreatic bud
• Week 7: main pancreatic duct Main pancreatic duct Pancreas: Anatomy and accessory pancreatic ducts fuse
• By week 13: alpha, beta, and delta cells present in islets

Spleen Spleen The spleen is the largest lymphoid organ in the body, located in the LUQ of the abdomen, superior to the left kidney and posterior to the stomach at the level of the 9th-11th ribs just below the diaphragm. The spleen is highly vascular and acts as an important blood filter, cleansing the blood of pathogens and damaged erythrocytes. Spleen: Anatomy

• Development begins in the 5th week
• Derived from mesenchymal cells (i.e., from mesoderm Mesoderm The middle germ layer of an embryo derived from three paired mesenchymal aggregates along the neural tube. Gastrulation and Neurulation, not the endoderm-derived foregut)
• Forms from mesenchymal buds off the dorsal mesogastrium
• Buds fuse together, forming the spleen Spleen The spleen is the largest lymphoid organ in the body, located in the LUQ of the abdomen, superior to the left kidney and posterior to the stomach at the level of the 9th-11th ribs just below the diaphragm. The spleen is highly vascular and acts as an important blood filter, cleansing the blood of pathogens and damaged erythrocytes. Spleen: Anatomy.
• Lobulates during fetal life; lobules Lobules Breasts: Anatomy disappear before birth.
• Develops within the dorsal mesogastrium
• Supplied by celiac artery

Midgut and Hindgut Derivatives

The midgut develops into the lining of the GI tract from the distal duodenum Duodenum The shortest and widest portion of the small intestine adjacent to the pylorus of the stomach. It is named for having the length equal to about the width of 12 fingers. Small Intestine: Anatomy (below the ampulla of Vater) to the midtransverse colon Colon The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix: Anatomy. The hindgut develops into the GI tract from the distal ⅓ of the transverse colon Transverse colon The segment of large intestine between ascending colon and descending colon. It passes from the right colic flexure across the abdomen, then turns sharply at the left colonic flexure into the descending colon. Colon, Cecum, and Appendix: Anatomy through the anus.

Midgut milestones

• Cranial portion: grows rapidly and forms small intestinal loop
• Develops cecal swelling Swelling Inflammation:
  • Forms the cecum Cecum The blind sac or outpouching area of the large intestine that is below the entrance of the small intestine. It has a worm-like extension, the vermiform appendix. Colon, Cecum, and Appendix: Anatomy and appendix Appendix A worm-like blind tube extension from the cecum. Colon, Cecum, and Appendix: Anatomy
  • Unequal growth of cecal walls → appendix Appendix A worm-like blind tube extension from the cecum. Colon, Cecum, and Appendix: Anatomy enters the cecum Cecum The blind sac or outpouching area of the large intestine that is below the entrance of the small intestine. It has a worm-like extension, the vermiform appendix. Colon, Cecum, and Appendix: Anatomy medially
• Week 5: obliteration of the vitelline duct Vitelline duct The narrow tube connecting the yolk sac with the midgut of the embryo; persistence of all or part of it in post-fetal life produces abnormalities, of which the commonest is meckel diverticulum. Meckel’s Diverticulum ( yolk sac Yolk Sac The first of four extra-embryonic membranes to form during embryogenesis. In reptiles and birds, it arises from endoderm and mesoderm to incorporate the egg yolk into the digestive tract for nourishing the embryo. In placental mammals, its nutritional function is vestigial; however, it is the source of intestinal mucosa; blood cells; and germ cells. It is sometimes called the vitelline sac, which should not be confused with the vitelline membrane of the egg. Embryoblast and Trophoblast Development is pinched off from the midgut)
• Week 6: physiologic herniation Herniation Omphalocele of the midgut through the umbilicus
  • The midgut naturally herniates through the umbilical ring.
  • Creates more space within the embryo Embryo The entity of a developing mammal, generally from the cleavage of a zygote to the end of embryonic differentiation of basic structures. For the human embryo, this represents the first two months of intrauterine development preceding the stages of the fetus. Fertilization and First Week for the the developing abdominal organs (especially the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy)
• Weeks 6–10:
  • Rapid growth of the midgut occurs outside the embryo Embryo The entity of a developing mammal, generally from the cleavage of a zygote to the end of embryonic differentiation of basic structures. For the human embryo, this represents the first two months of intrauterine development preceding the stages of the fetus. Fertilization and First Week
  • Rotation Rotation Motion of an object in which either one or more points on a line are fixed. It is also the motion of a particle about a fixed point. X-rays:
    • Midgut rotates 270 degrees counterclockwise around SMA.
    • Pulls the small intestine Small intestine The small intestine is the longest part of the GI tract, extending from the pyloric orifice of the stomach to the ileocecal junction. The small intestine is the major organ responsible for chemical digestion and absorption of nutrients. It is divided into 3 segments: the duodenum, the jejunum, and the ileum. Small Intestine: Anatomy into mature anatomic position: jejunum Jejunum The middle portion of the small intestine, between duodenum and ileum. It represents about 2/5 of the remaining portion of the small intestine below duodenum. Small Intestine: Anatomy begins in the LUQ 
    • Pulls the large intestine Large intestine The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix: Anatomy into its mature anatomic position: upside-down U shape surrounding the small intestines
• Week 10: midgut returns to the abdominal cavity
  • Growing abdominal cavity creates enough space for the developing midgut
  • Clinical relevance: Failure of the midgut to reenter the abdomen results in an omphalocele Omphalocele Omphalocele is a congenital anterior abdominal wall defect in which the intestines are covered by peritoneum and amniotic membranes. The condition results from the failure of the midgut to return to the abdominal cavity by 10 weeks’ gestation. Omphalocele (a congenital anterior abdominal wall Abdominal wall The outer margins of the abdomen, extending from the osteocartilaginous thoracic cage to the pelvis. Though its major part is muscular, the abdominal wall consists of at least seven layers: the skin, subcutaneous fat, deep fascia; abdominal muscles, transversalis fascia, extraperitoneal fat, and the parietal peritoneum. Surgical Anatomy of the Abdomen defect in which the intestines are covered by peritoneum Peritoneum The peritoneum is a serous membrane lining the abdominopelvic cavity. This lining is formed by connective tissue and originates from the mesoderm. The membrane lines both the abdominal walls (as parietal peritoneum) and all of the visceral organs (as visceral peritoneum). Peritoneum: Anatomy and amniotic membranes).

Hindgut milestones

The hindgut develops simultaneously and in close association with the urogenital system.

• Cloaca:
  • Distal portion of the hindgut
  • Adjacent to the “outside” of the developing embryo Embryo The entity of a developing mammal, generally from the cleavage of a zygote to the end of embryonic differentiation of basic structures. For the human embryo, this represents the first two months of intrauterine development preceding the stages of the fetus. Fertilization and First Week at the cloacal membrane, which:
    • Area where endoderm Endoderm The inner of the three germ layers of an embryo. Gastrulation and Neurulation and ectoderm Ectoderm The outer of the three germ layers of an embryo. Gastrulation and Neurulation met MET Preoperative Care at the ends of the primitive gut tube
    • Ultimately ruptures, becoming the anus
  • Early in development, empties into a drainage tube called the allantois 
• Allantois:
  • Drains both gut tube and developing urinary system early 
  • Travels through umbilical cord Umbilical cord The flexible rope-like structure that connects a developing fetus to the placenta in mammals. The cord contains blood vessels which carry oxygen and nutrients from the mother to the fetus and waste products away from the fetus. Placenta, Umbilical Cord, and Amniotic Cavity 
  • Called the urachus later in development
  • Ultimately obliterates, becoming the median umbilical ligament along the internal anterior abdominal wall Abdominal wall The outer margins of the abdomen, extending from the osteocartilaginous thoracic cage to the pelvis. Though its major part is muscular, the abdominal wall consists of at least seven layers: the skin, subcutaneous fat, deep fascia; abdominal muscles, transversalis fascia, extraperitoneal fat, and the parietal peritoneum. Surgical Anatomy of the Abdomen
  • Clinical relevance: incomplete obliteration can lead to a urachal fistula Fistula Abnormal communication most commonly seen between two internal organs, or between an internal organ and the surface of the body. Anal Fistula (infants may leak urine from the umbilicus at birth)
• Weeks 4‒7: cloaca divided by a growing urorectal septum:
  • Starts at the superior/proximal portion of the cloaca → grows distally until it reaches the outside of the body, where it becomes the perineal body
  • Separates the cloaca into 2 structures:
    • Urogenital sinus Urogenital Sinus Congenital Malformations of the Female Reproductive System (anterior/superior): connects to the ureters Ureters One of a pair of thick-walled tubes that transports urine from the kidney pelvis to the urinary bladder. Urinary Tract: Anatomy; develops into the bladder Bladder A musculomembranous sac along the urinary tract. Urine flows from the kidneys into the bladder via the ureters, and is held there until urination. Pyelonephritis and Perinephric Abscess, prostate Prostate The prostate is a gland in the male reproductive system. The gland surrounds the bladder neck and a portion of the urethra. The prostate is an exocrine gland that produces a weakly acidic secretion, which accounts for roughly 20% of the seminal fluid. (males), and urethra Urethra A tube that transports urine from the urinary bladder to the outside of the body in both the sexes. It also has a reproductive function in the male by providing a passage for sperm. Urinary Tract: Anatomy
    • Anal canal (posterior/inferior)
  • Clinical relevance: Failure of the anorectal septum to completely divide the cloaca can result in fistulas between the urogenital system and the anorectum.
• Anal pit:
  • Invagination of the ectoderm Ectoderm The outer of the three germ layers of an embryo. Gastrulation and Neurulation
  • Grows toward the anal canal
  • Pectinate line Pectinate line Rectum and Anal Canal: Anatomy:
    • The point where endoderm Endoderm The inner of the three germ layers of an embryo. Gastrulation and Neurulation (proximal) and ectoderm Ectoderm The outer of the three germ layers of an embryo. Gastrulation and Neurulation (distal) of the anal canal meet
    • Marks the prior location of the cloacal membrane 
  • Clinical relevance:
    • Imperforate anus: failure of the cloacal membrane to rupture
    • Anal atresia Atresia Hypoplastic Left Heart Syndrome (HLHS): failure of the anal pit to form/meet rectum Rectum The rectum and anal canal are the most terminal parts of the lower GI tract/large intestine that form a functional unit and control defecation. Fecal continence is maintained by several important anatomic structures including rectal folds, anal valves, the sling-like puborectalis muscle, and internal and external anal sphincters. Rectum and Anal Canal: Anatomy; more complicated repair because there are no sphincter muscles
• Hindgut/endoderm-derived: proximal ⅔ of the anal canal, lined by columnar epithelial cells
• Ectoderm-derived: distal ⅓ of the anal canal, lined by stratified squamous epithelial cells
• Hindgut is supplied by the IMA

Clinical Relevance

• Gastroschisis Gastroschisis Gastroschisis is a congenital abdominal wall defect characterized by the complete lack of closure of the abdominal musculature. A portion of intestine does not return to the abdominal cavity, thereby remaining in its early embryonic herniated state but with no coverings. Gastroschisis (see Gastroschisis Gastroschisis Gastroschisis is a congenital abdominal wall defect characterized by the complete lack of closure of the abdominal musculature. A portion of intestine does not return to the abdominal cavity, thereby remaining in its early embryonic herniated state but with no coverings. Gastroschisis): defect caused by incomplete lateral folding of trilaminar embryo Embryo The entity of a developing mammal, generally from the cleavage of a zygote to the end of embryonic differentiation of basic structures. For the human embryo, this represents the first two months of intrauterine development preceding the stages of the fetus. Fertilization and First Week, resulting in an incomplete “tube.” This incomplete tube results in a defect in the abdominal wall Abdominal wall The outer margins of the abdomen, extending from the osteocartilaginous thoracic cage to the pelvis. Though its major part is muscular, the abdominal wall consists of at least seven layers: the skin, subcutaneous fat, deep fascia; abdominal muscles, transversalis fascia, extraperitoneal fat, and the parietal peritoneum. Surgical Anatomy of the Abdomen; the intestines will be free floating in the amniotic sac, not covered by peritoneum Peritoneum The peritoneum is a serous membrane lining the abdominopelvic cavity. This lining is formed by connective tissue and originates from the mesoderm. The membrane lines both the abdominal walls (as parietal peritoneum) and all of the visceral organs (as visceral peritoneum). Peritoneum: Anatomy. 
• Omphalocele Omphalocele Omphalocele is a congenital anterior abdominal wall defect in which the intestines are covered by peritoneum and amniotic membranes. The condition results from the failure of the midgut to return to the abdominal cavity by 10 weeks’ gestation. Omphalocele: failure of midgut to reenter abdomen after physiologic herniation Herniation Omphalocele. This failure results in a congenital anterior abdominal wall Abdominal wall The outer margins of the abdomen, extending from the osteocartilaginous thoracic cage to the pelvis. Though its major part is muscular, the abdominal wall consists of at least seven layers: the skin, subcutaneous fat, deep fascia; abdominal muscles, transversalis fascia, extraperitoneal fat, and the parietal peritoneum. Surgical Anatomy of the Abdomen defect where the intestines are covered by peritoneum Peritoneum The peritoneum is a serous membrane lining the abdominopelvic cavity. This lining is formed by connective tissue and originates from the mesoderm. The membrane lines both the abdominal walls (as parietal peritoneum) and all of the visceral organs (as visceral peritoneum). Peritoneum: Anatomy and amniotic membranes.
• Esophageal atresia Atresia Hypoplastic Left Heart Syndrome (HLHS): portions of the esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus: Anatomy fail to fully form. Esophageal atresia Atresia Hypoplastic Left Heart Syndrome (HLHS) often coexists with a tracheoesophageal fistula Fistula Abnormal communication most commonly seen between two internal organs, or between an internal organ and the surface of the body. Anal Fistula.
• Tracheoesophageal fistula Fistula Abnormal communication most commonly seen between two internal organs, or between an internal organ and the surface of the body. Anal Fistula: connection between the trachea Trachea The trachea is a tubular structure that forms part of the lower respiratory tract. The trachea is continuous superiorly with the larynx and inferiorly becomes the bronchial tree within the lungs. The trachea consists of a support frame of semicircular, or C-shaped, rings made out of hyaline cartilage and reinforced by collagenous connective tissue. Trachea: Anatomy and esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus: Anatomy. This fistula Fistula Abnormal communication most commonly seen between two internal organs, or between an internal organ and the surface of the body. Anal Fistula occurs when the tracheoesophageal grooves fail to fully close in the midline (which should normally separate the trachea Trachea The trachea is a tubular structure that forms part of the lower respiratory tract. The trachea is continuous superiorly with the larynx and inferiorly becomes the bronchial tree within the lungs. The trachea consists of a support frame of semicircular, or C-shaped, rings made out of hyaline cartilage and reinforced by collagenous connective tissue. Trachea: Anatomy from the esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus: Anatomy). Tracheoesophageal fistula Fistula Abnormal communication most commonly seen between two internal organs, or between an internal organ and the surface of the body. Anal Fistula is often associated with esophageal atresia Atresia Hypoplastic Left Heart Syndrome (HLHS). Multiple different anatomic variations are possible.
• Hypertrophic pyloric stenosis Stenosis Hypoplastic Left Heart Syndrome (HLHS): narrowing of the pylorus Pylorus The region between the sharp indentation at the lower third of the stomach (incisura angularis) and the junction of the pylorus with the duodenum. Pyloric antral glands contain mucus-secreting cells and gastrin-secreting endocrine cells (g cells). Stomach: Anatomy of the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach: Anatomy due to the thickening of its pyloric sphincter muscle. Hypertrophic pyloric stenosis Stenosis Hypoplastic Left Heart Syndrome (HLHS) presents clinically with nonbilious vomiting Vomiting The forcible expulsion of the contents of the stomach through the mouth. Hypokalemia and an olive-shaped mass Olive-Shaped Mass Hypertrophic Pyloric Stenosis in the epigastric region Epigastric region Anterior Abdominal Wall: Anatomy.
• Congenital defects of duodenum Duodenum The shortest and widest portion of the small intestine adjacent to the pylorus of the stomach. It is named for having the length equal to about the width of 12 fingers. Small Intestine: Anatomy: failure of the duodenum Duodenum The shortest and widest portion of the small intestine adjacent to the pylorus of the stomach. It is named for having the length equal to about the width of 12 fingers. Small Intestine: Anatomy to fully recanalize, leading to either stenosis Stenosis Hypoplastic Left Heart Syndrome (HLHS) (narrowing) or obstruction. Duodenal atresia Atresia Hypoplastic Left Heart Syndrome (HLHS) presents with bilious vomiting Bilious Vomiting Congenital Duodenal Obstruction, and a double-bubble sign can be seen on x-ray X-ray Penetrating electromagnetic radiation emitted when the inner orbital electrons of an atom are excited and release radiant energy. X-ray wavelengths range from 1 pm to 10 nm. Hard x-rays are the higher energy, shorter wavelength x-rays. Soft x-rays or grenz rays are less energetic and longer in wavelength. The short wavelength end of the x-ray spectrum overlaps the gamma rays wavelength range. The distinction between gamma rays and x-rays is based on their radiation source. Pulmonary Function Tests; this defect is associated with Down syndrome Down syndrome Down syndrome, or trisomy 21, is the most common chromosomal aberration and the most frequent genetic cause of developmental delay. Both boys and girls are affected and have characteristic craniofacial and musculoskeletal features, as well as multiple medical anomalies involving the cardiac, gastrointestinal, ocular, and auditory systems. Down syndrome (Trisomy 21). 
• Annular Annular Dermatologic Examination pancreas Pancreas The pancreas lies mostly posterior to the stomach and extends across the posterior abdominal wall from the duodenum on the right to the spleen on the left. This organ has both exocrine and endocrine tissue. Pancreas: Anatomy (see Pancreas Pancreas The pancreas lies mostly posterior to the stomach and extends across the posterior abdominal wall from the duodenum on the right to the spleen on the left. This organ has both exocrine and endocrine tissue. Pancreas: Anatomy): results from the growth of the pancreatic bud around the duodenum Duodenum The shortest and widest portion of the small intestine adjacent to the pylorus of the stomach. It is named for having the length equal to about the width of 12 fingers. Small Intestine: Anatomy, which forms a pancreatic ring. May cause duodenal obstruction. Women are affected more frequently than men. 
• Pancreas Pancreas The pancreas lies mostly posterior to the stomach and extends across the posterior abdominal wall from the duodenum on the right to the spleen on the left. This organ has both exocrine and endocrine tissue. Pancreas: Anatomy divisum (see Pancreas Pancreas The pancreas lies mostly posterior to the stomach and extends across the posterior abdominal wall from the duodenum on the right to the spleen on the left. This organ has both exocrine and endocrine tissue. Pancreas: Anatomy): occurs when the ventral and dorsal ducts of the pancreas Pancreas The pancreas lies mostly posterior to the stomach and extends across the posterior abdominal wall from the duodenum on the right to the spleen on the left. This organ has both exocrine and endocrine tissue. Pancreas: Anatomy fail to fuse together to form the main pancreatic duct Main pancreatic duct Pancreas: Anatomy during the 8th week. May predispose individual to chronic pancreatic pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways. 
• Accessory spleens (see Spleen Spleen The spleen is the largest lymphoid organ in the body, located in the LUQ of the abdomen, superior to the left kidney and posterior to the stomach at the level of the 9th-11th ribs just below the diaphragm. The spleen is highly vascular and acts as an important blood filter, cleansing the blood of pathogens and damaged erythrocytes. Spleen: Anatomy): numerous splenic masses with functioning splenic tissue, which can occur when splenic buds fail to fully fuse. An accessory spleen Accessory Spleen Imaging of the Spleen usually exists near the hilum Hilum Lungs: Anatomy of the spleen Spleen The spleen is the largest lymphoid organ in the body, located in the LUQ of the abdomen, superior to the left kidney and posterior to the stomach at the level of the 9th-11th ribs just below the diaphragm. The spleen is highly vascular and acts as an important blood filter, cleansing the blood of pathogens and damaged erythrocytes. Spleen: Anatomy or the tail of the pancreas Pancreas The pancreas lies mostly posterior to the stomach and extends across the posterior abdominal wall from the duodenum on the right to the spleen on the left. This organ has both exocrine and endocrine tissue. Pancreas: Anatomy.
• Intestinal malrotation Malrotation Pediatric Gastrointestinal Abnormalities: failure of the GI tract to undergo normal rotation Rotation Motion of an object in which either one or more points on a line are fixed. It is also the motion of a particle about a fixed point. X-rays around the mesenteric vessels during embryologic development. Intestinal malrotation Malrotation Pediatric Gastrointestinal Abnormalities can result in a number of anatomic patterns characterized by abnormal location and attachments of the intestines within the abdominal cavity. These anomalies can be clinically silent or present with a number of complications, the most catastrophic of which is midgut volvulus Midgut Volvulus Intestinal Malrotation. 
• Volvulus Volvulus A volvulus is the twisting or axial rotation of a portion of the bowel around its mesentery. The most common site of volvulus in adults is the colon; most frequently the sigmoid volvulus. Patients typically present with symptoms of bowel obstruction such as abdominal pain, distension, vomiting, and constipation/obstipation. Volvulus: twisting or axial Axial Computed Tomography (CT) rotation Rotation Motion of an object in which either one or more points on a line are fixed. It is also the motion of a particle about a fixed point. X-rays of a portion of bowel around its mesentery Mesentery A layer of the peritoneum which attaches the abdominal viscera to the abdominal wall and conveys their blood vessels and nerves. Peritoneum: Anatomy. The most common site of volvulus Volvulus A volvulus is the twisting or axial rotation of a portion of the bowel around its mesentery. The most common site of volvulus in adults is the colon; most frequently the sigmoid volvulus. Patients typically present with symptoms of bowel obstruction such as abdominal pain, distension, vomiting, and constipation/obstipation. Volvulus encountered in adults is the colon Colon The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix: Anatomy. Presentation is typically with symptoms of bowel obstruction Bowel obstruction Any impairment, arrest, or reversal of the normal flow of intestinal contents toward the anal canal. Ascaris/Ascariasis, such as abdominal pain Abdominal Pain Acute Abdomen, distention, vomiting Vomiting The forcible expulsion of the contents of the stomach through the mouth. Hypokalemia and constipation Constipation Constipation is common and may be due to a variety of causes. Constipation is generally defined as bowel movement frequency < 3 times per week. Patients who are constipated often strain to pass hard stools. The condition is classified as primary (also known as idiopathic or functional constipation) or secondary, and as acute or chronic. Constipation/ obstipation Obstipation Large Bowel Obstruction. 
• Meckel diverticulum Diverticulum A pouch or sac opening from the colon. Diverticular Disease: failure of the vitelline duct Vitelline duct The narrow tube connecting the yolk sac with the midgut of the embryo; persistence of all or part of it in post-fetal life produces abnormalities, of which the commonest is meckel diverticulum. Meckel’s Diverticulum to obliterate, resulting in a “true” diverticulum Diverticulum A pouch or sac opening from the colon. Diverticular Disease, meaning that the outpouching from the ileum Ileum The distal and narrowest portion of the small intestine, between the jejunum and the ileocecal valve of the large intestine. Small Intestine: Anatomy contains all layers of the bowel wall. (In a “false” diverticulum Diverticulum A pouch or sac opening from the colon. Diverticular Disease, only the mucosa and submucosa protrude.) These diverticula arise from the antimesenteric surface of the middle-to-distal ileum Ileum The distal and narrowest portion of the small intestine, between the jejunum and the ileocecal valve of the large intestine. Small Intestine: Anatomy, anchoring it to the anterior abdominal wall Abdominal wall The outer margins of the abdomen, extending from the osteocartilaginous thoracic cage to the pelvis. Though its major part is muscular, the abdominal wall consists of at least seven layers: the skin, subcutaneous fat, deep fascia; abdominal muscles, transversalis fascia, extraperitoneal fat, and the parietal peritoneum. Surgical Anatomy of the Abdomen, and may lead to a fistula Fistula Abnormal communication most commonly seen between two internal organs, or between an internal organ and the surface of the body. Anal Fistula, volvulus Volvulus A volvulus is the twisting or axial rotation of a portion of the bowel around its mesentery. The most common site of volvulus in adults is the colon; most frequently the sigmoid volvulus. Patients typically present with symptoms of bowel obstruction such as abdominal pain, distension, vomiting, and constipation/obstipation. Volvulus, small bowel obstruction Bowel obstruction Any impairment, arrest, or reversal of the normal flow of intestinal contents toward the anal canal. Ascaris/Ascariasis, and/or necrosis Necrosis The death of cells in an organ or tissue due to disease, injury or failure of the blood supply. Ischemic Cell Damage.