Development of the Abdominal Organs

The abdominal organs are derived primarily from endoderm, which forms the primitive gut tube. The gut tube is divided into 3 regions: foregut, midgut, and hindgut. The foregut gives rise to the lining of the GI tract 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 to the upper duodenum, as well as 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, 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, and 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. The midgut gives rise to the GI tract lining between the midduodenum and 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. The hindgut gives rise to the GI tract lining from 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 through the upper anal canal. The mesoderm gives rise to the muscles of the GI tract wall, 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 (including the mesenteries and omenta), and the vasculature. The ectoderm gives rise to the nerve tissue and the lining of the lower anal canal.

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Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

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Review of Early Embryologic Development

Blastocyst and bilaminar disc

The morula (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 → trophoblast → placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity and membranes
  • Inner cell mass → embryoblast → bilaminar disc:
    • Epiblast
    • Hypoblast
  • Amniotic sac: a cavity of fluid that develops “above” the epiblast (between epiblast and trophoblast)
  • Primitive yolk sac: 
    • A cavity that forms “below” the hypoblast 
    • Important in GI development
Implanted blastocyst

Relationship of the bilaminar disc, yolk sac, and amniotic cavity in the early embryo

Image by Lecturio. License: CC BY-NC-SA 4.0

Trilaminar disc

The bilaminar disc 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. There are 3 layers of the trilaminar disc:

  • Ectoderm (continuous with the amnion)
  • Mesoderm:
    • Paraxial mesoderm
    • Intermediate mesoderm
    • Lateral plate mesoderm (LPM): 
      • Somatic layer of the LPM
      • Splanchnic layer of the LPM
    • Extraembryonic mesoderm:
      • Surrounds the amniotic cavity → continuous with somatic LPM
      • Surrounds the yolk sac → continuous with the splanchnic LPM
  • Endoderm 
    • Embryonic endoderm (usually just called the endoderm) → becomes the primitive gut tube
    • Extraembryonic endoderm → lines the yolk sac
    • Secondary yolk sac: the cavity between embryonic and extraembryonic endoderm
  • Folding of the trilaminar disc (folds in 2 directions):
    • Lateral folding: 
      • Creates a cylindrical structure surrounded by ectoderm and the amniotic cavity
      • Creates the primitive gut tube internally from endoderm
    • Cranial–caudal folding: 
      • Creates a cranial end and a caudal end (embryo becomes “bean-shaped”)

Forces the yolk sac farther from the body.→ The elongating stalk connecting the yolk sac to the gut tube is the vitelline duct.

Obliteration of the vitelline duct and yolk sac

  • As the embryo folds, the yolk sac is pushed farther and farther away from the body.
  • Vitelline duct: elongating stalk that connects the midgut to the yolk sac as it is pushed away
  • Growth of the amniotic cavity around weeks 4–6:
    • The amniotic cavity starts on the dorsal side of the embryo 
      • Grows and extends around the cranial and caudal ends of the embryo 
      • Edges of the growing cavity meet on the ventral side at the abdomen → pinch off the yolk sac/obliterate the vitelline duct
  • Clinical relevance: Failure of the vitelline duct to obliterate results in Meckel diverticulum:
    • A true diverticulum (contains all layers of the bowel wall) 
    • Arises from the antimesenteric surface of the middle-to-distal ileum, anchoring it to the anterior abdominal wall Anterior abdominal wall The anterior abdominal wall is anatomically delineated as a hexagonal area defined superiorly by the xiphoid process, laterally by the midaxillary lines, and inferiorly by the pubic symphysis. Anterior Abdominal Wall.
    • May lead to 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 Small Bowel Obstruction Small bowel obstruction (SBO) is an interruption of the flow of the intraluminal contents through the small intestine, and is classified as mechanical (due to physical blockage) or functional (due to disruption of normal motility). The most common cause of SBO in the Western countries is post-surgical adhesions. Small bowel obstruction typically presents with nausea, vomiting, abdominal pain, distention, constipation, and/or obstipation. Small Bowel Obstruction, and/or necrosis

Overview of Gut Tube Differentiation

Structures derived from the gut tube

The primitive gut tube is formed from endoderm at the completion of lateral folding. The gut tube can initially be divided into 3 areas:

  • Foregut: abdominal derivatives supplied by celiac artery
    • Esophagus 
    • Stomach
    • Proximal duodenum (above the ampulla of Vater)
    • Liver
    • Gallbladder and bile ducts
    • Pancreas
    • 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 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
      • Respiratory tract
  • Midgut: derivatives supplied by the superior mesenteric artery ( SMA SMA Spinal muscular atrophy (SMA) is a spectrum of autosomal recessive syndromes characterized by progressive proximal muscle weakness and atrophy, possibly due to degeneration of the anterior horn cells in the spinal cord and motor nuclei in the lower brainstem. Spinal Muscular Atrophy (SMA))
    • Distal duodenum (below the ampulla of Vater)
    • Jejunum
    • Ileum
    • Cecum
    • Appendix
    • Ascending 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
    • Proximal ⅔ of the transverse 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
  • Hindgut: derivatives supplied by the inferior mesenteric artery (IMA)
    • Distal ⅓ of the transverse 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
    • Descending 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
    • Sigmoid 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
    • 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
    • Superior portion of the anal canal
Development of the dorsal mesentery with the primitive gut tube

Development of the dorsal mesentery with the primitive gut tube

Image by Lecturio.

Structures derived from endoderm

Key structures derived from endoderm related to development of the abdominal organs:

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

Structures derived from mesoderm

Key structures derived from mesoderm 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
  • Vasculature
  • Visceral 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 and Retroperitoneum
  • Mesenteries:
    • Dorsal and ventral mesogastrium → greater and lesser omentum, falciform ligament
    • Mesentery 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
    • Transverse and sigmoid mesocolon
    • Numerous named abdominal ligaments

Somatic layer of the LPM: parietal 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 and Retroperitoneum

Structures derived from ectoderm

Key structures derived from ectoderm related to development of the abdominal organs: 

  • Neural crest cells: enteric nerve plexus (i.e., myenteric and submucosal plexuses)
  • Nerves: innervate the abdominal organs
  • Membranes:
    • At the cranial and caudal ends, the ectoderm and endoderm meet (with no mesoderm 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: lines the distal anal canal

Foregut Derivatives, Omenta, and Spleen

Esophagus

Separation from the respiratory system:

  • Lung bud (respiratory diverticulum): 
    • Buds off of the ventral side of the foregut around week 4
    • Simultaneously grows out (ventrally) and down (caudally)
    • Includes both endoderm and surrounding splanchnic LPM
  • Tracheoesophageal groove (or ridge):
    • As the lung bud grows out and down, the tracheoesophageal groove appears as lateral indentations between the new lung bud and the foregut.
    • The grooves/ridges move in medially, pinching off the lung bud, and forming the tracheoesophageal septum.
  • The tracheoesophageal septum creates 2 separate tubes:
    • Esophagus (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 (anteriorly, from the lung bud)
  • Clinical relevance: 
    • Tracheoesophageal fistula Tracheoesophageal fistula Tracheoesophageal fistula is an abnormal connection between the trachea and esophagus. Esophageal Atresia and Tracheoesophageal Fistula (TEF): occurs when the tracheoesophageal grooves fail to fully close in the midline 
    • Esophageal atresia Esophageal atresia Esophageal atresia is a congenital anomaly in which the upper esophagus is separated from the lower esophagus and ends in a blind pouch. The condition may be isolated or associated with tracheoesophageal fistula, which is an abnormal connection between the trachea and esophagus. Esophageal Atresia and Tracheoesophageal Fistula: 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 fail to form; often coexists with TEF

Growth and descent:

  • Esophagus 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
  • Completes development by 7th week
Embryonic development of the bronchial tree - intestinal epithelium

Embryonic development of the bronchial tree Development of the Bronchial Tree Lung development involves 5 stages: embryonic, pseudoglandular, canalicular, saccular, and alveolar. The inner respiratory epithelium arises from the endoderm, and the cartilage, bronchial muscles, connective tissue, and vasculature all arise from the mesoderm. Development of the Bronchial Tree

Image by Lecturio. License: CC BY-NC-SA 4.0

Stomach

  • 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
  • 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 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 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
  • Posterior wall grows more quickly than anterior wall → develops a C shape
    • Posterior wall → greater curvature
    • Anterior wall → lesser curvature
  • 90-degree rotation clockwise around longitudinal axis:
    • Greater curvature (dorsal):
      • Rotates anteriorly and to the left
      • Left vagus nerve innervates anterior wall
    • Lesser curvature (ventral):
      • Rotates posteriorly and to the right
      • Right vagus nerve innervates posterior wall
  • Smaller rotation 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 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, 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 from the dorsal wall
  • Originally located in the median plane
  • Moved to the left during 90-degree longitudinal rotation
  • 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 called the lesser sac (i.e., omental bursa)
  • Forms several important structures:
    • After rotation 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, bulges down to form the greater omentum
    • Gastrosplenic ligament: 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 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
    • Splenorenal ligament: 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 to the posterior abdominal wall (near the kidney)
  • Anterior to the greater omentum is the greater sac
  • Structures developing within the dorsal mesogastrium:
    • Spleen
    • Pancreas

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, duodenum, 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, and ventral abdominal wall
  • Originally located in the median plane
  • Moved to the right during 90-degree longitudinal rotation 
  • 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 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
  • Forms 2 primary structures:
    • Lesser omentum: 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 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
    • Falciform ligament: 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 and the anterior abdominal wall Anterior abdominal wall The anterior abdominal wall is anatomically delineated as a hexagonal area defined superiorly by the xiphoid process, laterally by the midaxillary lines, and inferiorly by the pubic symphysis. Anterior Abdominal Wall
  • Structures developing within the ventral mesogastrium:
    • Liver
    • Gallbladder
    • Bile ducts
Rotation of the stomach and gastric mesenteries

Rotation 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 and gastric mesenteries

Image by Lecturio.

Duodenum

  • 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 SMA Spinal muscular atrophy (SMA) is a spectrum of autosomal recessive syndromes characterized by progressive proximal muscle weakness and atrophy, possibly due to degeneration of the anterior horn cells in the spinal cord and motor nuclei in the lower brainstem. Spinal Muscular Atrophy (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 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 
      • Vacuoles grow and merge → lumen recanalization
    • Clinical relevance: failure of the lumen to fully recanalize results in either:
      • Duodenal stenosis: narrowing of the duodenum
      • Duodenal atresia: a complete obstruction persists; presents with bilious emesis and a double-bubble sign on radiography
Invertogram showing high arm and ap view showing duodenal atresia

Double-bubble sign on radiography (invertography) indicating duodenal obstruction:
The smaller bubble on the individual’s right is air in the duodenum, and the larger bubble on the left is air in 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.

Image: “Invertogram showing high ARM Arm The arm, or "upper arm" in common usage, is the region of the upper limb that extends from the shoulder to the elbow joint and connects inferiorly to the forearm through the cubital fossa. It is divided into 2 fascial compartments (anterior and posterior). Arm and AP view showing duodenal atresia” by Department of Pediatric Surgery, Gauhati Medical College, GUWAHATI, ASSAM, INDIA. License: CC BY 3.0

Liver and biliary ducts

  • Development begins in the 3rd‒4th week
  • Appears from the caudal portion of the foregut as the hepatic diverticulum, which goes on to form the:
    • Liver (endoderm differentiates into hepatocytes)
    • Intrahepatic ducts
    • Extrahepatic portions of the hepatic ducts
  • As the hepatic diverticulum enlarges, the connection to the duodenum narrows → common bile duct
  • A smaller bud grows off the caudal side of the hepatic diverticulum:
    • 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
    • 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 → cystic duct 
  • The hepatic diverticulum grows into the ventral mesogastrium.
  • Weeks 5–9: 
    • Hematopoietic stem cells (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:
      • Hematopoiesis begins 
      • Liver is the primary site of hematopoiesis up until about 28 weeks, when the primary job is taken over by the bone marrow Bone marrow Bone marrow, the primary site of hematopoiesis, is found in the cavities of cancellous bones and the medullary canals of long bones. There are 2 types: red marrow (hematopoietic with abundant blood cells) and yellow marrow (predominantly filled with adipocytes). Composition of Bone Marrow.
    • Synthesis of bile acid 
  • Week 12: beginning of bile secretion by hepatocytes
Model of the pancreatic ducts

Anatomy of the biliary and pancreatic ducts

Image by BioDigital, edited by Lecturio

Pancreas

  • Development begins in the 4th week
  • Develops primarily from a dorsal pancreatic bud off the foregut:
    • Accessory pancreatic duct
    • 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
  • A smaller ventral pancreatic bud contributes to the: 
    • Pancreatic head
    • Uncinate process
    • Main pancreatic duct
  • Both endocrine and exocrine cells arise from the pluripotent endoderm-derived cells of the pancreatic bud
  • Week 7: main pancreatic duct and accessory pancreatic ducts fuse
  • By week 13: alpha, beta, and delta cells present in islets

Spleen

  • Development begins in the 5th week
  • Derived from mesenchymal cells (i.e., from mesoderm, 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.
  • Lobulates during fetal life; lobules 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 (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. The hindgut develops into the GI tract from the distal ⅓ of the transverse 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 through the anus.

Midgut milestones

  • Cranial portion: grows rapidly and forms small intestinal loop
  • Develops cecal swelling:
    • Forms the cecum and appendix
    • Unequal growth of cecal walls → appendix enters the cecum medially
  • Week 5: obliteration of the vitelline duct (yolk sac is pinched off from the midgut)
  • Week 6: physiologic herniation of the midgut through the umbilicus
    • The midgut naturally herniates through the umbilical ring.
    • Creates more space within the embryo 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)
  • Weeks 6–10: 
    • Rapid growth of the midgut occurs outside the embryo
    • Rotation:
      • Midgut rotates 270 degrees counterclockwise around SMA SMA Spinal muscular atrophy (SMA) is a spectrum of autosomal recessive syndromes characterized by progressive proximal muscle weakness and atrophy, possibly due to degeneration of the anterior horn cells in the spinal cord and motor nuclei in the lower brainstem. Spinal Muscular Atrophy (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 into mature anatomic position: jejunum 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 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 Anterior abdominal wall The anterior abdominal wall is anatomically delineated as a hexagonal area defined superiorly by the xiphoid process, laterally by the midaxillary lines, and inferiorly by the pubic symphysis. Anterior Abdominal Wall 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 and Retroperitoneum and amniotic membranes).
Normal process of herniation during embryologic development

Diagram showing the normal process of intestinal rotation and herniation during embryologic development
A: The midgut (multicolored loop) before herniation.
B1–B3: As it grows rapidly, the midgut herniates through the umbilical ring and starts rotation.
C: The midgut returns to the abdominal cavity.

Image by Lecturio.

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 at the cloacal membrane, which:
      • Area where endoderm and ectoderm met 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 
    • Called the urachus later in development
    • Ultimately obliterates, becoming the median umbilical ligament along the internal anterior abdominal wall Anterior abdominal wall The anterior abdominal wall is anatomically delineated as a hexagonal area defined superiorly by the xiphoid process, laterally by the midaxillary lines, and inferiorly by the pubic symphysis. Anterior Abdominal Wall
    • Clinical relevance: incomplete obliteration can lead to a urachal 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 (anterior/superior): connects to the ureters; develops into the bladder, 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. Prostate and other Male Reproductive Glands (males), and urethra
      • 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
    • Grows toward the anal canal
    • Pectinate line: 
      • The point where endoderm (proximal) and ectoderm (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: failure of the anal pit to form/meet rectum; 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
Embryo between weeks 4 and 7

Between weeks 4 and 7, the urorectal septum begins growing into the cloaca, starting at its proximal end and growing distally until it reaches the outside of the embryo, fully separating the cloaca into the urogenital sinus and the anal canal.

Image by Lecturio.

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, resulting in an incomplete “tube.” This incomplete tube results in a defect in the abdominal wall; 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 and Retroperitoneum
  • Omphalocele: failure of midgut to reenter abdomen after physiologic herniation. This failure results in a congenital anterior abdominal wall Anterior abdominal wall The anterior abdominal wall is anatomically delineated as a hexagonal area defined superiorly by the xiphoid process, laterally by the midaxillary lines, and inferiorly by the pubic symphysis. Anterior Abdominal Wall 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 and Retroperitoneum and amniotic membranes.
  • Esophageal atresia Esophageal atresia Esophageal atresia is a congenital anomaly in which the upper esophagus is separated from the lower esophagus and ends in a blind pouch. The condition may be isolated or associated with tracheoesophageal fistula, which is an abnormal connection between the trachea and esophagus. Esophageal Atresia and Tracheoesophageal Fistula: 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 fail to fully form. Esophageal atresia Esophageal atresia Esophageal atresia is a congenital anomaly in which the upper esophagus is separated from the lower esophagus and ends in a blind pouch. The condition may be isolated or associated with tracheoesophageal fistula, which is an abnormal connection between the trachea and esophagus. Esophageal Atresia and Tracheoesophageal Fistula often coexists with a tracheoesophageal fistula.
  • Tracheoesophageal fistula Tracheoesophageal fistula Tracheoesophageal fistula is an abnormal connection between the trachea and esophagus. Esophageal Atresia and Tracheoesophageal Fistula: connection between the trachea 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. This fistula occurs when the tracheoesophageal grooves fail to fully close in the midline (which should normally separate the trachea 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). Tracheoesophageal fistula Tracheoesophageal fistula Tracheoesophageal fistula is an abnormal connection between the trachea and esophagus. Esophageal Atresia and Tracheoesophageal Fistula is often associated with esophageal atresia. Multiple different anatomic variations are possible.
  • Hypertrophic pyloric stenosis Hypertrophic pyloric stenosis Hypertrophic pyloric stenosis is hypertrophy and hyperplasia of the pyloric sphincter muscle. The condition is the most common cause of gastrointestinal obstruction in infants. Affected newborns typically present after the third to fifth week of life with progressive non-bilious vomiting and a firm, olive-like mass in the epigastrium. Hypertrophic Pyloric Stenosis: narrowing of the pylorus 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 due to the thickening of its pyloric sphincter muscle. Hypertrophic pyloric stenosis Hypertrophic pyloric stenosis Hypertrophic pyloric stenosis is hypertrophy and hyperplasia of the pyloric sphincter muscle. The condition is the most common cause of gastrointestinal obstruction in infants. Affected newborns typically present after the third to fifth week of life with progressive non-bilious vomiting and a firm, olive-like mass in the epigastrium. Hypertrophic Pyloric Stenosis presents clinically with nonbilious vomiting and an olive-shaped mass in the epigastric region.
  • Congenital defects of duodenum: failure of the duodenum to fully recanalize, leading to either stenosis (narrowing) or obstruction. Duodenal atresia presents with bilious vomiting, and a double-bubble sign can be seen on x-ray; 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
  • Annular 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 (see Pancreas): results from the growth of the pancreatic bud around the duodenum, which forms a pancreatic ring. May cause duodenal obstruction. Women are affected more frequently than men. 
  • Pancreas divisum (see Pancreas): 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 fail to fuse together to form the main pancreatic duct during the 8th week. May predispose individual to chronic pancreatic pain Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain
  • Accessory spleens (see Spleen): numerous splenic masses with functioning splenic tissue, which can occur when splenic buds fail to fully fuse. An accessory 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 usually exists near the hilum 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 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.
  • Intestinal malrotation Intestinal malrotation Intestinal malrotation is a congenital anomaly that results from failure of the GI tract to undergo normal rotation around the mesenteric vessels during embryologic development. This condition can result in several anatomic patterns characterized by abnormal location and attachments of the abdominal cavity's intestines. Intestinal Malrotation: failure of the GI tract to undergo normal rotation around the mesenteric vessels during embryologic development. Intestinal malrotation Intestinal malrotation Intestinal malrotation is a congenital anomaly that results from failure of the GI tract to undergo normal rotation around the mesenteric vessels during embryologic development. This condition can result in several anatomic patterns characterized by abnormal location and attachments of the abdominal cavity's intestines. Intestinal Malrotation 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 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
  • Volvulus: twisting or axial rotation of a portion of bowel around its mesentery. 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. Presentation is typically with symptoms of bowel obstruction, such as abdominal pain Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain, distention, vomiting 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. 
  • Meckel diverticulum: failure of the vitelline duct to obliterate, resulting in a “true” diverticulum, meaning that the outpouching from the ileum contains all layers of the bowel wall. (In a “false” diverticulum, only the mucosa and submucosa protrude.) These diverticula arise from the antimesenteric surface of the middle-to-distal ileum, anchoring it to the anterior abdominal wall Anterior abdominal wall The anterior abdominal wall is anatomically delineated as a hexagonal area defined superiorly by the xiphoid process, laterally by the midaxillary lines, and inferiorly by the pubic symphysis. Anterior Abdominal Wall, and may lead to a 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 Small Bowel Obstruction Small bowel obstruction (SBO) is an interruption of the flow of the intraluminal contents through the small intestine, and is classified as mechanical (due to physical blockage) or functional (due to disruption of normal motility). The most common cause of SBO in the Western countries is post-surgical adhesions. Small bowel obstruction typically presents with nausea, vomiting, abdominal pain, distention, constipation, and/or obstipation. Small Bowel Obstruction, and/or necrosis.

References

  1. Talathi, S., Zimmerman, R. (2021). Anatomy, abdomen and pelvis, pancreas. StatPearls. Retrieved October 28, 2021, from https://www.statpearls.com/articlelibrary/viewarticle/26567/ 
  2. Hundt, M., Wu, C.Y., Young, M. (2021). Anatomy, abdomen and pelvis, biliary ducts. StatPearls. Retrieved October 28, 2021, from https://www.ncbi.nlm.nih.gov/books/NBK459246/
  3. Mitchell, B., Biol, R. (2009). The digestive system. Chapter 7 of Embryology, 2nd ed. Churchill Livingstone. https://www.sciencedirect.com/science/article/pii/B9780702032257500105 
  4. University of Michigan Medical School. (n.a.) Anatomy: digestive system. Retrieved October 28, 2021, from https://www.med.umich.edu/lrc/coursepages/m1/embryology/embryo/10digestivesystem.htm 
  5. Mazziotti, M. (2021). Congenital anomalies of esophagus. Medscape. Retrieved October 28, 2021, from https://emedicine.medscape.com/article/934420-overview

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