Fertilization and First Week

Fertilization

Sperm enter the uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Uterus, Cervix, and Fallopian Tubes: Anatomy with sexual intercourse, subsequently entering the fallopian tubes Fallopian tubes The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The fallopian tubes receive an ovum after ovulation and help move it and/or a fertilized embryo toward the uterus via ciliated cells lining the tubes and peristaltic movements of its smooth muscle. Uterus, Cervix, and Fallopian Tubes: Anatomy and traveling upstream to the ampulla of the fallopian tube Fallopian Tube A pair of highly specialized canals extending from the uterus to its corresponding ovary. They provide the means for ovum transport from the ovaries and they are the site of the ovum’s final maturation and fertilization. The fallopian tube consists of an interstitium, an isthmus, an ampulla, an infundibulum, and fimbriae. Its wall consists of three layers: serous, muscular, and an internal mucosal layer lined with both ciliated and secretory cells. Uterus, Cervix, and Fallopian Tubes: Anatomy. The ampulla is the location of fertilization, which is a multistep process.

Step 1: 1st contact

• Sperm burrow through the external matrix to touch the oocyte.
• Facilitated by protein PH-20

Step 2: binding of sperm and oocyte

• Penetration Penetration X-rays of the zona pellucida by sperm
• Sperm receptor Receptor Receptors are proteins located either on the surface of or within a cell that can bind to signaling molecules known as ligands (e.g., hormones) and cause some type of response within the cell. Receptors binds ZP3 glycoprotein of zona pellucida → digestive enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body’s constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes (acrosin) release from the acrosome → sperm moves closer → sperm binds ZP2 glycoprotein → the head moves inside the zona pellucida

Step 3: release of sperm contents

• Sperm fuses with the egg membrane by an interaction between integrins Integrins A family of transmembrane glycoproteins (membrane glycoproteins) consisting of noncovalent heterodimers. They interact with a wide variety of ligands including extracellular matrix proteins; complement, and other cells, while their intracellular domains interact with the cytoskeleton. The integrins consist of at least three identified families: the cytoadhesin receptors(receptors, cytoadhesin), the leukocyte adhesion receptors (receptors, leukocyte adhesion), and the very late antigen receptors. Each family contains a common beta-subunit (integrin beta chains) combined with one or more distinct alpha-subunits (integrin alpha chains). These receptors participate in cell-matrix and cell-cell adhesion in many physiologically important processes, including embryological development; hemostasis; thrombosis; wound healing; immune and nonimmune defense mechanisms; and oncogenic transformation. Leukocyte Adhesion Deficiency Type 1 ( oocytes Oocytes Female germ cells derived from oogonia and termed oocytes when they enter meiosis. The primary oocytes begin meiosis but are arrested at the diplotene state until ovulation at puberty to give rise to haploid secondary oocytes or ova (ovum). Ovaries: Anatomy) and disintegrins (sperm) → DNA DNA A deoxyribonucleotide polymer that is the primary genetic material of all cells. Eukaryotic and prokaryotic organisms normally contain DNA in a double-stranded state, yet several important biological processes transiently involve single-stranded regions. DNA, which consists of a polysugar-phosphate backbone possessing projections of purines (adenine and guanine) and pyrimidines (thymine and cytosine), forms a double helix that is held together by hydrogen bonds between these purines and pyrimidines (adenine to thymine and guanine to cytosine). DNA Types and Structure released into the ovum
• Phospholipase C Phospholipase C A subclass of phospholipases that hydrolyze the phosphoester bond found in the third position of glycerophospholipids. Although the singular term phospholipase C specifically refers to an enzyme that catalyzes the hydrolysis of phosphatidylcholine, it is commonly used in the literature to refer to broad variety of enzymes that specifically catalyze the hydrolysis of phosphatidylinositols. Pseudomonas:
  • Released by sperm into the ovum
  • Breaks down PIP2 into IP3 IP3 Intracellular messenger formed by the action of phospholipase C on phosphatidylinositol 4, 5-bisphosphate, which is one of the phospholipids that make up the cell membrane. Inositol 1, 4, 5-trisphosphate is released into the cytoplasm where it releases calcium ions from internal stores within the cell’s endoplasmic reticulum. These calcium ions stimulate the activity of B kinase or calmodulin. Second Messengers and DAG DAG Second Messengers → calcium Calcium A basic element found in nearly all tissues. It is a member of the alkaline earth family of metals with the atomic symbol ca, atomic number 20, and atomic weight 40. Calcium is the most abundant mineral in the body and combines with phosphorus to form calcium phosphate in the bones and teeth. It is essential for the normal functioning of nerves and muscles and plays a role in blood coagulation (as factor IV) and in many enzymatic processes. Electrolytes signalling cascade → cortical reaction
  • Cortical reaction makes the egg impenetrable to further insemination.

1st Week of Human Development

During the 1st week of development, the fertilized egg completes meiosis II Meiosis II Meiosis II is a cellular division event wherein the number of chromosomes in the daughter cells is halved from that of the mother cell. Meiosis II: similar to meiosis I but not preceded by interphase (DNA replication) Meiosis within the ampulla of the fallopian tube Fallopian Tube A pair of highly specialized canals extending from the uterus to its corresponding ovary. They provide the means for ovum transport from the ovaries and they are the site of the ovum’s final maturation and fertilization. The fallopian tube consists of an interstitium, an isthmus, an ampulla, an infundibulum, and fimbriae. Its wall consists of three layers: serous, muscular, and an internal mucosal layer lined with both ciliated and secretory cells. Uterus, Cervix, and Fallopian Tubes: Anatomy. After completion of meiosis II Meiosis II Meiosis II is a cellular division event wherein the number of chromosomes in the daughter cells is halved from that of the mother cell. Meiosis II: similar to meiosis I but not preceded by interphase (DNA replication) Meiosis, the fertilized oocyte travels from the fallopian tube Fallopian Tube A pair of highly specialized canals extending from the uterus to its corresponding ovary. They provide the means for ovum transport from the ovaries and they are the site of the ovum’s final maturation and fertilization. The fallopian tube consists of an interstitium, an isthmus, an ampulla, an infundibulum, and fimbriae. Its wall consists of three layers: serous, muscular, and an internal mucosal layer lined with both ciliated and secretory cells. Uterus, Cervix, and Fallopian Tubes: Anatomy to the uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Uterus, Cervix, and Fallopian Tubes: Anatomy, which is the location of implantation.

• Days 1–3:
  • Fertilization of the oocyte
  • Completion of meiosis Meiosis The creation of eukaryotic gametes involves a DNA replication phase followed by 2 cellular division stages: meiosis I and meiosis II. Meiosis I separates homologous chromosomes into separate cells (1n, 2c), while meiosis II separates sister chromatids into gametes (1n, 1c). Meiosis
  • Early cell division Cell Division A type of cell nucleus division by means of which the two daughter nuclei normally receive identical complements of the number of chromosomes of the somatic cells of the species. Cell Cycle
• Day 4:
  • Morula formed: 16 cells
  • Zona pellucida breaks down → fluid moves in
• Day 5:
  • Blastocyst formed: 70–100 cells
  • Outer cell mass Mass Three-dimensional lesion that occupies a space within the breast Imaging of the Breast → trophoblast → supportive tissues of the 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 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
  • Inner cell mass Mass Three-dimensional lesion that occupies a space within the breast Imaging of the Breast → embryoblast Embryoblast Embryoblast and Trophoblast Development → definitive structures of the fetus
• Days 6–10:
  • Implantation of the embryo into the uterine wall
  • The blastocyst begins to secrete β-hCG.

Clinical Relevance

Ectopic pregnancy Ectopic pregnancy Ectopic pregnancy refers to the implantation of a fertilized egg (embryo) outside the uterine cavity. The main cause is disruption of the normal anatomy of the fallopian tube. Ectopic Pregnancy: implantation occurs outside the uterine cavity, most commonly in the ampulla of the fallopian tube Fallopian Tube A pair of highly specialized canals extending from the uterus to its corresponding ovary. They provide the means for ovum transport from the ovaries and they are the site of the ovum’s final maturation and fertilization. The fallopian tube consists of an interstitium, an isthmus, an ampulla, an infundibulum, and fimbriae. Its wall consists of three layers: serous, muscular, and an internal mucosal layer lined with both ciliated and secretory cells. Uterus, Cervix, and Fallopian Tubes: Anatomy. Patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship with an ectopic pregnancy Ectopic pregnancy Ectopic pregnancy refers to the implantation of a fertilized egg (embryo) outside the uterine cavity. The main cause is disruption of the normal anatomy of the fallopian tube. Ectopic Pregnancy present with severe lower abdominal pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways, which is often localized to the quadrant where the ectopic pregnancy Ectopic pregnancy Ectopic pregnancy refers to the implantation of a fertilized egg (embryo) outside the uterine cavity. The main cause is disruption of the normal anatomy of the fallopian tube. Ectopic Pregnancy is located. An ectopic pregnancy Ectopic pregnancy Ectopic pregnancy refers to the implantation of a fertilized egg (embryo) outside the uterine cavity. The main cause is disruption of the normal anatomy of the fallopian tube. Ectopic Pregnancy is a medical emergency and must be either medically managed ( methotrexate Methotrexate An antineoplastic antimetabolite with immunosuppressant properties. It is an inhibitor of tetrahydrofolate dehydrogenase and prevents the formation of tetrahydrofolate, necessary for synthesis of thymidylate, an essential component of DNA. Antimetabolite Chemotherapy) or surgically removed.