CH29 Concise

Fertilization is the fusion of two haploid gametes ( $23$ chromosomes) to form a normal diploid zygote ( $46$ chromosomes).
The process typically occurs in the uterine tube between the ampulla and the isthmus.
The sperm provides paternal chromosomes, while the oocyte provides all cellular organelles, nourishment, and genetic programming for early development.
A secondary oocyte is surrounded by the glycoprotein-rich zona pellucida and an outer protective layer of follicle cells called the corona radiata.
Sperm travel from the vagina to the ampulla in approximately $30$ minutes to $2$ hours.
Of roughly $200 \times 10^6$ sperm in an ejaculation, only about $10,000$ enter the uterine tube, and fewer than $100$ reach the isthmus; a sperm count below $20 \times 10^6/mL$ is categorized as sterile.

Sperm penetration is aided by the acrosome reaction, where enzymes like hyaluronidase break bonds between follicle cells in the corona radiata.
Once a sperm reaches the oocyte and binds to receptors in the zona pellucida, it triggers the rupture of the acrosome, releasing hyaluronidase and acrosin to digest a path to the oocyte membrane.
Oocyte activation occurs upon membrane fusion, leading to $Na^+$ ion influx, membrane depolarization, and the release of $Ca^{2+}$ from the smooth endoplasmic reticulum (SER).
The release of $Ca^{2+}$ triggers the cortical reaction, where vesicles release zonal inhibiting proteins (ZIPS) to inactivate sperm receptors and harden the zona pellucida, providing a block to polyspermy.
Activation also leads to the completion of meiosis II (forming an ovum) and a rapid increase in the metabolic rate for protein synthesis.
Moment of Conception: Includes amphimixis, where maternal and paternal chromosomes ( $23$ from the female pronucleus and $23$ from the male pronucleus) combine at metaphase of the first cleavage division.

Cleavage: A series of cell divisions that subdivide the zygote into smaller cells called blastomeres.
Morula: A solid ball of cells formed roughly $3$ days after fertilization.
Blastocyst: A hollow ball of cells formed by day $6$ , featuring an inner cavity (blastocoele), an outer layer (trophoblast) for nutrient absorption, and an inner cell mass (ICM) that becomes the embryo.
Implantation: Begins as the blastocyst contacts the endometrium. Trophoblast cells create the syncytiotrophoblast (a multinucleated cytoplasm layer) which erodes a path into the uterine lining using hyaluronidase.
Extra-embryonic membranes include the yolk sac (early nutrient source), amnion (fluid-filled chamber), allantois (base for the urinary bladder), and chorion.
Gastrulation: Occurs around day $15$ ; cells migrate toward the primitive streak to create three germ layers: ectoderm, mesoderm, and endoderm.

Ectoderm: Forms the nervous system, epidermis, hair, nails, glands of the skin, and the linings of the mouth, nose, and anus.
Mesoderm: Forms the dermis, skeletal and muscular systems, cardiovascular and lymphatic systems, genitourinary systems, and connective tissues.
Endoderm: Forms the linings of the digestive and respiratory tracts, the urinary bladder, urethra, and various endocrine organs.

Placentation: The formation of the placenta from embryonic and maternal tissues. By the $5th$ week, the endometrium is divided into the capsular decidua, basal decidua (concentrated placental functions), and parietal decidua.
Placental Circulation: Blood flows from the fetus to the placenta via paired umbilical arteries (carrying deoxygenated blood and wastes) and returns via a single umbilical vein (carrying oxygenated blood and nutrients).
Organogenesis: The formation of major organ systems, which are largely established in rudiment form by the end of the first trimester.
Second Trimester: Marked by the completion of organ systems, bone marrow formation, ossification, and fetal movement.
Maternal Changes: Include a $50\%$ increase in blood volume and glomerular filtration rate, increased respiratory rate, and mammary gland development.

Human chorionic gonadotropin (hCG): Maintains the corpus luteum for $3$ to $4$ months to prevent menstruation.
Human placental lactogen (hPL): Prepares mammary glands for lactation.
Relaxin: Increases pubic symphysis flexibility and dilates the cervix.
Oxytocin: Released from the fetal pituitary and maternal hypothalamus; stimulates myometrial contractions and the milk ejection reflex.
Stages of Labor:
- Dilation stage: ( $8+$ hours) Cervix dilates and the amniochorionic membrane ruptures.
- Expulsion stage: ( $<2$ hours) Cervix reaches $10\,cm$ dilation; contractions reach maximum intensity.
- Placental stage: (within $1$ hour) The placenta is ejected and uterine contractions control blood loss.

Neonatal Period: (Birth to $1$ month) Adjustments include filling lungs with air, closing the ductus arteriosus and foramen ovale, and stabilizing body temperature.
Puberty: Driven by GnRH from the hypothalamus, leading to FSH and LH secretion by the pituitary, which triggers gamete production and sex hormones.
Senescence: The process of aging, characterized by organ atrophy and decreased homeostatic capacity.
Twin Types:
- Dizygotic (fraternal): Two different oocytes fertilized by two different sperm.
- Monozygotic (identical): One zygote splits into two early in development.
Breech Birth: A delivery where the legs or buttocks enter the vaginal canal first.

Question: Where does fertilization occur?
- Answer: It occurs at the ampulla-isthmus junction within one day of ovulation.
Question: How many sperm are required for oocyte activation?
- Answer: Many sperm create a puncture in the corona radiata, but only one contacts the oocyte membrane for activation.
Question: What is the developmental fate of the inner cell mass?
- Answer: It becomes the embryo.
Question: Why is teratogen exposure most dangerous in the first trimester?
- Answer: This is the period when all major organs are forming (organogenesis).
Question: What does it mean to be homozygous for a trait?
- Answer: It means you will express that trait in your phenotype.
Question: What are the primary functions of hCG?
- Answer: It helps the corpus luteum persist throughout the first trimester.
Question: What is codominance?
- Answer: It means both alleles are equally expressed, such as in blood type AB.