Lecture 34- Oogenesis and ovarian cycle

Explain the phases of the uterine cycle and the
role of estrogens and progesterone in each one.

1. Menstrual Phase (Days 1–5)

What happens:

• The endometrial lining sheds, leading to menstrual bleeding.

• This occurs if fertilization has not happened, and hormone levels drop.

Hormone roles:

• Estrogen & progesterone levels are low, which triggers the shedding of the lining.

• The drop in hormones causes the blood vessels in the endometrium to constrict, breaking down tissue and leading to menstruation.

---

2. Proliferative Phase (Days 6–14)

What happens:

• The endometrium rebuilds itself, becoming thicker and more vascular.

• This phase ends with ovulation (around day 14).

Hormone roles:

• Estrogen levels rise (produced by developing follicles in the ovary).

• Estrogen stimulates the regeneration and thickening of the endometrial lining.

• Progesterone remains low in this phase.

---

3. Secretory Phase (Days 15–28)

What happens:

• The endometrium becomes even thicker and more glandular, preparing to support a fertilized egg.

• If no fertilization, the cycle starts over.

Hormone roles:

• After ovulation, the corpus luteum (remains of the follicle) secretes high levels of progesterone and some estrogen.

• Progesterone maintains and enhances the endometrium to support a possible pregnancy.

• If no pregnancy, progesterone and estrogen levels drop, leading to the next menstrual phase.

What happens to the corpus luteum and
endometrium if fertilization does not occur?

. Corpus Luteum:

• After ovulation, the corpus luteum (the leftover follicle in the ovary) produces progesterone (and some estrogen) to maintain the endometrium.

• If no fertilized egg implants, the corpus luteum degenerates after about 10–14 days.

• As it breaks down, progesterone and estrogen levels drop sharply.

---

2. Endometrium:

• Without progesterone, the thickened endometrial lining can no longer be maintained.

• The blood vessels constrict, tissues break down, and the lining is shed.

• This marks the beginning of a new menstrual cycle—the menstrual phase, where the endometrium is expelled as menstrual blood.

---

Summary:

• Corpus luteum → degenerates → hormone levels drop

• Endometrium → breaks down → menstruation begins

What happens to the corpus luteum and
endometrium if fertilization occurs?

1. Corpus Luteum:

• After ovulation, the corpus luteum continues to produce progesterone (and some estrogen).

• If a fertilized egg implants in the uterus, the developing embryo releases a hormone called hCG (human chorionic gonadotropin).

• hCG "rescues" the corpus luteum, keeping it alive and functional.

• The corpus luteum keeps producing high levels of progesterone to maintain the endometrium and prevent menstruation.

• This continues until the placenta forms (around 8–12 weeks), at which point the placenta takes over hormone production.

---

2. Endometrium:

• Under the influence of sustained progesterone, the endometrium:

  • Stays thick, nutrient-rich, and well-vascularized.

  • Becomes a suitable environment for the implantation and nourishment of the embryo.

• The endometrium does not shed, so menstruation does not occur.

---

Summary:

• Fertilization → embryo implants → hCG released

• hCG keeps corpus luteum alive → progesterone stays high

• Endometrium maintained → no menstruation → pregnancy continues

What are the roles of HCG, estrogens, and
progesterone during pregnancy?

1. hCG (Human Chorionic Gonadotropin)

Produced by: The embryo (specifically, the cells that become the placenta)
Roles:

• Saves the corpus luteum, keeping it alive so it can keep producing progesterone.

• Helps maintain the endometrium and prevent menstruation.

• Stimulates production of progesterone and estrogen in early pregnancy.

• Is the hormone detected by pregnancy tests.

Levels peak around 8–11 weeks, then decrease as the placenta takes over hormone production.

---

2. Estrogens

Produced by: Corpus luteum (early on), then placenta later in pregnancy
Roles:

• Promotes growth of the uterus and blood supply to support the fetus.

• Stimulates development of breast tissue for lactation.

• Helps regulate other hormones and maintain the uterine lining.

• In later stages, it helps prepare the body for labor by increasing uterine sensitivity to oxytocin.

---

3. Progesterone

Produced by: Corpus luteum at first, then placenta
Roles:

• Maintains the endometrial lining so the embryo can stay implanted.

• Suppresses uterine contractions, preventing early labor.

• Helps develop the placenta and supports fetal development.

• Works with estrogen to prepare breasts for milk production.