Concerning the structural and functional changes in the ovary and endometrium during the menstrual cycle, all of the following are true EXCEPT:

Explanation

## Endometrial and Ovarian Changes Across the Menstrual Cycle ### Endometrial Transformation in the Luteal Phase **Key Point:** The endometrium does NOT remain proliferative during the luteal phase. Instead, it undergoes **secretory differentiation** under the influence of progesterone. ### Three Phases of Endometrial Change | Phase | Timing | Hormone | Endometrial State | Thickness | |-------|--------|---------|-------------------|----------| | **Menstrual** | Days 1–5 | Low E₂, low P₄ | Shedding of functionalis layer | <2 mm | | **Proliferative** | Days 5–14 | Rising E₂ | Mitotic activity, gland elongation, stromal edema | 8–12 mm at ovulation | | **Secretory** | Days 15–28 | High P₄ (+ E₂) | Glandular secretion, stromal edema, spiral artery development | 10–14 mm (peak) | **High-Yield:** Progesterone transforms the proliferative endometrium into a **secretory endometrium** characterized by: - Subnuclear vacuoles in glandular epithelium (days 17–19) - Stromal edema and vascularization - Spiral artery elongation and coiling - Secretion of glycogen, mucus, and growth factors (e.g., IGFBP-1, prolactin) This secretory phase is **essential for implantation**; without it, a blastocyst cannot implant. **Warning:** Do NOT confuse "secretory phase" with "secretion only after implantation." Secretory changes begin 2–3 days post-ovulation, *before* implantation, in preparation for it. ### Corpus Luteum Function (Options 1 & 3 — Both Correct) **Mnemonic: "CL = P₄ + E₂"** — The corpus luteum secretes both progesterone (dominant, ~25 mg/day) and estradiol (~5 mg/day). If pregnancy does not occur: - Corpus luteum lifespan = 14 ± 2 days - Luteolysis (apoptosis) is triggered by prostaglandin F₂α and loss of LH support - Progesterone and estradiol levels fall sharply - Endometrial shedding (menstruation) follows