Menstrual Cycle - Part 2

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Menstrual cycle

The cycle lasts 21–35 days on average
The cycle consists of two phases:
• Follicular phase: accounts for most of the variability in the length of the menstrual cycle
• Luteal phase: lasts 14–15 days
The cycle changes with age:
First few years following menarche → irregular menstrual cycles (caused by the immaturity of the hypothalamic-pituitary-gonadal axis)
Days 1–14

Ovaries: the follicular phase
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1. FSH stimulates the development of several follicles in the ovaries → granulosa cells of follicles produce estrogen → suppresses the release of FSH via a negative feedback loop
2. Selection of a dominant follicle (Graafian follicle)
3. High levels of estrogen trigger a positive feedback loop → release of FSH → subsequent LH surge initiates ovulation
Endometrium: desquamation and proliferative phase
• Desquamation (menstruation at days 1–4)
 Absence of a pregnancy → resolution of corpus luteum → progesterone concentration decreases → induces vasospasms in the uterine spiral arteries, ischemia, and sloughing off of the functional layer of the endometrium
• Proliferation (days 4–14): growing follicles produce estrogen (granulosa cells express aromatase, which converts androgens to estrogens) → stimulates proliferation of the endometrium

Histology
• Proliferation of endometrial epithelial cells (cells show high mitotic activity)
• Endometrial glands are straight, tubular, and lined by simple columnar epithelium.
• Stromal cells start to divide, enlarge, and accumulate glycogen.
• Uterine spiral arteries start to regenerate and extend two-thirds of the way into the endometrium.

Days 14–28
Ovaries: luteal phase
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• Ovulation: rupture of Graafian follicle → oocyte is released
• Following ovulation, the granulosa cells produce LH receptors → LH-induced transformation of the Graafian follicle into the corpus luteum → produces progesterone → inhibits LH release via a negative feedback loop
Endometrium: secretory phase
• Endometrial differentiation promoted by progesterone → preparation of the functional layer of the endometrium for oocyte implantation

Histology
 Increased endometrial gland tortuosity
 Glycogen-rich secretions
 Edematous stromal cells
 Uterine spiral arteries extend the full length of the endometrium.
In the absence of pregnancy → ↓ progesterone levels → induces apoptosis of the functional layer of the endometrium (seen as shrinkage and fragmentation of endometrial cell nuclei) → menstruation

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