Phases of the Menstrual Cycle
The menstrual cycle prepares the ovaries for ovulation and the endometrium (uterine lining) for implantation of the fertilized egg (ovum).
Follicular Development
[Shown in the top row of the above figure.] Every 28 days or so, new follicles (clusters of cells that each enclose a developing egg) begin to grow inside the ovaries. Usually just one follicle fully matures and ovulates. The empty follicle becomes the corpus luteum, which secretes large quantities of progesterone and estrogen. It soon degenerates, hormonal secretion declines sharply, and menstruation starts.
Endometrial Development
[Shown in the bottom row of the above figure.] Here are the phases and the activities during endometrial development:
- Menstruation - The outer layers of the endometrium slough off.
- Postmenstrual phase - Endometrial cells proliferate and the lining thickens.
- Midcycle - An egg is expelled from the ovary (ovulation) into the pelvic cavity.
- Premenstrual phase - The endometrium continues to mature until a sudden drop in hormone levels triggers menstruation.
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