Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Genitourinary System
Reproductive System
Female Reproductive Organs — Ovulation and Menstrual Cycle

A mature vesicular (tertiary) follicle, or Graafian follicle, in one or the other Ovary elevates and eventually ruptures its surface epithelium. Through this rupture, the oocyte (secondary oocyte) is released into the peritoneal cavity near the abdominal opening of the Uterine tube. The process of the follicular rupture and the release of the oocyte is known as ovulation. In place of the ruptured follicle, a corpus luteum forms from its follicular Cells, temporarily functioning as an endocrine gland. The hormone of the corpus luteum, progesterone, inhibits the next ovulation. Under METABOLISM/18.html">The Influence of progesterone, the uterine mucosa thickens, preparing to receive a fertilized egg. If Fertilization does not occur, the corpus luteum (menstrual) regresses within 12–14 days, and its hormone-producing function ceases. As a result, the uterine mucosa is shed, its Blood Vessels rupture, leading to bleeding commonly referred to as menstruation. The next menstruation typically occurs every 28 days on average. The period from the first day of the previous menstruation to the first day of the next is termed the menstrual (ovarian-menstrual) cycle (Fig. 70). The duration of the cycle varies individually, generally ranging from 21 to 30 days. Menstruation begins in girls during Puberty (at 11–16 years of age) and continues until 40–50 years. The average duration of menstruation is 2–3 days, which constitutes the menstrual phase. In the subsequent postmenstrual phase, driven by the hormone estrogen, the uterine mucosa (endometrium) is restored. Then, starting from the 14th–15th day of the Menstrual cycle, coinciding with ovulation, the premenstrual phase begins, during which the uterine mucosa prepares to receive the fertilized egg.

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Fig. 70. Ovarian-Menstrual Cycle:

I — menstrual, II — postmenstrual, III — premenstrual phases; 1 — primordial follicle in the ovary; 2 — primary (growing) follicles; 3 — mature (Graafian) follicle; 4 — ovulation; 5 — fully developed corpus luteum; 6 — regression of the corpus luteum; 7 — anterior pituitary; 8 — posterior pituitary; 9 — fertilization; 10 — corpus luteum of Pregnancy; 11 — implanted embryo. The arrows indicate the effects of follitropin (FSH) on growing follicles, lutropin (LH) on ovulation and corpus luteum formation, prolactin (LTH) on the mature corpus luteum, the stimulatory effect of estrogen (E) on endometrial growth in the Uterus (postmenstrual or proliferative phase), and progesterone (Pg) on the endometrium (premenstrual phase)

If the egg is fertilized and pregnancy occurs, the ruptured vesicular follicle develops into a large corpus luteum of pregnancy, measuring up to 5 cm in diameter. This corpus luteum persists for 6 months, performing an essential endocrine function before eventually regressing.