Functional Considerations Summary of Hormonal Regulation of the Ovarian Cycle

During each menstrual cycle, the ovary undergoes cyclic changes that involve two phases:

• Follicular phase

Ovulation occurs between the two phases (Fig. 22.14).

The follicular phase begins with the development of a small number of primary follicles (10 to 20) under the influence of FSH and LH. During the first 8 to 10 days of the cycle, FSH is the principal hormone influencing the growth of the follicles. It stimulates the granulosa and thecal cells, which begin to secrete steroid hormones, principally estrogens, into the follicular lumen. Late in the follicular phase, before ovulation, progesterone levels begin to rise under the influence of LH. Estrogens continue to accumulate in the follicular lumen, finally reaching a level that makes the follicle independent of FSH for its continued growth and development. The amount of estrogens in the circulating blood inhibits further production of FSH by the adenohypophysis. Ovulation is induced by a surge in the LH level, which occurs concomitantly with a smaller increase in the FSH level.

The luteal phase begins immediately after ovulation, as the granulosa and thecal cells of the ruptured follicle undergo rapid morphologic transformation to form the corpus luteum. Estrogens and large amounts of progesterone are secreted by the corpus luteum. Under the influence of both hormones, but primarily progesterone, the endometrium begins its secretory phase, which is essential for the preparation of the uterus for implantation in the event that the egg is fertilized. LH appears to be responsible for the development and maintenance of the corpus luteum during the menstrual cycle. If fertilization does not occur, the corpus luteum degenerates within a few days as the hormonal levels drop. If fertilization does occur, the corpus luteum is maintained and continues to secrete progesterone and estrogens. hCG, which is initially produced by the embryo and later by the placenta, stimulates the corpus luteum and is responsible for its maintenance during pregnancy.

menstrual cycle pituitary hormones (in blood)

ovarian cycle

menstrual

proliferative

secretory

phase

phase

phase

ovarian hormones (in blood)

pituitary hormones (in blood)

ovarian cycle

menstrual cycle

ovarian hormones (in blood)

Was this article helpful?

0 0

Post a comment