Endocrine Regulation of the Cycle - Reproduction
You have learnt above that the reproductive cycles are governed by the interplay of pituitary and gonadal hormones. According to current concepts, a feedback mechanism operates whereby the pituitary release of FSH and LH is controlled by the levels of estrogen and progesterone in the circulation. It is not known what factors are originally responsible for the activation of the pituitary-ovarian axis, but it has been postulated that very low levels of estrogens, coming from the immature follicles or extragonadal sources may stimulate the pituitary to augment its release of FSH and LH.
When the level of estrogen in the blood becomes high, indicating that the ovarian follicles are full-grown, it acts to prevent a greater release of FSH by the hypophysis and to promote an augmented release of LH. Under the influence of rising titers of LH, preovulatory swelling ensures and definite lutein changes occur in the walls of the mature follicles. The preovulatory follicle secretes some progesterone as well as large quantities of estrogens. Ovulation occurs while LH is in ascendancy and there is an immediate fall in the circulating estrogens after ovulation. The ruptured follicle becomes transformed into a corpus luteum, which becomes functional under the influence of prolactin, a hormone from pituitary which is also known as luteotropic hormone. The discharge of LH from the pituitary seems to be inhibited by rising titers of progesterone. The corpora lutea remain functional for only a short period unless pregnancy or pseudopregnancy supervenes, but the ovaries of cyclic rats always contain several sets of corpora lutea in different stages of disintegration.
Changes in the ovaries must be regarded as resulting from the interaction of the gonadotropins and changes in the sex accessories as consequences of the interaction of the various ovarian hormones. There is ample evidence that in many mammalian species the secretion of progesterone by the follicle begins before ovulation has occurred during the period of preovulatory swelling. Even in species that ovulate spontaneously it is an interesting fact that sexual receptivity precedes ovulation. In the cow ovulation is spontaneous, but it does not occur until 13 to 15 hours after the end of heat, The secretion of progesterone by the ovarian follicles of the rat, guinea pig, and perhaps other species probably coincides with the onset of sexual receptivity. The uteri of the rat become quite small and anaemic during diestrus, indicating that while the corpora lutea persist they secrete progesterone only for a brief time in the reproductive cycle. When pregnancy or pseudopregnancy follows a period of estrus, the corpora lutea remain functional much longer, probably owing to the action of prolactin.
The progestational changes in rats and mice are much less extensive than those that occur in the uteri of such forms as the rabbit. However, the progestational uteri conditioned by estrogen plus progesterone, are equally sensitive to implanting blastocytes or to endometrial trauma.