Endocrinology of the Ovarian Cycle

Selected topic from "Novas Perspectivas em Ginecologia"

Nelson Soucasaux
Nelson Drawing 1993

In my book mentioned above, I have observed that the complexity of the neuroendocrine, endocrine, biochemical and psychical phenomena that occur in the female organism at each ovarian and menstrual cycle is so great that we can even say that it is very difficult to find two exactly equal menstrual cycles. Even from the exclusively endocrine point of view, there are lots of possible patterns in the secretion of estrogens ( estrone and estradiol ) and progesterone - as well as of androgens ( androstenedione and testosterone ), which, curiously, are precursors of the estrogens in the sexual steroids synthesis.

There are also anovulatory cycles, in which there is no progesterone ( except on the minimal basal levels, that have no clinical importance, and only exist because progesterone synthesis is one of the first stages in the synthesis of all other sexual steroids, both the gonadal and adrenal ones ). Even in ovulatory cycles, the interrelations between estrogens and progesterone levels exhibit great variations from cycle to cycle. Here we must remember that during the first phase of a typical ovulatory cycle, there is an increasing production of estrogens by the growing ovarian follicles - mostly by the dominant follicle, the one that will ovulate. It is only after ovulation, in the second phase of the cycle, with the luteinization of the ruptured follicle and formation of the corpus luteum, that this structure begins to produce higher levels of progesterone. Together with progesterone, the corpus luteum also produces estrogens.

Therefore, throughout ovulatory cycles ( considered as biphasic ), women produce good levels of estrogens during the two phases of the cycle, while progesterone is only produced in adequate levels in the second phase. On the other hand, during anovulatory cycles ( considered as monophasic ) there is only a significant production of estrogens ( which can be moderate, normal or even elevated, a fact that can create situations of relative or absolute hyperestrogenism ). At the end of each ovarian cycle (ovulatory or anovulatory), there is a sudden fall in the hormonal levels, which cause the regression and necrosis of the endometrium and the consequent menstrual desquamation of this tissue.

The beginning of menstruation is the most evident external event that indicates the end of a cycle and the beginning of a new one. For clinical purposes, of a practical order, it was stipulated in a simplified way to consider the first day of menstruation as being the first day of the new cycle that initiates, and this is how we usually proceed in Gynecology. However, it must be said that, in the intimacy of the complex phenomena that occur in the ovaries ( both from the histologic and endocrine points of view ), it is impossible to determine with precision the end of one cycle and the beginning of the next one.

All estrogenic and/or estrogenic-progesteronic cyclical actions produce innumerable cyclical transformations in the female sexual organs ( genitals and breasts ), in women's physiology and in other parts of their bodies. These rhythmically arranged changes can also generate, through the somatopsychic pathways, psychological alterations. The ovarian cycle is controlled by the hypothalamus-pituitary system by means of feedback mechanisms and, because of this, it also controls this system. There is a continuous two-way interaction between both, by extremely intricate mechanisms.

Simplifying too much the subject for didactical reasons, this is what happens: the hypothalamus ( part of the brain which the pituitary is attached to ), by means of the production of a neurohormone known as Gn-RH ( gonadotropin releasing hormone ), controls the pituitary (hypophyseal) release of the gonadotropins, which are the follicle stimulating hormone ( FSH ) and the luteinizing hormone ( LH ). FSH and LH are the pituitary (hypophyseal) hormones that control the ovarian function, commanding the processes of follicular maturation, ovulation, formation of the corpus luteum and synthesis of the sexual steroids. The estrogens produced in the ovaries under the command of the pituitary gonadotropins cause, in turn, negative and positive retroactive effects upon the hypothalamic centres responsible for the Gn-RH production.

There is also an interference in these mechanisms due to prolactin, another pituitary hormone whose main action takes place on the breasts. The release of prolactin by the pituitary ( hypophysis ) is also controlled by a hypothalamic factor formerly known as PIF ( prolactin inhibiting factor ), presently identified as the neurohormone dopamine. Furthermore, the hypothalamus is always receiving influences from other parts of the brain, as the limbic system and the cortex. The interaction between the psyche and the cerebral cortex demonstrates how women's emotional problems interfere on the hypothalamus-pituitary-ovaries axis, being capable of generating various endocrine disorders, including many of those that are usually seen in gynecologic practice.

Considering the multiplicity of effects produced by the ovarian hormones on the female body, the more evidently manifested ones are: 1) the remarkable puberal changes, through which the girl's body acquires the typical features of the adult woman's one; 2) the innumerable phenomena that occur in the female sexual organs and organism along each cycle - among them, the periodic uterine bleeding which is menstruation.

Nelson Soucasaux is a gynecologist dedicated to Clinical, Preventive and Psychosomatic Gynecology. Graduated in 1974 by Faculdade de Medicina da Universidade Federal do Rio de Janeiro, he is the author of several articles published in medical journals, and of the books "Novas Perspectivas em Ginecologia" ("New Perspectives in Gynecology") and "Os Órgãos Sexuais Femininos: Forma, Função, Símbolo e Arquétipo" ("The Female Sexual Organs: Shape, Function, Symbol and Archetype"), published by Imago Editora, Rio de Janeiro, 1990, 1993.

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