The Physiology of Menstruation
Dr. Nelson Soucasaux, Brazilian gynecologist
Of all organs of the body, the uterus is the only one whose inner lining
mucosa is almost entirely expelled and reconstructed periodically, both
phenomena taking place at each ovarian cycle. With the purpose of facilitating
the periodic elimination of the endometrium that undergoes regression, shrinkage
and necrosis at the end of each cycle, the uterus also exhibits the unique
peculiarity of physiologically bleeding. The endometrial histo-physiology
is entirely controlled by the ovarian hormones along the cycle. Of all tissues
of the woman's body, the endometrium is the one that, throughout de ovarian
cycle, more perfectly reflects the levels in the production of estrogens
The estrogens are responsible for the process of proliferation and growth
of the endometrium, a process that begins immediately after menstruation
and by means of which a new endometrium is constructed at each cycle. As
it is widely known, the estrogens cause a strong proliferative effect upon
the tissues of the woman's sexual organs. The estrogenic action on the endometrium
results on great glandular proliferation and great increase in thickness,
as well as the development of its vascularization.
Progesterone is mostly responsible for the secretory transformation
of the endometrium, that occurs during the second phase of the ovulatory
cycles. Under the effect of progesterone, the proliferative activity of
the endometrial glands is greatly reduced, and these glands begin producing
nourishing substances, which also begin to appear in the endometrial connective
tissue. The progesteronic action upon the endometrium has two purposes:
the first can be regarded as mostly "medical", and the second
as essentially reproductive. The "medical" purpose is to oppose
to some excessive proliferative effects of the estrogens, preventing the
appearance of endometrial hyperplasic alterations ( progesterone has a strong
anti-proliferative effect on the epithelium of the endometrial glands ).
The reproductive purpose is to create, in the endometrium, the ideal conditions
for the implantation and development of the egg.
In the absence of pregnancy, at the end of each ovulatory cycle, the
menstrual desquamation of the endometrium is triggered by the sudden fall
in the levels of progesterone and estrogens that result from the involution
of the corpus luteum. On the other hand, at the end of each anovulatory
cycle, the exclusive estrogenic fall, all by itself, also causes the menstrual
desquamation of the endometrium. In both ovulatory and anovulatory cycles,
the sudden fall of the ovarian hormones gives rise to regressive and necrotic
phenomena in the endometrium. By means of complex biochemical and vascular
mechanisms, these phenomena result on the menstrual disintegration and elimination
of this tissue. It is important to emphasize that, though menstruation is
triggered by the sudden hormonal fall, its occurrence is only possible due
to the very peculiar pattern that characterize the endometrial vascularization
and blood supply.
We have already said that, in anovulatory cycles (monophasic), only
the estrogenic fall, all by itself, triggers menstruation, while in ovulatory
cycles (biphasic), the menstrual phenomenon is triggered by the progesteronic-estrogenic
sudden fall that results from the involution of the corpus luteum. Clinically
we also verify that, in the absence of pregnancy and acting upon an endometrium
previously stimulated by the estrogens, the exclusive progesterone withdrawal
equally causes the coming of menstruation - even if there is no simultaneous
fall in the estrogenic levels. The usual medicinal triggering of menstruation
a few days after interrupting the use of progesterone or synthetic progestogens
is based on this fact.
The endometrium is the tissue that covers the cavity of the uterine
corpus, being endowed with a highly complex and organized histological structure.
It is constituted by a stroma of connective tissue containing an enormous
number of tubular-shaped glands, and its blood supply is provided by an
intricate vascular system. The glands are lined by a single layer of columnar
epithelial cells and open on the endometrial surface, which is lined by
the same kind of epithelium. Under the influence of progesterone, its stroma
also acquires secretory features. Two layers can be distinguished in the
endometrium, the functional and the basal.
The functional layer is the one that is entirely expelled and reconstructed
at each menstrual cycle, being able to reach considerable thickness when
fully developed. It is highly responsive to the proliferative effect of
the estrogens and to the secretory transformation induced by progesterone.
Its blood supply (also developed by these hormones) is constituted by an
intricate system of spiral arteries, capillary networks, veins, venous lakes
and arteriovenous anastomosis. Of all structures that integrate this vascular
system, the spiral arteries are the most important ones.
When the blood levels of the ovarian hormones fall, the endometrial
functional layer immediately exhibits signs of regression, shrinkage and
atrophy. This results in the formation of toxic and vasoactive substances
which, by local action, trigger the ischemic and hemorrhagic vascular phenomena
that cause the necrosis and menstrual desquamation of this layer of the
endometrium. Menstruation happens as a consequence of the very peculiar
way through which the mentioned vascular system reacts to the endometrial
regressive changes that result from the sudden fall of the ovarian hormones.
The basal layer of the endometrium possesses small thickness and is
situated deeply, in direct contact with the myometrium. This layer does
not desquamate during menstruation and provides the histologic elements
from which a new functional layer grows when the menstrual bleeding ceases.
The basal layer does not exhibit significative modifications during the
cycle, and seems to be less responsive to the hormonal stimuli. Its blood
supply is provided by another vascular system, constituted by small straight
arteries. In this way, the basal layer does not undergo the necrotic, ischemic
and hemorrhagic alterations of the menstrual period and, because of this,
is always preserved.
The menstrual phenomenon depends on two fundamental factors, one of
which is endocrine and the other vascular. The endocrine one, that consists
on the sudden fall in the blood levels of the ovarian hormones, is the main
triggering factor. The vascular reactions are a consequence of the involutive
endometrial changes caused by the hormonal sudden fall.
The text above is an excerpt from my book "Os Órgãos
Sexuais Femininos: Forma, Função, Símbolo e Arquétipo"
("The Female Sexual Organs: Shape, Function, Symbol and Archetype").
For more information on the book, see page http://www.nelsonginecologia.med.br/orgaos.htm
of my Website http://www.nelsonginecologia.med.br
©Nelson Soucasaux, 1993, 2001
Nelson Soucasaux is a gynecologist especially dedicated to clinical,
preventive and psychosomatic gynecology. Graduated in 1974 by Faculdade
de Medicina da Universidade Federal do Rio de Janeiro, Brazil, 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.
© 2001 Harry Finley. It is illegal
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