Physiological anatomy of the female sexual system
- Gastroepato
- Clinical sexology
notes by dr Claudio Italiano
The
sexual and reproductive function in women can be considered of two phases: the
first one consisting in the preparation of the organism for conception and
gestation, the second in the gestation itself. Approximately every 28 days the
gonadotropic hormones of the adenohypophysis start the maturation of the new
follicles in the ovary, one of which, at the end, ovulates on the 14th day of
the cycle. During the maturation of follicles, estrogens are secreted. After
ovulation, the secretory cells of the follicle are transformed into a corpus
luteum that secretes considerable quantities of ovarian hormones, progesterone
and estrogen. Two weeks later, the corpus luteum degenerates; then the ovarian
hormones, progesterone and estrogens decrease strongly, and the menstruation
intervenes: another ovarian cycle begins again.
Physiology of female reproduction
The female reproductive system includes the ovary, the fallopian tubes, the
uterus and the vagina. The reproductive process begins with the development of
follicles in the ovary. At the middle of every monthly sexual ode, only one egg
is usually expelled from an ovarian follicle in the abdominal cavity. This egg
then passes through one of the Falloppio trumpets descending into the uterus
and, if it has been fertilized by a sperm, it implants into the uterus where it
gives rise to the fetus, to the placenta and to the fetal membranes. During
fetal life, the outer surface of the ovary is covered by a germinative
epithelium, which embryologically derives directly from the epithelium of the
germinative ridges. When developing the fetus, the primordial eggs differ from
the germinative epithelium and migrate in the thickness of the cortical of the
ovary, dragging behind a layer of epithelioid granulosa cells. The egg
surrounded by a single layer of epithelioid granulosa cells, is what is called a
primordial follicle. At birth there are about 750,000 primary follicles in both
ovaries, but this number is rapidly reduced, so that only 400,000 remain at
puberty. During the fertile age of the woman only about 450 of these follicles
mature enough to expel their egg, while the rest degenerate (become Atresic). At
the end of the fertile period, at the menopause, only a few primordial follicles
remain in the ovary, which are also destined to degenerate soon after.
The female hormonal system
The female hormonal system, like the male one, consists of three different
hierarchies of big man:
1. The liberating factors of the hypothalamus, the liberating factor of the
follicle-stimulating hormone (FRF) and the liberating factor of the luteinizing
hormone (LRF) (which could be the same factor).
2 Adenohypophysis hormones, follicle-stimulating hormone (FSH) and luteinizing
hormone LH
secreted in response to the factor, or factors, of the hypothalamus.
3. Ovarian hormones, estrogen and progesterone, secreted ovary in response to
adenohypophyseal hormones.
The various hormones are not secreted in a constant and continuous way, but
rather at drastically variable rhythms during the different phases of the
woman's cycle. Although no detailed measurements have been made, it is
reasonable to assume that hypothalamic liberating factors also undergo cyclical
variations.
Before discussing the connections between these different hormones, it is
convenient to describe the correlations in relation to the ovarian function.
Monthly ovarian cycle and function of gonadotropic hormones
The normal fertile life of the woman is characterized by monthly rhythmic
variations related to the secretion of female sex hormones and the corresponding
changes in the sexual organs themselves. This rhythmic picture is what is
defined as a woman's sexual cycle (or menstrual cycle). Each cycle lasts an
average of 28 days. This duration may be reduced to 20 days, or move up to 45
days even in completely normal women, although an abnormal duration of the cycle
is often accompanied by a decrease in fertility.
The two most significant events of the female sexual cycle are: first, that at
each cycle only one mature egg is released from the ovaries, so that only one
fetus can begin to develop at a time; second, that the uterine endometrium is
prepared at the right time of the cycle for implantation of the fertilized egg.
Gonadotropic hormones. The sexual cycle depends entirely on the gonadotropic
hormones secreted by the anterior pituitary. The ovaries, if not stimulated by
the gonadotropic hormones, remain completely inactive, something that usually
occurs during childhood, when gonadotropic hormones are almost not secreted. But
at the age of about eight years, the pituitary begins to secrete increasing
quantities of gonadotropic hormones and this culminates with the beginning of
the monthly sex cycles, at an age between eleven and fifteen. This is the
so-called period of puberty.
The anterior pituitary secretes two different hormones, notoriously essential
for a full function of the ovaries:
1) the follìcolo-stimulating hormone (FSH),
2) luteinizing hormone (LH).
Both of these hormones are low molecular weight glycoproteins of 30,000 and
26,000, respectively. LH in the male is also called interstitial cell
stimulating hormone (ICSH), as it stimulates the activity of these cells.
In every month of the woman's sexual cycle, there is a cyclical increase and
decrease of FSH and LH. These cyclic variations, in turn, determine the ovaries
cyclical changes, which will be described later.
Both FSH and LH stimulate ovarian cells by combining with specific receptors of
cell membranes, which in turn activate adenyl cyclase. This increases cyclic AMP
in cells, leading to the development and secretion of special ovarian cells. So
the mechanism of action of these hormones resembles the usual cyclic AMP system
for hormone control.
Maturing of the follicle
FSH hormone
During childhood, primordial follicles do not ripen at all but, at puberty, when
FSH begins to be secreted in large quantities, the ovaries in full and
especially the follicles inside are beginning to develop. The first stage of
follicular development is represented by an enlargement of the egg itself,
followed by a proliferation of the granulosa cells and the development of cell
layers of the theca, all around the surface of the granulosa cells. The cells of
the theca originate from the ovary stroma and soon take on epithelioid
characteristics. Taken together, these cells constitute the so-called inner
casing. It is mainly these cells that secrete most of the female hormones,
estrogens and progesterone. A capsule of connective tissue forms around the
inner case. This is called an external casing and forms the capsule of the
evolving follicle. The vesicular follicles. At the beginning of each woman's
monthly sex cycle, almost at the beginning of menstruation, the concentrations
of FSH and LH are increased. These increases accelerate the development of the
granulosa cells and the case of about twenty ovarian follicles every month. The
cells of the casket and granulosa secrete a follicular fluid, rich in estrogens,
one of the important female sex hormones, which will be explained later. The
accumulation of this liquid in the follicle causes the appearance of an antrum
in the mass of the cells of the case and of the granular. After the formation of
the antrum, the cells of the theca and granulosa continue to proliferate, the
secretion is accelerated, and each evolving follicle is transformed into the
so-called vesicular follicle. This faster growth and more abundant secretion is
determined not only by the follicle-stimulating hormone, but also by two other
factors. First, a small basal amount of luteinizing hormone is secreted by the
anterior pituitary; this hormone has a synergistic and reinforcing effect
compared to the stimulating action of FSH. Secondly, the estrogen secreted in
the follicle also has a similar synergistic effect. As the vesicular follicle
grows, the cells of the case and those of the granulosa continue to develop at
one of the follicle's poles. The egg is housed in this cellular mass (cumulus
oophorus).
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