The normal reproductive years of the female are
characterized by monthly rhythmical changes in the values of secretion of the releted hormones and equivalent physical changes in the ovaries and other sexual
organs. This rhythmical pattern is called the female monthly sexual cycle (or,
the menstrual cycle). The duration of the cycle averages 28 days. It may be as short
as 20 days or as long as 45 days in some women, although abnormal cycle length
is frequently related with decreased fertility.
There are mainly two important results of the female sexual cycle.
Gonadotropic
Hormones and Their Effects on the Ovaries:
Follicular
Phase:
Development Follicles
Ovulation:
Monthly Endometrial Cycle and Menstruation
There are mainly two important results of the female sexual cycle.
1.
Only a single ovum
is normally released from the ovaries each month, so normally only a single
fetus will begin to grow at a time based on fertilization.
2.
The uterine
endometrium is prepared in advance for implantation of the fertilized ovum at
the necessary time of the month.
There are four phases to complete a full hormonal regulation. These
are:
1.
Follicular Phase
2.
Ovulation
3.
Luteum Phase
4.
Menstruation
The ovarian changes that occur during the menstrual
cycle depend completely on the gonadotropic hormones FSH and LH, secreted by
the anterior pituitary gland. Absence of these hormones, the ovaries still remain
inactive, which is the case throughout childhood, when almost no pituitary
gonadotropic hormones are secreted. The pituitary begins
to secrete progressively more FSH and LH at the age 9 to 12 years, which leads to onset of normal
monthly menstrual cycles beginning between the ages of 11 and 15 years. This
period of the change is called puberty, the time of the first menstrual cycle
is called menarche. Both FSH and LH are stimulate their ovarian target cells by
linking with highly specific FSH and LH receptors in the ovarian target cell membranes.
In turn, the activated receptors raise the cells’ rates of secretion and normally the growth and proliferation of the cells as well.
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Figure: Regulation of the hypothalamic-pituitary ovarian axis in females. Stimulatory effects are shown by (+)and
negative feedback inhibitory effects are shown by (-). For large view Please click here
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A newly born female child, each ovum is
surrounded by a single layer of granulosa cells; the ovum, with this granulosa
cell scabbard, is called a primordial
follicle. Throughout childhood, the granulosa cells are provide nutrition
for the ovum. Then, after getting puberty, when FSH and LH release from the anterior
pituitary gland, some of the follicles among them begin to grow, its called primary follicles. At puberty
only 60000 to 80000 primary follicles are left in each ovary.
Development Follicles
During the first few days of each monthly female sexual cycle, the concentrations
of both FSH and LH increase mini to moderately, FSH increase little bit greater than that of LH. These hormones, specially FSH, cause accelerated
growth of 6 to 12 primary follicles in every month. The primary follicles get
surrounded by more layers of granulosa cells and a new theca, called secondary
follicles. The secondary follicle soon transforms into a tertiary follicle
which is characterized by a fluid filled cavity called antrum. The theca layer is organized into 2 theca, one is an inner theca interna
and another is an outer theca externa.
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| Figure: Stage of follicular growth in the ovary, also presenting the formation of the corpus luteum after ovulation. |
After the early proliferative phase of growth, extended for a few days, the mass of granulosa cells secretes a follicular fluid what contains a high concentration of estrogen, one of the significant female sex
hormones.
Once the antral follicles(A developing stage of follicle) begin to grow, their
growth occurs almost explosively. The ovum, itself also enlarges in diameter
another threefold to fourfold, a total ovum diameter increase up to
10 fold, or a mass increase of 1000 fold. As the follicle enlarges, the ovum still remains set in a mass of granulosa cells located at one pole of the
follicle.
Only One Follicle Fully Matures Each Month, and the
Remainder Undergo Atresia (the rest 5 to 11 ovarian follicles which do not
ovulate during the menstrual cycle) The single follicle reaches a diameter almost 1
to 1.5 cm at the time of ovulation and is called the mature follicle.
Ovulation:
In a woman who has a normal 28 days female sexual cycle, ovulation fall out 14 days after the onset of menstruation. Surge of LH is necessary for final follicular growth
and ovulation. Without LH, even when large quantities of FSH are
available, the follicle will not develop to the stage of ovulation. About 2
days before ovulation, the rate of LH secretion by the anterior pituitary gland
increases notably, rising 6 fold to 10 fold and before
ovulation peaking about 16 hours. FSH also increases about twofold to threefold at the same time, and
the FSH and LH act exaggeratingly to cause rapid swelling of the follicle during
the last few days before ovulation. While progesterone
begin to be secreted of increasing amounts. On the granulosa and
theca cells, the LH also has a specific effect to conver them mainly to progesterone-secreting cells. Therefore,
the rate of estrogen secretion begins to fall about 1 day before ovulation. Within
a few hours, two events occur
1.
Swelling of the
entire follicle
2.
Growth of new blood
vessels into the follicle wall
Finally, the combination of above 2 the follicle
be ruptured, with discharge of the ovum.
Luteum Phase:
During the first few hours after discharge of
the ovum from the follicle, the remaining granulosa and theca interna cells
change rapidly into lutein cells. They enlarge in diameter two or more times
and become filled with lipid inclusions that give them a yellowish (LH
responsible for this) appearance. This method is called Latinization, and
the total amount of cells together is called the corpus luteum. In the corpus luteum the granulosa
cells develop extensive intracellular smooth endoplasmic
reticula that form large amounts of estrogen and progesterone (more progesterone than estrogen during the luteal phase). The corpus luteum
normally grows to about 1.5 cm in diameter, reaching this stage of development 7
to 8 days after ovulation. Then it begins to involute and ultimately miss out its
secretory function, lipid characteristic and its yellowish about 12 days after
ovulation, becoming the corpus albicans
which absorber over the months. Once LH has acted on the granulosa and the
newly formed lutein cells seem to be programmed to go through a preordained
sequence of 1. proliferation, 2. enlargement 3. secretion and then 4. degeneration. All this occurs in about the 12 days.
During the luteal phase the estrogen and
progesterone to a lesser extent, secreted by the corpus luteum. Final involution of carpus luteum normally happens
at the end of almost exactly 12 days of corpus luteum life, which is around the
26th day of the normal female sexual cycle that means 2 days before menstruation
begins. At this time, the sudden stop of
secretion of estrogen, progesterone, and inhibin by the corpus luteum removes
the feedback inhibition of the anterior pituitary gland, permitting it to
secret increasing amounts of FSH and LH
again. Beginning a new ovarian
cycle, FSH and LH initiate the growth of the new follicles. The secretion of progesterone and estrogen at this time also leads to
menstruation by the uterus.
Monthly Endometrial Cycle and Menstruation
Associated with the monthly cyclical production
of estrogens and progesterone by the ovaries is an endometrial cycle in the
lining of the uterus that operates through the following stages:
1.
Proliferation of the
uterine endometrium;
2.
Development of
secretory changes in the endometrium; and
3.
Desquamation of the
endometrium, which is known as menstruation.
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| Figure: Phases of endometrial growth and during each month human female sexual cycle |
Proliferative
Phase: It’s also called the Estrogen Phase of the Endometrial
Cycle, happen before vulation. At the beginning of each monthly cycle, most of
the endometrium has been come off by menstruation. After menstruation, only
a thin layer of endometrial stroma and epithelial cell remains. Under the
influence of estrogens, secreted in increasing quantities by the ovary at the initial time of monthly sexual circle. After begening of menstruation the
endometrial surface is re-epithelialized within 4 to 7 days. Then, during the next week and a half, before ovulation occurs,
the endometrium increases greatly in thickness by 3 to 5 mm owing to increasing
numbers of stromal cells and
Development
of secretory changes: It is known as Progestational
Phase of the Endometrial Cycle, happen
after ovulation. This is mainly endometrial building phase. About a week after
ovulation, the endometrium has a thickness of 5 to 6 mm. The purpose of endometrial
changes is to produce a secretory endometrium that contains large amounts of
stored nutrients to provide appropriate conditions for implantation of a fertilized
ovum. A ovum can be fertilized after 3 to 4 days of ovulation. A fertilized ovum
enters the uterine cavity from the fallopian tub. Until the ovum
implants (7 to 9 days after ovulation), the uterine secretions (uterine
milk), provide nutrition for the early dividing ovum. Then, once the ovum
implants in the endometrium, as a result making great quantities of nutrients available
to the early implanting embryo.
Menstruation: If the ovum is not fertilized after 3 to 4 days of
ovulation, the corpus luteum in the ovary suddenly involutes and decrease estrogens
and progesterone to low levels of secretion. End of monthly sexual circle these
2 hormones, especially progesterone reduction is the main cause of menstruation.
Sudden decoration of these 2 hormones decrease the stimulation of endometrial
cell then reduce almost 65 percent of its previous thickness. Gradually, the
necrotic outer layers of the endometrium separate from the uterus at the sites
of the bleedings until, about 48 hours after starting menstruation. Once only
endometrial stroma and epithelial cells that are remains and all acting
together, initiate uterine contractions that expel the uterine contents. During
normal menstruation, approx 40 ml of blood and an additional 35
ml of serous fluid are lost. Menstruation remain up to 4 to 7 days
after starts, by this time the endometrium has become re- epithelialized.
