At
the beginning of each menstrual cycle, several oocytes start developing
in their follicle in both ovaries, but only one of them will achieve
complete growth, while others are doomed to regress.
Maturation of the oocyte depends on different hormonal secretions.
These hormones are released by endocrine glands into the blood to act,
at a distance on their target organs, either by stimulating other hormonal
production, or by preparing a given organ for its function.
Among endocrine glands, the pituitary located underneath the brain
regulates the cycle by secreting to main hormones under the influence
of the hypotalamus :
Schematically,
the cycle may be divided into 3 periods :
1.Before
ovulation (=follicular phase)
Normally,
this period extends from the 1st to the 14th day of the cycle
and corresponds to FS H secretion by the pituitary gland. This hormone
directs the development of follicles containing an oocyte and induces
the production, by the follicle itself, of a class of ovarian hormones
called "oestrogens". Under FSH action, the follicle grows
and forms a small cyst containing the maturing oocyte at the surface
of the ovary. (Fig.2)
Oestrogens
act at the level of the uterus by :
-
inducing
thickening of the endouterine mucosa ("endometrium")
in view of its preparation to the reception of an implanting
embryo. (Fig. 3);
-
preparing
the uterine cervix to become permeable to spermatozoa by stimulating
the production of an adequate type of cervical muccus.
On the
other hand, a mechanism of intra-ovarian control prevents the maturation
of more than one follicle per cycle.
At the end of this first phase, i.e. around day 12, the pituitary
gland, under the influence of increasing levels of oestrogen in the
blood, will rapidly release high quantities of the LH hormone. This
event is known as the mid cycle LH peak which induces ovulation.
(Fig.4)
2.
Ovulation
Some
37-38 hrs after the ovulatory discharge of LH, rupture of the dominant
follicle occurs, releasing the mature oocyte into the Fallopian
tube where it starts its journey toward the uterine cavity. This
process of ovulation occurs around the 14-15th day after the onset
of menstruation.
3.
After ovulation (luteal phase)
This
period normally lasts 14 days.
Under the action of LH, the ruptured follicle is transformed into
a "corpus luteum". This structure not only secretes some
oestrogen but also increases quantities of another hormone called
progesterone which completes maturation and receptivity of the endometrian.
If no
implantation occurs, the corpus luteum regresses and rapidly ceases
to secrete hormones. Disintegration of the endometrium ensues,
bringing on the menstrual flow.. The pituitary gland then resumes
FSH secretion and another cycle is started.
If,
in contrast, a fertilized oocyte develops into an embryo which
normally implants around day 21, the function of the corpus luteum
is maintained under the action of an embryonic hormone called hCG
(chrorionic gonadotrophin) known as the "hormone of pregnancy".
