The menstrual cycle is a term used to describe
monthly events that occur within a woman’s body in preparation for the possibility of
pregnancy. Each month, an egg is released from an ovary in a process called ovulation.
At the same time, the lining of the uterus thickens, ready for pregnancy. If fertilization
does not take place, the lining of the uterus is shed in menstrual bleeding and the cycle
starts over. An ovary contains hundreds of thousands of
primary oocytes – immature eggs, or ova. Each of these is enclosed in a structure called
a follicle, and at this stage — a primordial follicle.
The menstrual cycle is under control of hormones secreted by the pituitary gland and the ovaries.
The pituitary itself is under control of the hypothalamus.
The hypothalamus produces a hormone called the gonadotropin-releasing hormone or GnRH.
GnRH stimulates the anterior lobe of the pituitary to secrete follicle-stimulating hormone (FSH).
FSH travels in the bloodstream to the ovaries and stimulates a group of follicles to grow.
These primordial follicles develop into primary follicles and then secondary follicles. These
produce a hormone named estrogen which acts to stimulate the growth of the endometrium
— the inner lining of the uterus. The secondary follicles compete with each other and only
one of them will survive and become a mature follicle, the rest atrophy and die.
The increasing level of estrogen also acts on the hypothalamus and the anterior pituitary
to increase the level of GnRH and induce the production of another hormone — luteinizing
hormone (LH). A surge in LH secretion triggers ovulation – the release of the egg from the
follicle and the ovary. The egg is then swept up by the fimbriae and taken into the uterine
tube. Fertilization by a spermatozoon, when it occurs,
usually takes place in the ampulla, the widest section of the fallopian tube. The fertilized
egg immediately begins the process of development while travelling toward the uterus. After
6 days it becomes a blastocyst and is implanted into the endometrium of the uterus.
Meanwhile, the left-over of the ruptured follicle has become a corpus luteum which secretes
progesterone. Progesterone further stimulates uterine development making it a nutritious
bed for the embryo in the event of pregnancy. In the absence of pregnancy, the corpus luteum
atrophies and progesterone level falls. This leads to the breakdown of the endometrium,
menstruation begins and the cycle starts over.