See also superovulation.
At the beginning of the cycle the GnRH from the hypothalamus stimulates
the anterior pituitary cells to produce a mixture of two hormones FSH (Follicle
Stimulating Hormone) and LH (Luteinizing Hormone). The effect of these hormones
is to recruit a group of primordial follicles so that follicular growth
begins.
[ In men the the LH stimulates the release of testosterone from the testis which with FSH acts on the Sertoli cells in the seminiferous tubules and initiates the production of spermatozoa in a process known as spermatogenesis.]
As the follicles grow they become surrounded by several types of cells, blood vessels, nerves and muscle cells. Some of these cells also secrete steroid hormones from which estradiol and progesterone are derived.
The cells around the follicle increase in number and begin to secrete follicular fluid which collects in the antrum (central cavity) of the follicle.
The strength of the estradiol signal increases as the follicles enlarge - it is this hormone which acts as a 'messenger' to the pituitary gland providing a measure of the follicular growth.
When the pituitary 'senses' that the follicle has reached maturity a surge of LH will initiate the release of the egg - see the follicle erupting below right.
The collapsed follicle becomes a structure known as the corpus luteum (brown collapsed structure right) which continues to secrete estradiol and begins the production of progesterone which will support a pregnancy if that occurs.
The embryo hatches and begins to implant in the lining of the uterus at about 6 days. By 12 days it has 'burrowed' beneath the surface and already the beginnings of the blood vessel connections for the placenta are being made. Progesterone support begins in the corpus luteum. After 12 weeks the progesterone support is provided by the placenta.