Formation of Ova
Primordial germ cells multiply during fetal development. At birth, the ovary contains around 400 000 primordial follicles which contain primary oocytes. These primary oocytes do not undergo further mitotic division, and they remain arrested in the prophase stage of meiotic division I, until sexual maturity (see Meiosis, in the topic cell). Compare this to male gametogenesis.
At sexual maturity, two hormones, produced by the pituitary gland: follicle stimulating hormone (FSH) and lutenising hormone (LH) cause these primordial follicles to develop. In each ovarian cycle, about 20 primordial follicles are activated to begin maturation. however, normally only one follicle fully matures, and the rest contribute to the endocrine function of the ovary.
When activated, the first meiotic division is completed. When this happens, the primary follicle has matured into a secondary follicle. The second division then starts, and a Graafian follicle is formed. This contains a secondary oocyte. This second division is not completed, unless the ovum is fertilised.
This diagram shows how primary oocytes in the primordial follicles are diploid, and are starting their first meiotic division. The pairs of homologous chromosomes (one maternal, one paternal) pair up on the spindle, and genetic material can be swapped over. Even the X and Y chromosome (if present) will pair up, and form a partial bivalent.
At completion of first meiotic division: one of the chromosome pairs is segregated to each of the daughter cells. For example, if there is an XY pairing, then one cell will receive the X, and one the Y chromosome pair.Thus when this division is completed, the resulting secondary oocyte has 'diploid' DNA, but the chromosome copies are only derived from one of the original chromosomes in the parent cell. (i.e. either maternal or paternal). These cells are called 'haploid'. One of the daughter cells degenerates and forms a 'polar body'. These are small and degenerate rapidly.
At completion of the second meiotic division: this division occurs without the DNA being replicated prior to division. Each daughter cell receives one chromatid from the original chromosome pair to form the gamete producing cells that only have a single copy of each chromosome. Again one of the daughter cells forms a polar body.
Thus only one gamete is formed from one primary oocyte (compare this to male gametogenesis).
For more information compare normal mitotic division with meiotic division.
Hormonal changes during the menstrual cycle
Development of the follicles is stimulated by production of follicle stimulating hormone (FSH) by the pituitary gland. Ripening of the follicles then results in an increase in oestrogen levels, as oestrogen is secreted by follicular cells. This increase in oestrogen levels feeds back to the pituitary, and suppresses further release of FSH (negative feedback). The follicles also release a second hormone called inhibin, which also suppresses further production of FHS. As the oestrogen levels rise, this triggers a a mid cycle surge in a second pituitary hormone called Lutenising hormone (LH), which causes the follicle to rupture (ovulation). LH also causes ruptured follicles to lutenise, forming a transitory endocrine organ called the corpus luteum. This looks yellow, due to its pigmented lutein cells (luteus is latin for yellow). The corpus lutein secretes progesterone and oestrogen. The progesterone levels feed back to the pituitary and suppress further release of LH. If fertilisation does not occur, the corpus luteum degenerates into a small white fibrous scar called the corpus albicans. The resulting decline in progesterone (and to some extent oestrogen) levels precipitate menstruation. The decline in oestrogen levels, feeds back to the pituitary and there is a corresponding increase in FSH to being the cycle all over again.