Reproductive function in females
Reproductive function in females
Oogenesis
Primitive egg cells in the female are called oogonia. They divide rapidly during fetal life to reach a maximum number of approximately 7 million by the sixth month of pregnancy. They then remain suspended partway through the first meiotic division and remain in that state for up to 50 years. At this stage, they are known as oocytes. The first meiotic division is only finally completed at ovulation and the second meiotic division follows fertilisation. From 24 weeks of fetal life until the menopause there is a rapid loss of oocytes by atresia (degeneration). A baby is born with 2 million oocytes but by puberty only 400 000 remain.
These oocytes are surrounded by a single layer of granulosa cells, and the oocyte granulosa complex at this stage is called a primordial follicle. Some follicles start development during childhood but they will all degenerate. During reproductive life, follicles develop in waves from small primary follicles through a series of eight stages to preovulatory follicles, which will rupture and release the egg in response to the appropriate gonadotrophin stimulation. It has been estimated that the duration of time for a small primary follicle to develop into a preovulatory follicle is approximately 85 days (i.e. nearly three menstrual cycles).
The entry of groups of follicles into the cycle of follicular growth is not dependent on gonadotrophins. However, when FSH levels rise at the beginning of the menstrual cycle there will be a group of follicles of a certain size containing a critical number of granulosa cells that are able to ‘catch the wave’ and become recruited for that particular menstrual cycle. In natural cycles, one of this developing subgroup will be selected to become dominant. The others will advance only partway along the wave and then degenerate. In this way, 99.9% of the 400 000 eggs present in the human ovary at the onset of reproductive life will degenerate and only an average of 360 eggs will be ovulated and stand a chance of being fertilised to commence a new human life.
Hormonal control of ovulation
Hormonal changes during a typical menstrual cycle are illustrated in this article Each month, FSH levels begin to rise just before the onset of menstruation in the late luteal phase. FSH stimulates the granulosa cells of follicles between 1 and 5 mm in diameter to produce oestradiol (E2). As the oestradiol level rises in the second half of the follicular phase it exerts a negative feedback effect on the pituitary gland production of FSH. 
Only the follicle destined to ovulate (dominant follicle) is able to retain the reduced level of FSH selectively and continue to produce oestradiol. This is probably because it has the most abundant blood supply. The other follicles that had been recruited start to degenerate. Once oestradiol production from the dominant follicle reaches a critical level, it triggers a surge in LH production from the pituitary by positive feedback. The LH surge induces a number of significant alterations to the follicle.
Resumption of first meitoic division. As described previously, the oocyte has been in a state of suspended animation (arrested halfway through the first meiotic division) since embryonic life. During this arrested stage, it contained the diploid number of chromosomes packed into the nucleus (germinal vesicle). The LH surge induces the resumption of the first meiotic division.
The chromosome number is divided into two equal halves, with one portion remaining within the egg cytoplasm (which now becomes haploid) and the second portion being expelled as the first polar body. At the completion of this division, meiosis then arrests again. The second meiotic division only occurs if fertilisation of the egg by the sperm takes place. Otherwise the ovulated egg degenerates. Luteinisation of granulosa cells. Protein synthesis induced by the LH surge causes a change in the secretory product of granulosa cells from oestradiol to progesterone. Progesterone influences the endometrium making it receptive to embryo implantation should a fertilised egg arrive in the uterine cavity.
Follicular rupture. LH induces an increase in follicular blood flow, which leads to a rise in follicular volume but no change in intrafollicular pressure. There is rapid development of oedema and increased vascular permeability of the follicular wall. This is accompanied by the release of prostaglandins, histamines and noradrenaline. The net effect of these changes is the release of plasmin, which breaks down the collagen in the follicular wall leading to rupture of the follicle and release of the oocyte.
