When a girl is born she has two ovaries containing about two million immature eggs. But, by the time the girl reaches puberty and starts menstruating, many of the eggs that she started off with, will have disappeared.
The long-established theory is that women are born with all the eggs they will ever have was challenged by lead researcher Dr Jonathan Tilly, from Massachusetts General Hospital, USA who found that women's ovaries contain stem cells that can divide and make eggs (Journal Nature Medicine, 2012)
Each month an egg is usually produced by one of the ovaries. Before the egg is shed from the ovary (ovulation) it must be matured. The egg is matured inside a fluid filled sac called a follicle. The egg is surrounded by granulosa cells and fluid, these cells produce the female hormone estrogen. The development of the follicles is controlled by chemical messages called hormones, which are produced by the pituitary gland. At the beginning of menstruation, the pituitary gland releases follicle stimulating hormone (FSH) into the bloodstream to stimulate a selected group of immature follicles to grow. One of these follicles grows faster than the others and is called the dominant follicle, it is from this follicle that the egg will be released. The other follicles undergo follicular atresia i.e. shrink in size and disappear. As the dominant follicle grows, its granulosa cells produce an increasingly large quantity of the hormone estrogen that prepares the lining of the womb (known as the endometrium) to receive an embryo.
When the hormone estrogen level reaches its peak, the hypothalamus will send a chemical message (releasing hormones) to the pituitary gland to slow the production of FSH and trigger the release of a hormone called luteinizing hormone (LH). The ovary will detect this, and ovulation is set in motion. The follicle usually ruptures about 24 hours after the level of LH reaches its maximum. The follicle grows to a diameter of about an inch before it bursts and releases the egg. When the egg is released, it is swept into the Fallopian tube.
Another effect of the Luteinizing hormone is to convert the granulosa cells to luteal cells. Once the follicle has released the egg, it collapses and becomes the corpus luteum. The corpus luteum secretes estrogen and progesterone in steadily increasing amounts; and about seven days after ovulation the progesterone level reaches its peak. The progesterone prepares the endometrium to receive the fertilized egg. The endometrium becomes thickened, (a maximum thickness of 8 to 12 millimeters) and more glandular (the endometrial cells secretes a fluid rich in nutrients). Ovulation usually precedes menstruation by about two weeks.
In most months, conception will not occur, and after about 10 days of ovulation, the corpus luteum gradually degenerates and the progesterone production will start to fall, this will result in cessation of growth and shedding of the endometrium (menstruation).