Ovaries And Egg Production

The eggs (ova) for which the uterus prepares itself every month come from one or other of the ovaries. These are the female gonads or sex glands and are responsible for egg production and the secretion of female sex hormones. They are ovoid in shape, averaging around 4cm in length and 2cm in breadth, and lie roughly 7cm either side of the midway point between the vagina and navel.

In infancy the ovaries are small, delicate, structures which enlarge after puberty to adult proportions. Egg cells start to be released at puberty. The female ovary however already has the precursors of some 2,000,000 eggs at birth. During childhood many of these will perish, until approximately 200,000 are left at the onset of puberty. Of these only around 400 eggs will grow to full maturity and leave the ovary during a woman’s reproductive years.

The ovary itself is made up of two distinct layers of tissue, the central medulla and the surrounding cortex which is a framework of connective tissue covered by a germinal epithelium. This outer layer contains the ovarian follicles, each of which contains an egg. Once a month, under the influence of hormones, hundreds of follicles mature. Eventually one of these follicles ruptures and releases its egg. This process, known as ovulation, usually takes place 13 to 17 days after the start of a menstrual period. The increase in the production of certain hormones during ovulation, for example progesterone, may produce other bodily effects such as a slight rise in temperature.

The hormonal factors

The stage leading up to the rupture of the follicle is initiated by follicle-stimulating hormone (FSH) which is released from the anterior pituitary gland just below the brain. The maturing follicle secretes the hormone oestrogen which stimulates the proliferation of the uterine lining in preparation for the reception of a fertilized egg. The rise of oestrogen in the blood eventually causes reduction in FSH and the release of luteinizing hormone (LH) from the pituitary. This hormone brings about ovulation; it causes the follicle to burst and to release its egg.

The remaining empty follicular ‘shell’, the yellow body or corpus luteum, starts to produce progesterone which is involved in ‘ripening’ the uterus. If the egg is eventually fertilized it embeds in the wall of the uterus where it grows and secretes the pregnancy sustaining hormone (human chorionic gonadotropin; HCG). This hormone keeps the corpus luteum intact which is essential for the supply of progesterone during the first weeks of pregnancy. Eventually, the placenta grows and takes over the production of progesterone as well as producing oestrogen. The corpus luteum then degenerates into a fibrous body called the corpus albicans (white body).

If the egg is not fertilized then the corpus luteum degenerates in less then two weeks after ovulation. Menstruation then takes place, and the menstrual cycle starts all over again.