The ovarian cycle and ovulation
Ovarian tissue contains immature (resting) ova (eggs) and stromal cells. Each ovum is surrounded by a ball of closely packed, specialised stromal cells, which is separated from the surrounding, looser ovarian tissue by a thin membrane.
An ovum and its surrounding cells form an ovarian follicle. We are born with about 300 000 follicles in each ovary. They remain in a resting state until puberty. From then on, between 10 and 20 follicles will begin to mature each month, of which only one is likely to reach full maturity and be released.
The maturing follicle
As we have seen, the trigger that starts a testing follicle on the path to maturity is FSH from the pituitary. Under the influence of FSH the ovum enlarges and its nucleus prepares to halve its number of chromosomes. At the same time the cells surrounding the ovum multiply and produce fluid that separates them from each other. Ovarian stromal cells outside the follicular membrane multiply and become packed together to form a capsule for the follicle. The cells in the inner layers of this capsule produce progressively more oestrogen as the ovum matures. The outer capsule has an important function after ovulation, as we shall see.
As maturation proceeds, the fluid between the cells surrounding the ovum runs into a central lake that pushes the ovum to one side. A mature follicle is a fluid-filled ball up to 2 cm in diameter that bulges from the surface of the ovary. Development of a follicle from resting to maturity takes about 14 days.
I have mentioned that only one ovum reaches maturity in each cycle. What happens to the other 9-19 ova? They fall by the wayside. The follicle that grows fastest has more cells to attract FSH and keep its growth going, and knocks the others out of the race – the survival of the fittest.
You may also be wondering about the 300 000 resting ova per ovary with which we are born. Even with an ovarian cycle every month for 35 years (which doesn’t often happen because the ovarian cycle is suspended during pregnancy and lactation), only about 8000 of these would ever begin to mature, and at the most around 450 would be released as mature eggs. What happens to the rest? The answer is that from birth to the menopause immature ova are constantly dying, disintegrating and disappearing. The few that a left by the age of 50 are mostly defective and none can be found after the menopause.
Ovulation
As the follicle matures it produces mol and more oestrogen. Shortly before the ovum is ready to be released (usual about 12 days after the beginning q the cycle), the amount of oestrogen in the blood reaches a certain level, which stimulates the pituitary to release a surge of LH. The LH surge causes the follicular wall to produce chemicals that make it weaken and rupture. The ovum is pushed out of the follicle.
This is ovulation. The fronds at the ovarian end of the tube are waiting to sweep up the ovum and send it on its journey towards the uterus. If it is not fertilised within about 12 hours after ovulation, the ovum dies and disintegrates.
The ovary after ovulation
The follicle collapses after its fluid and the ovum are ejected, and the break in its wall seals. The inner layer of its capsule continues to produce some oestrogen, and the outer cells are now stimulated by LH to make the other important hormone of the ovarian cycle, progesterone. This hormone gives the cells a yellowish colour, and the follicle now becomes the corpus luteum (which means ‘yellow body’): hence, also, the name ‘luteinising hormone’. The main function of progesterone is to prepare the lining of the uterus for pregnancy (see below under menstrual cycle).
During the first 3-4 days after ovulation the corpus luteum produces increasing amounts of progesterone. The combined effect of oestrogen and progesterone in the blood has a feedback effect on the pituitary, which stops releasing LH. Without LH, the corpus luteum gradually stops making progesterone over the next 8-9 days, and shrivels up and dies. The ovarian cycle has finished. However, unless pregnancy has occurred, a new cycle begins at once.
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