Function of the ovaries
The function of the ovaries is mainly the production of oocytes. In a newborn girl, there are about one to two million eggs in both ovaries after birth, which are present as primary follicles (small follicles). Most of the eggs die during a woman’s lifetime.
Every month, one or two follicles mature into a “mature follicle”, which contains hormone-producing cells in addition to the egg. When it has grown to about 24 millimetres in size, it bursts open and the egg cell is ejected, taken up into the fimbrial funnel of the fallopian tube and transported to the uterus. The rest of the follicular tissue remains in the ovary, from which the corpus luteum is formed.
The corpus luteum lasts for about two weeks and mainly produces progesterone, which prevents the premature breakdown of the lining of the uterus. If fertilisation takes place at this time, pregnancy can develop. If fertilisation does not take place, the egg is ejected together with the lining of the uterus and menstruation occurs.
Another important function of the ovaries is the production of female sex hormones (oestrogens and gestagens). They are fundamental for the development of a woman’s secondary sexual characteristics, as well as for the regulation of the female menstrual cycle and the development of pregnancy. Furthermore, the female sex hormones strengthen the bones, have a protective effect on the cardiovascular system and strengthen the immune system.
The sexually mature, fertile phase of life of every woman begins with the menarche and ends with the menopause. During this period, one follicle matures every month, with one egg inside, in the ovary. Oestrogens and progesterone cause the lining of the uterus to build up and prepare for the implantation of the egg.
The female cycle consists of two cycles that run parallel to each other and are connected to each other – the mucous membrane cycle and the ovarian cycle. The mucous membrane cycle affects the lining of the uterus and begins on the first day of menstruation. It is repeated approximately every 28 days if fertilisation has not taken place.
The first phase of the mucosal cycle is called the “menstrual phase” and includes the first five days of menstruation. If fertilisation has not occurred, the corpus luteum recedes and the concentration of progesterone in the blood drops. As a result, the uterine lining is less supplied with blood and is rejected.
During menstruation about 50-150 millilitres of blood with tissue residues and mucus are excreted. From day 6 until about ovulation (day 14) we speak of the “build-up phase”. During this time, oestrogen is released and the uterine lining is rebuilt.
The menstrual phase and the build-up phase can vary in time for each woman, so that they can extend beyond 14 days. The last phase is the “secretion phase” and includes days 15-28. After ovulation, in the middle of the cycle, the corpus luteum is formed, which mainly produces progesterone and, in small amounts, oestrogen.
This leads to further maturation and thickening of the uterine lining. Blood vessels grow in – everything is prepared for the implantation of the egg. If fertilisation does not take place, the corpus luteum recedes within a few days and hormone production ceases.
The cycle then begins anew with the menstrual phase. The ovarian cycle runs parallel to this. It runs in three phases and begins with the “follicle maturation phase” (days 1-10).
The FSH secreted centrally by the pituitary gland stimulates the ovaries to mature follicles. The follicles produce oestrogens, which are released into the blood. However, during the follicle maturation phase, one follicle in particular grows and reaches full maturity, while the other follicles die and are absorbed.
From day 11-14 the ovulation phase takes place. During this phase there is a strong increase in the centrally secreted LH, which leads to ovulation. The egg cell leaves the ovary and travels via the fallopian tube towards the uterus.
From this day on, the egg is fertilisable for 24 hours. This is followed by the “corpus luteum phase” (day 15-28). The follicle transforms into the corpus luteum and begins to produce progesterone and, in small quantities, oestrogens.
If fertilisation does not occur, the corpus luteum recedes and the hormone concentration in the blood drops. Menstruation is initiated and the cycle starts again.
