16 - Premature ovarian failure

Summary

Premature ovarian failure is defined as the cessation of ovarian function under the age of 40 years and occurs in approximately 1% of women. In order to have an understanding of the aetiology of ovarian failure, which is still unknown in many cases, it is important first to have an idea of the factors that influence ovarian ageing.

The control of ovarian ageing

The control of ovarian ageing is still one of the greatest enigmas in reproductive biology. The function of the ovary depends upon the total number of oocytes contained within primordial follicles. Primordial follicles and oocytes are derived during fetal life and the oogonial stem cell line is lost before birth (see Chapter 1). The final number of oocytes is determined by three factors:

1. • the maximum number achieved by mitotic divisions

2. • the time at which they enter meiosis, preventing further increase in number

3. • the rate of atresia.

The factors that affect the number of mitotic divisions and the transition from mitosis to meiosis are unknown.

More germs cells die during fetal life than survive in primordial follicles. The maximum number of germs cells is approximately seven million and this is achieved at 20 weeks of gestation. By birth, this is reduced to between one and two million. It is thought that the eliminated germ cells might have a higher rate of chromosomal abnormalities than those that remain, although this has never been conclusively proven.