The ovary is a unique complex organ uniting somatic cells and germ cells in the follicles. By contrast with the male germ cells, female germ cells are already present at birth as a pool of 1-2 million primordial follicles which cannot be renewed by stem cells and consequently cannot be re-produced once destroyed by cytotoxic therapies. From the age of sexual maturity onwards, primary and secondary follicles develop continuously from numerous primordial follicles. The Anti-Müllerian hormone (AMH) increasingly produced in the granulosa cells prevents the recruitment of an excessive number of primordial follicles. From a small pool of preantral secondary follicles, sonographically visible antral tertiary follicles develop cyclically under the influence of follicle-stimulating hormone (FSH). The whole process of folliculogenesis from the primary to tertiary follicle takes around 3-6 months. The recruitment of multiple mature tertiary follicles is prevented by the secretion of Inhibin by granulosa cells.

Due to the importance of AMH, Inhibin B and FSH in folliculogenesis, it was suggested that these parameters could be chosen as markers of the ovarian reserve. With the exception of Inhibin B, the parameters AMH and FSH as well as the number of antral follicles visible under ultrasound can be used to estimate the ovarian reserve. However, such an estimate is of limited use if the woman wants to plan the time when she wants to get pregnant. Until there are more reliable data on what damage chemotherapy does to the ovarian reserve, we have to assume that radio- and chemotherapy cause premature aging of the ovary by several years, i.e. premature onset of the menopause. Therefore, getting pregnant should not be put off too long after cytotoxic therapy.

By contrast with the ovaries, the testis can constantly produce new sperm due to its stem cells. Alongside the hematopoetic system, the adult testicular epithelium is the most productive self-renewing tissue in humans with a production rate of up to 100 million sperm cells per day. The stem cells, also called spermatogonia, are divided into Ad (A dark) and Ap (A pale) spermatogonia. Ad spermatogonia are currently thought to be the real testicular stem cells, the regenerative reserve of the testicular epithelium.

Endocrine regulation is based on the pituitary hormones LH (luteinising hormone) and FSH (follicle-stimulating hormone). LH stimulates the extratubular Leydig cells and promotes testosterone production, while FSH acts upon the intratubular Sertoli cells, thereby stimulating spermatogenesis. Furthermore, FSH stimulates the secretion of Inihibin, which, as in women, acts upon the hypothalamus, triggering a feedback inhibition of GnRH release.