Endocrine Abstracts

There can be few mechanisms subject to more powerful evolutionary forces than ovulation. In all species the number of ovulatory follicles is carefully regulated to match the optimum number of offspring which can be successfully reared. In monovulatory species like the cow and our own it is essential that there is only a single ovulatory (Graafian) follicle if the disastrous consequences of multiple pregnancy are to be avoided. Folliculogenesis involves initial recruitment from a finite pool of primordial follicles which are laid down in fetal and/or early neonatal life into a process of growth and differentiation which terminates in ovulation or atresia. This process involving extensive changes in structure and function is controlled by a number of genes which are also expressed during embryonic and fetal development including members of the TGβ super family. The mechanism by which primordial follicles are recruited is not known but involves local paracrine rather than endocrine factors. Because the proportion recruited each day is inversely related to the total number of follicles in the ovary it is assumed that some local factors such as AMH and activin derived from adjacent developing follicles are involved. The follicle becomes increasingly responsive to gonadotrophins as it enlarges and shortly after antrum formation becomes dependant on FSH. The ovulatory follicle is selected finally only shortly before ovulation probably because it happens to be at the appropriate stage of development to benefit from the transient intercycle rise in FSH. Once selected it suppresses FSH to a level below that necessary to maintain the development of subordinate follicles by secreting large quantities of inhibin A and oestradiol. Although the number of ovulatory follicles is carefully regulated there is little evidence that follicular survival is dictated solely by the quality of the oocyte. In humans a significant proportion of ovulated eggs are chromosomally abnormal resulting in aneuploid embryos the majority of which are discarded at implantation. Thus natural selection occurs mainly after fertilisation as the embryonic genome is expressed