Hormonal Control of Folliculogenesis: The Key to Successful Reproduction

Abstract

Folliculogenesis is the process by which primordial follicles are promoted through a series of developmental steps until a mature oocyte is ovulated. The process of folliculogenesis is not fully understood and involves initial or primary recruitment from the primordial pool, development of small primary and secondary preantral follicles, formation of an antral cavity, and finally the selection of the ovulatory follicle(s) (Hirshfield 1991; Gougeon 1996). The growth and differentiation of the oocyte and the surrounding somatic cells which make up the follicle are subject to various carefully regulated steps which ensure that the number of eggs which are ovulated is reduced to that which is appropriate for the species. The disastrous consequences of dysregulation of this system are illustrated by sheep carrying the Booroola fecundity gene (Baird and Campbell 1998). Due to a mutation in the BMPRI_β receptor, these sheep ovulate up to ten eggs instead of one or two (de Souza et al. 2001; Mulsant et al. 2001; Wilson et al. 2001). The subsequent fetuses die due to either abortion or following premature birth. As a result, under natural circumstances the mutation only rarely persists in those few ewes carrying a single copy of the gene which ovulates twins.