Growth and paracrine factors regulating follicular formation and cellular function

The purpose of this paper is to review, using fetal sheep as the animal mod el, aspects of ovarian development related to follicular formation and to r eport on the identity of growth and paracrine factors which might be involv ed in this process. Before follicular formation there is a massive and sust ained colonisation of the fetal ovary by mesonephric cells, which become a precursor source of follicular cells. From within the ovarian medulla, soma tic 'cell-streams' branch into the cortex around nests of oogonia and oocyt es. These 'cell-streams', which contain elongated cells with either flatten ed or cuboidal shaped nuclei, express steroidogenic factor-1 (SF-1), steroi d acute regulatory protein (StAR), 3 beta-hydroxysteroid dehydrogenase (3 b eta-HSD), cytochrome P450(scc), and P450(aromatase) mRNA and/or protein. Fo llicles form from the association of an oocyte with the 'cell-stream' with either a single layer of flattened cells (i.e. type 1 follicle) or with a m ixture of flattened and cuboidal cells (i.e. type 1a follicle). These newly -formed follicles have between 3 and 57 somatic cells (i.e. granulosa cells ) and contain oocytes which vary in diameter between 23 and 52 mu m. Newly formed and early growing follicles have been identified with growth factors or growth factor receptors in either the oocytes or granulosa cells. Many of the growth factors an from the TGF beta superfamily and are expressed in a cell- and stage-specific manner. (C) 2000 Published by Elsevier Science Ireland Ltd. All rights reserved.