Expression of mRNA encoding growth differentiation factor 9 and bone morphogenetic protein 15 during follicular formation and growth in a marsupial, the brushtail possum ( Trichosurus vulpecula)

The oocyte derived growth differentiation factor (GDF) 9 and bone morphogenetic protein 15 (BMP15; also known as GDF9b) are essential for normal follicular growth. However, little is known about expression of these factors during ovarian development. Therefore, we determined the ontogeny of expression of GDF9 and BMP15 mRNA in the developing ovary of the brushtail possum. Ovaries were collected from pouch young ( n=3–5 per group) around times of key developmental events namely: (1) morphological sexual differentiation (i.e. days 1–5 following birth), (2) after sexual differentiation (i.e. days 10–15), (3) before and during initiation of germ-cell meiosis (i.e. days 22–45), (4) shortly after initiation of follicular growth (i.e. days 78–85), (5) during preantral follicular growth (i.e. days 96–113) and (6) during antral follicular growth (i.e. days 155–190). Ovaries were also collected from three juvenile and four adult animals and gene expression was determined by in situ hybridization. The mRNAs encoding GDF9 and BMP15 were first observed in oocytes of newly-formed primordial follicles (i.e. days 78–85). Expression of both mRNAs was restricted to the oocyte and was present in follicles irrespective of whether they were non-growing primordial follicles or undergoing preantral or antral development. Thus, since the mRNAs encoding GDF9 and BMP15 were not observed until follicular formation, it is unlikely that these proteins have any role in early germ cell development. Nevertheless, the findings that the mRNAs encoding both proteins were observed in oocytes from the primordial stage of follicular formation suggest a possible role for these proteins in the maintenance of primordial follicles as well as a key role during follicular development. These results highlight important species differences in the ontogeny of expression of GDF9 and BMP15 between possums and other species such as the human, sheep or rat.