MORPHOLOGY AND FUNCTION OF THE OVARY DURING FETAL AND EARLY NEONATAL LIFE - A COMPARISON BETWEEN THE SHEEP AND BRUSHTAIL POSSUM (TRICHOSURUS-VULPECULA)

In this review comparisons are made between gonadal development and fu nction in two species, the sheep and the brushtail possum (Trichosurus vulpecula), which represent examples of eutherians and metatherians, respectively. In sheep most stages of gonadal development (e.g. sexual differentiation, meiosis, follicular formation and initiation of foll icular growth) occur during fetal life, whereas in the brushtail possu m these stages occur after birth. However, the time frames (e.g. days) over which these developmental stages occur are similar between the t wo species. Ovarian function between sheep and brushtail possums appea rs to differ with respect to the ability of somatic cells to synthesiz e steroids; this may be related to the relative contribution of the ov arian rete and medullary cords to the developing gonad in sheep and br ushtail possums, respectively. In both species the timing of expressio n of mRNA and/or peptide for c-kit, c-kit ligand and alpha-subunit of inhibin during follicular growth appear to be the same. Nevertheless, there are many gaps in our knowledge of growth factors during gonadal development, especially in the brushtail possum. It is concluded that the pouch-young brushtail possum has several advantages over the sheep as an experimental model for the study of gonadal development.