Gene expression profiling during early folliculogenesis in the mouse ovary.

Objective To evaluate genes involved in ovarian primordial-to-primary follicle transition. Design Experimental animal study. Setting Research institute. Animal(s) Day-2 and day-4 female Swiss mice. Intervention(s) We conducted a complementary DNA array study using ovarian messenger RNAs from day-2 and day-4 mice. Main Outcome Measure(s) The expression profiles of 1,176 genes in neonatal mouse ovaries on day 2 and day 4, which contain primordial and primary follicles, respectively, were compared. Result(s) Twenty-six percent of genes were differentially expressed between day-2 and day-4 ovaries, with 19% being up-regulated and 7% down-regulated in day 4. Analysis of differentially expressed genes revealed that the primordial-to-primary follicle stage transition is associated with induction in the expression of mainly growth factors, immune-related factors, hormone and hormone receptors, and signal transducers. The transition is also associated with proliferation of granulosa cells and absence of apoptosis. In addition, our studies demonstrated that the primary follicles express estrogen receptor β and are responsive to estrogen actions in vitro in terms of increase in the number of primary follicles and granulosa cell proliferation. Conclusion(s) The transition of primordial to primary follicles is associated with the participation of multiple pathways in regulating gene expression.