Morphological characteristics observed during early follicular development in perinatal MRL/MpJ mice

In perinatal mice, the ovary undergoes drastic morphological changes, as clusters of oocytes called nests break into smaller cysts and subsequently form individual follicles. We studied perinatal oocyte development in MRL/MpJ mice, and compared it to that observed in C57BL/6 mice between embryonic day 18.5 and postnatal day 4. Throughout the observation period, compared to C57BL/6 mice, MRL/MpJ mice displayed significantly fewer oocytes in their ovaries. Morphologically, there were no clear differences between the strains at embryonic day 18.5. However, the beginning of folliculogenesis, as evidenced by the expression of NOBOX oogenesis homeobox (Nobox) transcript and protein, was more enhanced in MRL/MpJ mice than in C57BL/6 mice at embryonic day 18.5 and postnatal day 0. In addition, developed follicles were more frequently observed in MRL/MpJ mice than in C57BL/6 mice between postnatal days 0 and 4. In conclusion, the oocyte development during nest breakdown and folliculogenesis was accelerated in MRL/MpJ mice when compared to that observed in C57BL/6 mice.