THE CORTEX-MEDULLA OOCYTE GROWTH-PATTERN IS ORGANIZED DURING FETAL LIFE - AN IN-VITRO STUDY OF THE MOUSE OVARY

The cortex-medulla growth pattern of oocytes in the developing ovary i s formed by a rim of small nongrowing oocytes at the periphery and gro wing oocytes at the inner part of the cortex and in the medulla. In th is study we aimed to: (i) develop an in-vitro model using fetal mouse ovaries to evaluate when the cortex-medulla growth pattern is organize d during ovarian differentiation; and (ii) study the interaction betwe en small, non-growing and growing oocytes. Fetal mouse ovaries of days 11, 13 and 16 post coitus (E11, E13 and E16) were cultured for 14, 12 and 9 days respectively, corresponding to post-partum day 7 (P7). A c ortex-medulla growth pattern had developed by P7 in most ovaries from E16 and in half of those from E13. No ovaries from E11 developed a cor tex-medulla pattern. The organization of this pattern within the ovary is concurrent with the initiation of meiosis. Ovaries with a cortex-m edulla growth pattern had the same number of growing follicles (simila r to 87), regardless of the number of small, non-growing oocytes prese nt. In contrast, in most ovaries without a cortex-medulla pattern, alm ost all oocytes began to grow. Hence, the geographically determined gr owth pattern of oocytes in the developing mouse ovary may be organized at the onset of meiosis and is sustained by a balanced interaction be tween small and growing follicles.