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 is formed by a rim of small non-growing oocytes at the periphery and growing oocytes at the inner part of the cortex and in the medulla. In this study we aimed to: (i) develop an in-vitro model using fetal mouse ovaries to evaluate when the cortex-medulla growth pattern is organized during ovarian differentiation; and (ii) study the interaction between 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 cortex-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 cortex-medulla pattern. The organization of this pattern within the ovary is concurrent with the initiation of meiosis. Ovaries with a cortex-medulla growth pattern had the same number of growing follicles (approximately 87), regardless of the number of small, non-growing oocytes present. In contrast, in most ovaries without a cortex-medulla pattern, almost all oocytes began to grow. Hence, the geographically determined growth pattern of oocytes in the developing mouse ovary may be organized at the onset of meiosis and is sustained by a balanced interaction between small and growing follicles.