Figure 7

Schematic representation of the study results towards better understanding of postnatal oogenesis and follicle assembly. Small insert represents a model proposed earlier by us [34]. Blue line represents sheep ovarian surface epithelium (OSE). Gentle scraping of OSE shows the presence of epithelial cells (ECs), red blood cells (RBCs) and stem cells (VSELs, OGSCs and cysts). A crucial pluripotent marker OCT-4 (represented in brown) shows nuclear expression in VSELs and cytoplasmic in OGSCs and cyst. FSH acts via FSH receptor isoform R3 (rather than the canonical R1 isoform) on the stem cells and regulates self-renewal of VSELs increased (Oct-4A and Sox-2) and clonal expansion of the OGSCs (increased Oct-4) and formation of cysts (rapid proliferation of OGSCs with incomplete cytokinesis). The OGSCs get surrounded by somatic granulosa cells (formed by epithelial-mesenchymal transition of OSE cells) resulting in primordial follicle (PF) assembly. PF undergo transition into primary follicle and OCT-4 is observed in the ooplasm in the developing oocytes whereas the surrounding granulosa cells remain distinctly negative. In pre-antral to antral follicles, OCT-4 expression gradually decreases in the ooplasm whereas cytoplasmic OCT-4 is observed in the surrounding granulosa cells and interestingly the staining is relatively dark in cumulus granulosa cells compared to distal granulosa cells (please refer to Figure 5). In vitro matured sheep oocytes show cytoplasmic OCT-4 staining in surrounding granulosa cells which is reduced in MII compared to MI oocytes. Nuclear OCT-4 reappears in the developing embryo post-fertilization providing it a pluripotent state. In addition to OCT-4, FSHR which is also required for initial stem cell function during oogenesis exhibited a similar staining pattern.