Notch Signaling during Ovarian Follicle Formation and Development

Morphogenesis of the mammalian ovary requires the precise spatial and temporal organization and function of multiple cell types, which is coordinated by autocrine, juxtacrine, paracrine, and endocrine signaling mechanisms. In order to better understand the cellular movements and processes required for the development of the ovary, we used mouse lines that express fluorescent proteins in specific ovarian cell types, as well as multiple imaging modalities, to visualize the cellular organization and interactions during the formation, growth, maturation, ovulation, and luteinization of ovarian follicles. Further, we have applied these techniques to investigate the role of the Notch pathway, a highly conserved juxtacrine signaling system, during follicle histogenesis and development.;At birth, Notch active somatic cells are arranged in an intricate cage-like pattern around germ cell syncytia and can be observed sending projections around individual germ cells. Notch active cells are seen in close association with collagen fibrils, detected by second harmonic generation, which could potentially facilitate pregranulosa cell migration and cellular reorganization during follicle histogenesis. Time-lapse imaging of ex vivo cultured ovaries reveals diverse cellular behaviors, including ovigerous cord fragmentation, cell-cell interactions, cell migration, and germ cell apoptosis. Additionally, Notch active cells can be visualized within growing follicles, cumulus oocyte complexes, and corpora lutea, suggesting important roles for Notch signaling between the various cell types of these structures.;Complementary expression of the Notch ligand, Jagged1, in germ cells and the Notch receptor, Notch2, in pregranulosa cells, suggests a role for Notch signaling in mediating many of the cellular interactions during follicle assembly. In addition, increased expression of the Notch ligands, Jagged1 and Jagged2; the Notch receptor, Notch2; and the Notch target genes, Hes1 and Hey2 coincides with a significant increase in the fluorescent activity of a Notch reporter line. Histological examination of ovaries from mice with conditional deletion of Jagged1 within germ cells (J1KO) or Notch2 within granulosa cells (N2KO) reveals changes in follicle dynamics, including perturbations in the primordial follicle pool and antral follicle development. J1KO and N2KO ovaries also contain multi-oocytic follicles (MOFs), which represent a failure to resolve germ cell syncytia, and follicles with enlarged oocytes but lacking somatic cell growth, signifying a potential role of Notch signaling in follicle activation and the coordination of follicle development. We also observed decreased cell proliferation and increased apoptosis in the somatic cells of both conditional knockout lines. As a consequence of these defects, J1KO female mice are subfertile; however, N2KO female mice remain fertile. These studies demonstrate important functions for Jagged1 and Notch2 in the resolution of germ cell syncytia and the coordination of somatic and germ cell growth within follicles of the mouse ovary. New approaches in imaging and ex vivo culture of ovarian explants will undoubtedly provide new means by which to follow the identity and behavior of multiple cell types in the ovary in a dynamic fashion, facilitating investigations into the cellular interactions, processes, and organization necessary for follicle formation and development.