The role of the cumulus oocyte complex during ovulation.

Abstract

Ovulation is fundamentally crucial to the reproductive success of all mammals. Despite this fact there remain major knowledge gaps in our understanding of how the Luteinizing Hormone (LH) surge, which initiates ovulation, controls this process. There have been numerous theories regarding this phenomenon, yet the underlying mechanisms involved remain relatively unknown. In this thesis I sought to elucidate mechanisms involved in ovulation, with a particular focus on the role played by the expanded cumulus oocyte complex (COC). Specifically, I investigate whether the cumulus cells and their associated matrix following expansion could contribute actively to its own extrusion from the ovarian follicle during ovulation. I developed a novel hypothesis whereby the cumulus cells transition to an adhesive, motile and invasive cell phenotype in response to an ovulatory stimulus, hCG an analog of LH. I investigate whether the cumulus cells from expanded COCs are capable of cell adhesion to various extracellular matrices found in the follicle wall, and whether this is dependent upon hormonal stimulation by comparison to cumulus cells from unexpanded COCs, not receiving such stimulation. Further, I investigate whether the cumulus oocyte complex is capable of transitioning to a migratory cell phenotype. I tested this with established methods used in the study of cancer cell metastasis. I determine whether this phenotype is firstly dependent on an ovulatory stimulus, and whether it is cumulus cell specific. I attempt to elucidate the molecular mechanisms involved by investigating expression of the well-characterised CD44 cell migration pathway in COCs, during an ovulation time-course. I then use specific antagonists to this pathway, to inhibit cell migration. The final step in our hypothesis involves the investigation of the invasive capacity of the expanded COC. I analyse whether the expanded COCs are capable of degrading an extracellular matrix barrier during migration assays, and I compare this ability to characterised invasive and non-invasive breast cancer cell lines. I also investigate possible mechanisms involved in the invasive phenotype by inhibiting the matrix metalloprotease system, proposed to play an important role in the degradation of the follicle wall during follicle rupture, and by examining the Adamts1 null mouse, as Adamts1 is a protease shown to be crucial during ovulation. This thesis demonstrates novel and exciting properties of the cumulus oocyte complex during ovulation; offering new insight into our understanding of this complex process. It shows that the oocyte and its surrounding cumulus cells are not merely a passive entity, as previously thought, but rather may play an active role during this vital reproductive process.