Abstract
The ovary contains oocytes developing within ovarian follicles. The purpose of the ovarian follicle is to assist the oocyte maturation and to prepare the oocyte for ovulation. These follicles grow through the primary, secondary, and tertiary stages before becoming an ovulatory follicle. An ovary will contain more developing follicles than are required for ovulation and the excess follicles are removed via a degenerative process called atresia. Ovulatory follicle selection is governed by the positive and negative feedback between the brain, pituitary ovarian hormones. However, the differences between follicles destined to ovulate and the follicles that will become atretic are poorly understood. The aim of this thesis was to investigate whether follicle development speed can predict which follicles will be selected in mice. The progression of follicle development was tracked and growth rate from the primary to preovulatory stages were calculated in a mouse ovary to characterise the innate differences between follicles. Growing follicles in mice were labelled with bromodeoxyuridine (BrdU), which labels the DNA in dividing cells by two different methods: 48hour administration in drinking water and, short term single intra-peritoneal injection. Labelled follicles were examined at different post-BrdU euthanasia time points to study the growth rate of follicles at the time of labelling. Immunohistochemistry and machine learning software were used to analyse and generate a mitotic index (MI) for follicles (n=828) across the ovarian cycle. Follicles were classified into atretic and non-atretic. Primary follicles were found to have a slow development relative to other advanced follicle stages. At the preantral stage, a medium development-rate was followed by a burst in follicle growth in the early antral phase. Follicles became increasingly susceptible to atresia beyond 170 µm in metestrus and diestrus when the FSH levels had fallen suggesting that the onset of FSH-sensitivity occurs at this diameter. Follicle growth modelling suggested that an antral follicle needs to be ̴ 250 µm at the beginning of the cycle if developing at a median speed. However, if the growth rate is higher, the minimum size required for an antral follicle that can potentially be selected is smaller. Two types of preovulatory follicles sequentially emerge from different developmental cohorts namely early-arriving follicles at the beginning of the cycle and late-arriving follicles that reach the preovulatory stage later in the cycle. In conclusion, follicle development speed was found to be heterogenous throughout follicle growth. A small follicle developing fast enough can overtake larger follicles that are developing slowly and become an ovulatory follicle. Since speed determines the fate of a follicle, it can be used as a predictor of follicle selection to further investigate why a particular follicle gets selected to ovulate over all other follicles.