17 - Metabolism of the follicle and oocyte in vivo and in vitro

Summary

Introduction

Oocyte metabolism reflects other aspects of the unique biology of this important cell type. The protracted process of mammalian oogenesis exacts a huge metabolic toll on the presumptive gamete. To ensure that the nutritional needs of oocytes are met oogenesis occurs in concert with folliculogenesis. Folliculogenesis is a lengthy process beginning with a primordial oocyte surrounded by a small number of flattened pregranulosa cells and ending with the ovulation of a fully grown, metaphase II oocyte, some weeks or months later. Throughout their development, oocytes and follicle cells are physically and metabolically linked via a complex network of homologous and heterologous gap junctions [1]. Metabolic coupling of oocytes and somatic cells facilitates the transfer of molecules of <1kDa, including ions, amino acids, pyruvate and glucose, molecules such as adenosine triphosphate (ATP) [2], and other signaling molecules and meiosis-arresting signals from the somatic compartment of the follicle to the oocyte and vice versa to provide the physiological basis for oocyte and follicle development [3]. While the metabolic cooperativity between oocytes and their companion granulosa cells is dynamic, discrete differences exist between the nutritional needs of oocytes and somatic granulosa cells and throughout their development oocytes are exposed to a changing nutritional environment as the follicular cells undergo proliferation, antral cavity formation, differentiation, and ovulation. In turn, oocytes have been shown to regulate apoptosis and cholesterol biosynthesis and metabolism by the follicular cells and so impact on follicular development [4].