4 - Function of the Egg's Extracellular Matrix

Abstract

This chapter discusses function of the egg's extracellular matrix and focuses on the two egg-associated structures—that is, cumulus layer and zona pellucida, specifically in regard to their involvement in the process of fertilization. Before membrane fusion between mammalian gametes can occur, the sperm must first negotiate two major extracellular layers that surround the egg. These layers are the cumulus layer, which is made up of cumulus cells, hyaluronic acid, and a number of minor components; and the zona pellucida, which is a matrix made up of three to five glycoproteins that are secreted by the developing oocyte. The cumulus oophorus plays many essential roles in maturation, ovulation, and fertilization of the mammalian oocyte. The chapter reviews the functions of the cumulus as related to the overall success of fertilization and look specifically at three interrelated functions of the cumulus matrix: (1) production of soluble factors, (2) transport into the oviduct for fertilization, and (3) creation of a selective barrier or filter for sperm. Many studies have suggested that the cumulus matrix is a chemical and mechanical barrier to sperm progress toward the oocyte. The zona pellucida plays critical roles in sperm recognition and adhesion, initiation of acrosomal exocytosis, sperm penetration of the matrix, and, subsequently, protection of the fertilized egg and embryo. The structure of the zona pellucida, especially the carbohydrate composition of the individual glycoproteins and the precise arrangement of those molecules, is important in determining biological function. Following sperm-egg fusion and egg activation, the zona pellucida undergoes molecular changes, collectively known as the zona reaction, which function to block polyspermy.