Fertilization and egg activation

Introduction: Sexual reproduction is a survival strategy employed by species for procreation, and is characterized by the transfer of haploid genetic material from each parent to produce diploid offspring, ensuring continued genetic diversity. In mammals, males produce vast numbers of gametes called sperm in the testes (˜53–55µm in length), which are deposited in the female reproductive tract in a liquid medium (semen) via the process of ejaculation. Females produce much larger gametes, eggs (˜100µm in diameter), which are surrounded by a thin glycoprotein layer, the zona pellucida (ZP), and generated in follicles contained in the ovaries which, following maturation, are released and transported through the fallopian tube to the uterus. Sexual reproduction involves the concerted release of these two components, which subsequently fuse to re-form the diploid chromosome number, resulting in a new, genetically unique individual, a process termed fertilization. Fertilization involves a number of key, sequential steps, these are: (1) the acquisition of sperm motility and chemotaxis; (2) capacitation and the acrosome reaction; (3) sperm-egg fusion; and (4) egg activation and the initiation of embryo development. This chapter will summarize these key events. Sperm motility and chemotaxis Sperm generated in the testis are immature and immotile, and need to undergo a variety of modifications in order to obtain fertilization competency. The first step of maturation occurs within the testis, where sperm acquire proteins and cholesterols secreted from the epididymis [1]. It is thought that motility is brought about by changes in ambient conditions of the sperm such as temperature, osmotic stimulation, ionic concentration, or following ejaculation, factors secreted from the female reproductive tract [1]. Following the acquisition of motility, sperm begin to make their way up the female reproductive tract towards the egg, employing a process termed chemotaxis.