Gametes and Fertilization

Gametes are mature, haploid cells that develop from germ cells following meiosis, mitosis, and cell differentiation. Oocyte-secreted growth factors, such as bone morphogenic protein 15 (BMP-15) and growth differentiation factor 9 (GDF-9), play critical roles in gamete maturation and female fertility. During the process of fertilization, a series of reactions triggers the fusion of gametes to produce a diploid cell called a zygote. Once the sperm reaches the egg, presumably by following a gradient of chemoattractants, cell surface receptors bind to proteins within the glycoprotein-rich zona pellucida that surrounds the oocyte. Sperm-egg binding is thought to induce the acrosome reaction, in which the sperm cell releases enzymes that degrade the zona pellucida and prepares the gamete cell membranes for fusion. A complimentary process, the cortical reaction, occurs in the egg. During the cortical reaction, a Ca2+ wave activates the exocytosis of secretory vesicles that modify the zona pellucida and prevent polyspermy. Understanding the molecular mechanisms that control gamete maturation and fertilization are important for contraception and assisted reproductive technologies.