Bedford, Mammalian Fertilization Misread? Spermatozoa and oocytes of many eutherian placental mammals, including the human, are now manipulated successfully in a variety of ways for reasons having to do particularly with human infertility and domestic animal production, as well as basic research. Even so, several evolutionarily novel features of their function and form largely defy explanation still [ 1 ]. Most obvious in the male is the absolute dependence of eutherian spermatozoa on the environment of the upper epididymis for final maturation, and on an androgen- and usually a scrotal temperature-regulated system in the distal epididymis for their storage.
In fertilization, the sperm binds to the egg, allowing their membranes to fuse and the sperm to transfer its nucleus into the egg. Fertilization is the process in which gametes an egg and sperm fuse to form a zygote. The egg and sperm are haploid, which means they each contain one set of chromosomes; upon fertilization, they will combine their genetic material to form a zygote that is diploid, having two sets of chromosomes. A zygote that has more than two sets of chromosomes will not be viable; therefore, to ensure that the offspring has only two sets of chromosomes, only one sperm must fuse with one egg. In mammals, the egg is protected by a layer of extracellular matrix consisting mainly of glycoproteins called the zona pellucida. When a sperm binds to the zona pellucida, a series of biochemical events, called the acrosomal reaction, take place.
We'd like to understand how you use our websites in order to improve them. Register your interest. The apex of the sperm head which has undergone the acrosome reaction comes in contact with the plasma membrane of the ovum. After the entire surface of the inner acrosomal membrane has come into close contact with the plasma membrane of the ovum, the two membranes fuse to form a continuous membrane. All parts of the spermatozoon that are devoid of plasma membrane penetrate into the ooplasm.
During fertilization , a sperm must first fuse with the plasma membrane and then penetrate the female egg cell in order to fertilize it. Fusing to the egg cell usually causes little problem, whereas penetrating through the egg's hard shell or extracellular matrix can present more of a problem to the sperm. Therefore, sperm cells go through a process known as the acrosome reaction which is the reaction that occurs in the acrosome of the sperm as it approaches the egg. The acrosome is a cap-like structure over the anterior half of the sperm's head. As the sperm approaches the zona pellucida of the egg, which is necessary for initiating the acrosome reaction, the membrane surrounding the acrosome fuses with the plasma membrane of the sperm's head, exposing the contents of the acrosome.