Fertilization

Because it has medical, sociological, agricultural and economic implications, fertilization is one of the most fascinating and intensely-studied aspects of development. You should study carefully your textbook and the links listed below to understand fertilization. Be aware of distinct mechanisms that are used by different organisms. In particular, study the activation of sperm motility and the role of chemotaxis in fertilization in your textbook.

The sea urchin acrosome reaction

Sperm-egg contact requires that the sperm penetrate surface coats that surround the egg. This is facilitated by the acrosome reaction, in which the membranes enclosing the acrosome are shed, releasing the contents of the acrosome. The acrosome reaction has been studied intensively in sea urchin sperm. In sea urchins, the acrosome reaction is stimulated by contact with the egg jelly coat. Study this process in your textbook.

Mechanisms of sperm-egg recognition and contact in mammals

Mammalian sperm must reside in the female reproductive tract before they are able to undergo the acrosome reaction. This maturation process is called capacitation.

The mammalian egg is surrounded by an extracellular envelope called the zona pellucida, to which sperm must bind and penetrate before they can make contact with the surface of the egg itself. The zona pellucida of the mouse egg contains three glycoproteins called ZP-1, ZP-2 and ZP-3 that polymerize to form a gel. The zona of newly-ovulated eggs is also surrounded by a constellation of follicle cells in a matrix of hyaluronic acid. Figure 1 shows a mouse ovum accompanied by polar body I surrounded by the zona pellucida.