Bizarre Facts in Biology

Home > Cell Membrane Structure and Function > Bizarre Facts in Biology

Bizarre Facts in Biology

When Egg Meets Sperm

The process of fertilization is really a chain of events, beginning with complex changes that occur in the plasma membrane of the sperm. Fertilization requires the specific recognition of the sperm by the egg, followed by fusion of the egg with only one sperm. But before it can fuse with the egg, the sperm must travel through the female reproductive tract and pass through outer layers of follicle cells, the corona radiata and zona pellucida, that surround the egg and form a barrier to sperm penetration (see chapter 36, Animal Reproduction, for a detailed discussion of reproduction). New strategies for contraception and for treatment of infertility can be devised once the molecular processes of these complex events are understood.

When a sperm cell leaves the testis, no more new lipid synthesis occurs. While in the female reproductive tract, the sperm cell undergoes a series of changes called capacitation. One of the events that occurs during capacitation is a reorientation of the existing plasma membrane lipids and proteins and a depletion of cholesterol lipids in the sperm head membrane, causing membrane destabilization. At this point, the sperm is said to be hyperactivated. The destabilization that takes place in the sperm cell membrane prepares it for the acrosome reaction that occurs when it encounters the layers surrounding the egg. When the sperm comes in contact with the zona pellucida, it recognizes and binds to specific glycoproteins on the surface, releasing enzymes via exocytosis. The enzymes digest the zona pellucida in front of the sperm to make a pathway to the egg. The mechanism of sperm-egg binding is not fully understood, but may involve a transmembrane protein in the plasma membrane of the egg called fertilin, that specifically recognizes the sperm. After fusion, the egg releases enzymes that alter the zona pellucida and prevent fusion of additional sperm.

As the molecules that mediate the various steps of the fertilization process are better understood, it may be possible to immunize males or females with molecules that are necessary for reproduction. In this scenario, antibodies would be produced that would bind to and inactivate molecules needed for steps in the fertilization process. See a nice animation of the fertilization process.

1. During capacitation, changes occur in the sperm head membrane. Among these changes are depletion of cholesterol. What function does cholesterol have in an animal cell membrane? What will occur if cholesterol is depleted?
To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.

2. What are the three categories of membrane proteins that exist in the plasma membrane? Binding of the sperm to the zona pellucida and fusion with the egg is very specific. Which category of membrane proteins would you expect to be involved in this process?
To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.




1.	During capacitation, changes occur in the sperm head membrane. Among these changes are depletion of cholesterol. What function does cholesterol have in an animal cell membrane? What will occur if cholesterol is depleted? To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



2.	What are the three categories of membrane proteins that exist in the plasma membrane? Binding of the sperm to the zona pellucida and fusion with the egg is very specific. Which category of membrane proteins would you expect to be involved in this process? To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.

Chapter 4: Cell Membrane Structure and Function

Bizarre Facts in Biology