Fertilization 2

Fertilization

INTRODUCTION 

Fertilization, the process in which gametes—a male's sperm and a female's egg or ovum—fuse together, producing a single cell that develops into an adult organism. Fertilization occurs in both plants and animals that reproduce sexually—that is, when a male and a female are needed to produce an offspring

INTRODUCTION The first cell of a new human being, called a zygote, is formed when a sperm fertilizes an egg. In the egg cell shown here, the nuclei of the sperm and egg appear as two yellowbrown, irregular shapes. When these two nuclei fuse, fertilization is accomplished.

THE FERTILIZATION PROCESS 

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Fertilization is complete when the sperm's nucleus fuses with the egg's nucleus. Researchers have identified several specific steps in this process. The first step is the sperm approaching the egg. In some organisms, sperm just swim randomly toward the egg (or eggs). In others, the eggs secrete a chemical substance that attracts the sperm toward the eggs. For example, in one species of sea urchin.

THE FERTILIZATION PROCESS  The second step of fertilization

is the attachment of several sperm to the egg's surface coat. All animal eggs have surface coats, which are variously named the vitelline envelope (in abalone and frogs) or the zona pellucida (in mammals). This attachment step may last for just a few seconds or for several minutes.

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The attachment of several sperm to the egg's surface coat

THE FERTILIZATION PROCESS  The third step is a complex

process in which the sperm penetrate the egg’s surface coat. The head, or front end, of the sperm of almost all animals except fish contains an acrosome, a membrane-enclosed compartment. The acrosome releases proteins that dissolve the surface coat of an egg of the same species.

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THE FERTILIZATION PROCESS  The next step in fertilization—the

fusion of sperm and egg cell membranes—is poorly understood. When the membranes fuse, a single sperm and the egg become one cell. This process takes only seconds, and it is directly observable by researchers. Specific proteins on the surface of the sperm appear to induce this fusion process, but the exact mechanism is not yet known.

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The fusion of sperm and egg cell membranes

 Finally,

in a process that is also poorly understood, the egg and sperm nuclear envelopes (outer membranes) fuse, permitting the chromosomes from the egg and sperm to mix within a common space. A zygote is formed, and development of an embryo begins.

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A zygote is formed, and development of an embryo begins.

 Fertilization

is not instantaneous—it may take 30 minutes in sea urchins and up to several hours in mammals.  These stages show how human and animal reproduce sexually. 