Fertilization

When the sperm encounters an egg, it undergoes an acro-somal reaction—exocytosis of the acrosome, releasing the enzymes needed to penetrate the egg. But the first sperm to reach an egg is not the one to fertilize it. Sperm must first penetrate the granulosa cells and zona pellucida that surround it (fig. 29.1). It may require hundreds of sperm to clear a path for the one that penetrates the egg proper.

Two of the acrosomal enzymes are hyaluronidase, which digests the hyaluronic acid that binds granulosa cells together, and acrosin, a protease similar to the trypsin of pancreatic juice. When a path has been cleared, a sperm binds to the zona pellucida and releases its enzymes, digesting a pathway through the zona until it contacts the egg itself. The sperm head and midpiece enter the egg, but the egg destroys the sperm mitochondria and passes only maternal mitochondria on to the offspring.

Fertilization combines the haploid (n) set of sperm chromosomes with the haploid set of egg chromosomes and produces a diploid (2n) set. Fertilization by two or more sperm, called polyspermy, would produce a triploid (3n) or larger set of chromosomes and the egg would die. The egg has two mechanisms for preventing this: a fast block and slow block to polyspermy. In the fast block, binding of the sperm to the egg opens Na+ channels in the egg membrane. The rapid inflow of Na+ depolarizes the

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Sperm

Corona radiata Zona pellucida

Sperm

Corona radiata Zona pellucida

Zona pellucida

Extracellular space

Granulosa cells of corona radiata

Acrosomal reaction

Acrosome Function

Cortical granule Egg cytoplasm

Figure 29.1 Fertilization and the Slow Block to Polyspermy.

Zona pellucida

Extracellular space

Granulosa cells of corona radiata

Acrosomal reaction

Acrosome Sperm nucleus

Cortical granule Egg cytoplasm

Figure 29.1 Fertilization and the Slow Block to Polyspermy.

membrane and inhibits the binding of any more sperm to it. The slow block involves secretory vesicles called cortical granules just beneath the membrane. Sperm penetration triggers an inflow of Ca2+; this, in turn, triggers a cortical reaction in which the cortical granules release their secretion beneath the zona pellucida. The secretion swells with water, pushes any remaining sperm away from the egg, and creates an impenetrable fertilization membrane.

_Think About It_

What similarity can you see between the slow block to polyspermy and the release of acetylcholine from synaptic vesicles of a neuron? (Compare p. 465.)

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