Fertilization:

Introduction

Fertilization is the "great awakening" of the oocyte. Dr. Jerry Schatten (Dept. of Zoology, Univ. of Wisconsin-Madison), has likened fertilization to the classic fairy tale, "Sleeping Beauty", since the oocyte is "awakened" from "slumber" by the events of fertilization, resulting in the process of egg activation. Taken in its complete context, fertilization accomplishes at least four things:

1. Restoration of a diploid genome by uniting the two haploid genomes of the oocyte and sperm;
2. Species-specific sperm-egg binding;
3. Egg activation, beginning with the cortical reaction, a transient calcium wave that passes across the egg, and the onset of nucleic acid and protein synthesis.
4. Contribution of paternal centrosomal material by the sperm, required for proper mitosis.

Fertilized Xenopus oocytes viewed from the animal pole. After fertilization, the cortical reaction results in modifications to the vitelline envelope, permitting the reorientation of the eggs via gravity. The different densities of yolk in the egg result in the consistent orientation seen here.