The Questions of Developmental Biology

Lying immediately beneath the plasma membrane of the egg is a thin shell (about 5 μm)
of gel-like cytoplasm called the cortex. The cytoplasm in this region is stiffer than the internal
cytoplasm and contains high concentrations of globular actin molecules. During fertilization,
these actin molecules polymerize to form long cables of actin known as microfilaments.
Microfilaments are necessary for cell division, and they also are used to extend the egg surface
into small projections called microvilli, which may aid sperm entry into the cell (see Figure 7.6B;
also see Figure 7.19). Also within the cortex are the cortical granules (see Figures 7.4 and 7.6B).
These membrane-bound structures, which are homologous to the acrosomal vesicle of the sperm,
are Golgi-derived organelles containing proteolytic enzymes. However, whereas each sperm
contains one acrosomal vesicle, each sea urchin egg contains approximately 15,000 cortical
granules. Moreover, in addition to digestive enzymes, the cortical granules contain
mucopolysaccharides, adhesive glycoproteins, and hyalin protein. The enzymes and
mucopolysaccharides are active in preventing other sperm from entering the egg after the first
sperm has entered, and the hyalin and adhesive glycoproteins surround the early embryo and
provide support for the cleavage-stage blastomeres.
Many types of eggs also have an egg jelly outside the vitelline envelope (see Figure 7.4).
This glycoprotein meshwork can have numerous functions, but most commonly is used either to
attract or to activate sperm. The egg, then, is a cell specialized for receiving sperm and initiating
development.
*The contents of the egg vary greatly from species to species. The synthesis and placement of these materials will be
addressed in Chapter 19, when we discuss the differentiation of germ cells
Recognition of Egg and Sperm
The interaction of sperm and egg generally proceeds according to five basic steps
(Figure 7.8; Vacquier 1998):
1. The chemoattraction of the sperm to the egg by soluble molecules secreted by the egg
2. The exocytosis of the acrosomal vesicle to release its enzymes
3. The binding of the sperm to the extracellular envelope (vitelline layer or zona pellucida) of the
egg
4. The passing of the sperm through this extracellular envelope
5. Fusion of egg and sperm cell plasma membranes