The Questions of Developmental Biology

The calcium ions responsible for the cortical granule reaction are stored in the
endoplasmic reticulum of the egg (Eisen and Reynolds 1985; Terasaki and Sardet 1991). In sea
urchins and frogs, this reticulum is pronounced in the cortex and surrounds the cortical granules
(Figure 7.26; Gardiner and Grey 1983; Luttmer and Longo 1985). In Xenopus, the cortical
endoplasmic reticulum becomes ten times more abundant during the maturation of the egg and
disappears locally within a minute after the wave of cortical granule exocytosis occurs in any
region of the cortex. Once initiated, the
release of calcium is self-propagating.
Free calcium is able to release
sequestered calcium from its storage
sites, thus causing a wave of calcium
ion release and cortical granule
exocytosis.
The Activation of Egg Metabolism
Although fertilization is often depicted as merely the means to merge two haploid nuclei,
it has an equally important role in initiating the processes that begin development. These events
happen in the cytoplasm and occur without the involvement of the nuclei.*
The mature sea urchin egg is a metabolically sluggish cell that is activated by the sperm.
This activation is merely a stimulus, however; it sets into action a preprogrammed set of
metabolic events. The responses of the egg to the sperm can be divided into "early" responses,
which occur within seconds of the cortical reaction, and "late" responses, which take place
several minutes after fertilization begins (Table 7.1; Figure 7.27).
Early responses
As we have seen, contact between sea urchin sperm and egg activates the two major
blocks to polyspermy: the fast block, initiated by sodium influx into the cell, and the slow block,
initiated by the intracellular release of calcium ions. The activation of all eggs appears to depend
on an increase in the concentration of free calcium ions within the egg. Such an increase can
occur in two ways: calcium ions can enter the egg from outside, or calcium ions can be released
from the endoplasmic reticulum within the egg.