The Questions of Developmental Biology

eticulum (McPherson et al. 1992). The binding of calcium ions to these receptors releases more
calcium, and this released calcium binds to more receptors, and so on. The resulting wave of
calcium release is propagated throughout the cell, starting at the point of sperm entry; and the
cortical granules, which fuse with the cell membrane in the presence of high calcium
concentrations, respond in a wave of exocytosis that follows the calcium ions. Mohri and
colleagues (1995) have shown that IP 3 -released calcium is both necessary and sufficient for
initiating the wave of calcium release.
IP 3 is similarly found to release calcium ions in vertebrate eggs. As in sea urchins, waves
of IP 3 are thought to mediate calcium release from sites within the endoplasmic reticulum
(Lechleiter and Clapham 1992; Miyazaki et al. 1992; Ayabe et al. 1995). Blocking the IP 3
receptor in hamster eggs prevents the release of calcium at fertilization. Xu and colleagues (1994)
found that blocking the IP 3 -mediated calcium release blocks every aspect of sperm-induced egg
activation, including cortical granule exocytosis, mRNA recruitment, and cell cycle resumption.
The question then becomes, what initiates the production of IP 3 ? In other words, what
activates the phospholipase C enzymes? This question has not been easy to address, since (1)
there are numerous types of PLC, (2) they can be activated through different pathways, and (3)
different species can use different mechanisms to activate them. Results from recent studies of
sea urchin eggs suggest that the active PLC is a member of the family of PLCs and is activated
by a protein tyrosine kinase
(Carroll et al. 1997, 1999;
Williams et al. 1998).
The next question, then,
is, what protein tyrosine kinase
is activated? This question has
not yet been satisfactorily
answered. According to one
model, the sperm receptor
protein crosses the egg plasma
membrane and has a protein
tyrosine kinase activity in its
cytoplasmic domain (Figure
7.29A).