16
In frog eggs there is a local, slow increase in Ca2+ concentration at the site of
sperm entry that lasts for 20–60 s (Nuccitelli et al. 1993 ). It is followed by a propa-
gating Ca2+ wave that, as in medaka, spreads around the entire egg with a higher
wave velocity in the cortex (8.9 μm/s) than in the center (5.7 μm/s) of the egg.
During this time the resting Ca2+ concentration of ~300 nM rises to about 2.2 μm,
with the average peak concentration in the center being ~60 % of that in the cortex.
This elevation is maintained for 5–10 min (Fontanilla and Nuccitelli 1998 ). By con-
trast, in physiologically polyspermic urodele eggs, only a very small increase in the
cytosolic Ca2+ level is observed. A representative of this group is the newt Cynops
pyrrhogaster. Up to 20 sperm enter the newt egg at fertilization (Grandin and
Charbonneau 1992 ). The first sperm that binds to the egg triggers an initial Ca2+
spike (Harada et al. 2011 ). The spike is then followed by a Ca2+ wave that starts at
the site of sperm entry but travels only partially across the animal hemisphere.
Although the exact Ca2+ concentration is yet to be determined, the peak level is
much lower in Cynops than in Xenopus eggs. The additional sperm that enter the
egg also trigger partially propagating waves so that the Ca2+ concentration in the
newt ooplasm remains elevated for 30–40 min. The multiple Ca2+ waves triggered
by the numerous penetrating sperm are probably necessary for complete egg activa-
tion. Although physiological polyspermy is a feature in other animals such as carti-
laginous fishes, reptiles, and birds, the Ca2+ changes that follow gamete interaction
remain unknown in these species (Iwao 2012 ).
Mammalian eggs, on the other hand, show a series of low-frequency Ca2+ oscilla-
tions at the time of fertilization that persist for several hours (Stricker 1999 ). The pattern
of the Ca2+ signal varies across species; the interval between the transients is around
3 min in mice and can be as long as 50 min in cow eggs (Kashir et al. 2013 ). In all spe-
cies studied, the first sperm-induced Ca2+ transient arises near the site of sperm attach-
ment and propagates as a wave across the entire egg. In mouse and hamster, it travels
with a velocity of ~20 μm/s and crosses the ooplasm in about 5 s (Miyazaki et al. 1986 ;
Deguchi et al. 2000 ). In the mouse, the initial Ca2+ elevation lasts longer than subse-
quent ones, which probably reflects the transition of the ooplasm from a “non-excitable”
to an “excitable” state. As the oscillations continue, the initiation site of subsequent
waves changes with time; in the mouse it has been shown to translocate from the point
of sperm entry to the cortex of the vegetal hemisphere (Deguchi et al. 2000 ).
1.3.3 The Signaling Cascade
Early observations that the fertilizing sperm is able to generate a propagating Ca2+
wave even in the absence of external Ca2+ suggested that the source of Ca2+ is intracel-
lular (Gilkey et al. 1978 ). Since then it has been firmly established that Ca2+ is mobi-
lized from the intracellular stores that reside in the smooth endoplasmic reticulum.
Ca2+ is loaded into the store by sarcoplasmic/endoplasmic reticulum Ca2+ ATPases
(SERCA pumps). Their activity results in a considerable elevation in the Ca2+ concen-
tration inside the lumen of the endoplasmic reticulum that can reach approximately
Z. Machaty et al.