29the periodic increases in the rate of Ca2+ influx (Halet et al. 2004 ). PKC activation
with phorbol esters such as 12-O-tetradecanoylphorbol-13-acetate (TPA) or phor-
bol-12-myristate-13-acetate (PMA) causes Ca2+ oscillations in mouse eggs
(Cuthbertson and Cobbold 1985 ; Ducibella et al. 1991 ). In addition, PMA treatment
of fertilized eggs during the ongoing Ca2+ oscillations markedly increases oscilla-
tion frequency, whereas inhibition of PKC with bisindolylmaleimide I suppresses
the sperm-induced Ca2+ oscillations (Halet et al. 2004 ). A potential candidate to
mediate the PKC-controlled Ca2+ influx is the trp protein that is known to function
as a Ca2+ entry channel in a number of cell types. TRP channels are present in eggs
(Petersen et al. 1995 ; Machaty et al. 2002 ), and certain TRP isoforms are modulated
by PKC (Hardie 2007 ), which would explain the stimulatory effect of PKC on the
sperm-induced Ca2+ signal. However, the TRP channel does not seem to be essential
during fertilization. Although selective stimulation of TRPV3 channels mediates
Ca2+ entry that leads to activation, eggs of transgenic mice lacking TRPV3 channels
show normal Ca2+ oscillations at fertilization (Carvacho et al. 2013 ). This indicates
that TRPV3 is not required for the maintenance of the sperm-induced Ca2+ tran-
sients. Currently, it is still unclear what type of Ca2+ entry operates in mouse eggs at
fertilization.
Oscillatory Ca2+ signals seem to have physiological advantages over single
monotonic Ca2+ elevations. The repetitive behavior provides a means to deliver pro-
longed Ca2+ signals to targets without the deleterious effects of sustained Ca2+ eleva-
tions. The pattern of the sperm-induced Ca2+ signal encodes crucial information and
has a profound effect not only on the immediate events of egg activation but also on
peri-implantation development (Ozil and Huneau 2001 ). Although a single rise in
the intracellular Ca2+ concentration can promote parthenogenetic development,
freshly ovulated eggs show limited cell cycle progression following activation with
such a stimulus. Recruitment of mRNAs is also abnormal in these eggs, and only
after aging can a single Ca2+ rise stimulate these critical events (Jones 1998 ; Ozil
et al. 2005 ). By manipulating the number of Ca2+ transients in fertilized mouse eggs,
it has been demonstrated that the first few Ca2+ rises are able to induce development
to the blastocyst stage, but the developmental competence of these blastocysts is
compromised. Microarray analysis of global gene expression patterns in these
embryos has revealed that approximately 20 % of the genes are misregulated, par-
ticularly those involved in RNA processing, polymerase II transcription, cell cycle,
and cell adhesion (Ozil et al. 2006 ). It is not clear how exactly the Ca2+ oscillations
at fertilization have their effects on many cell divisions later. It is possible that an
inadequate Ca2+ signal does not sufficiently trigger the degradation of cell cycle
regulatory proteins that maintain the metaphase II arrest and that the presence of
such proteins hampers embryonic cell divisions later during development.
Alternatively, it may be that the suboptimal signal is not able to turn on gene expres-
sion properly. These two possibilities are not mutually exclusive, however, and they
may both contribute to the reduced developmental potential of embryos whose
development was stimulated by suboptimal Ca2+ signals (Jones 2007 ).
It is interesting to speculate why multiple Ca2+ oscillations became the norm for
egg activation in mammals. Physiological polyspermy might have arisen in animals
1 Egg Activation at Fertilization