15embryo development (Stricker 1999 ). The elevation of Ca2+ level is an evolutionarily
conserved mechanism for egg activation and is considered a hallmark of fertilization
in all species. The sperm-induced Ca2+ increase was observed directly for the first time
in the eggs of the medaka fish (Ridgway et al. 1977 ). Two lines of evidence support
the notion that Ca2+ is a key second messenger in the stimulation of embryo develop-
ment. First, artificially increasing Ca2+ concentration in the ooplasm triggers early
events of egg activation (Whittingham 1980 ). Second, treatment of eggs with chela-
tors that bind to Ca2+ and prevent the rise in its concentration obliterates all changes
related to egg activation (Whitaker and Steinhardt 1982 ). These findings indicate that
a Ca2+ rise is both necessary and sufficient to induce development at fertilization.
In lower vertebrates, as in most invertebrates, the Ca2+ change involves a single
transient elevation in the cytosolic Ca2+ levels, while mammalian eggs display a
series of low-frequency Ca2+ oscillations at fertilization (Fig. 1.1). The need for a
single versus multiple Ca2+ transients for activation seems to depend on the cell
cycle stage at which the egg awaits fertilization (eggs of invertebrate animals may
arrest at stages other than metaphase II), the time needed for completion of meiosis
after sperm-egg fusion, and the extent of changes the intracellular Ca2+ stores
undergo during oocyte maturation (Jones 1998 ).
Upon fertilization, eggs of teleost fishes display a transient rise in their intra-
cytoplasmic Ca2+ concentration. In medaka eggs the Ca2+ transient originates at
the site of sperm entry, the micropyle. The elevated Ca2+ level then travels as a
0.05–0.1 mm wide band across the yolk-deficient peripheral ooplasm at an
average velocity of ~12.5 μm/s, reaching the vegetal pole after about 2 min
(Iwamatsu 2000 ; Fig. 1.2). In zebrafish eggs, the Ca2+ signal is made up of two
components. First, a Ca2+ wave is initiated at the micropyle at the animal pole
and propagates through the cortical cytoplasm to the vegetal pole at a velocity
of ~9 μm/s (Lee et al. 1999 ). The cortical Ca2+ wave is then followed by a slower
elevation of Ca2+ in the center of the egg (Sharma and Kinsey 2008 ).
Fig. 1.1 Sperm-induced Ca2+ changes at fertilization. In Xenopus (and other vertebrates except
mammals), a single Ca2+ rise is generated at fertilization (a). Mammalian eggs, on the other hand,
display a series of low-frequency Ca2+ oscillations. Shown here is a train of Ca2+ spikes detected in
a fertilized mouse egg (b). The eggs were loaded with a Ca2+ indicator dye and inseminated (From
Nuccitelli et al. 1993 ; Swann and Jones 2002 ; with permission)
1 Egg Activation at Fertilization