14
In other animals such as mammals, there is no indication for an action potential
after sperm-egg fusion. Physiologically polyspermic eggs of urodele amphibians
lack the ability to mount a positive-going fertilization potential. Additionally, volt-
age clamping of the plasma membrane to positive levels does not affect sperm entry
into such eggs, indicative of an absence of a fast block to polyspermy (Iwao and
Jaffe 1989 ). Mouse and hamster eggs can generate action potentials only under
nonphysiological circumstances, after being artificially hyperpolarized to −70 mV
(Miyazaki and Igusa 1981 ; Peres 1986 ). In the eggs of these species, membrane
potential changes of another kind can be measured following gamete fusion.
Hamster oocytes show recurring hyperpolarization responses from the original
−40 mV to −70 mV. These responses are due to Ca2+-activated K+ conductance, and
they reflect the underlying repetitive elevations in the cytosolic Ca2+ levels (Miyazaki
and Igusa 1982 ). In fertilized medaka eggs, the initial depolarization is followed by
a large hyperpolarization phase that is associated with increased K+ permeability
(Nuccitelli 1980 ). Finally, similar hyperpolarizations were reported in mouse, rab-
bit, and human eggs as well, indicative of underlying Ca2+ oscillations in the ooplasm
(Jaffe et al. 1983 ; McCulloh et al. 1983 ; Homa and Swann 1994 ).
1.3.2 Increase in the Cytosolic Ca2+ Level
Sperm-egg fusion is followed by an interval known as the latent period. The term was
coined while studying sea urchin fertilization, and it refers to the time between the
sperm-egg membrane fusion and the next observable event of fertilization, the initia-
tion of the cortical reaction (Allen and Griffin 1958 ). Because the cortical reaction is
induced by an increase in cytosolic Ca2+ levels, the term latent period is now used to
indicate the time between the fusion and the onset of the Ca2+ signal that activates the
egg (Whitaker and Swann 1993 ). The most accurate measurements regarding the
length of the latent period were performed in sea urchin. Gamete fusion was con-
trolled by holding the eggs at membrane potentials that successively allowed first
sperm adhesion only and then gamete fusion (Shen and Steinhardt 1984 ). This
revealed that the time between fusion and initiation of the Ca2+ wave was about 15 s.
In frog eggs the latent period, defined as the time between the action potential (which
indicates gamete fusion) and the Ca2+ wave, was found to be longer, at about 1 min
(Busa and Nuccitelli 1985 ). In the absence of an action potential in mammalian eggs,
the transfer of a fluorescent dye from the egg into the sperm was used to establish the
moment of fusion. The length of the latent period determined this way in mouse eggs
was around 1–3 min (Lawrence et al. 1997 ). In hamster, the cessation of sperm motil-
ity was used to characterize the latent period. Sperm tail movement stops when the
membranes of the gametes fuse; using this as an indicator, the latent period in ham-
ster eggs was found to be less than 10 s (Miyazaki and Igusa 1981 ).
The end of the latent period is marked by an increase in the egg’s intracellular free
Ca2+ concentration. In all animals that have been examined so far, the rise in the cyto-
solic Ca2+ levels is the signal the fertilizing sperm use to activate the egg and stimulate
Z. Machaty et al.