13urodeles show physiological polyspermy, where several sperm enter each egg (see
below). In birds the sperm must pass through pores in the inner perivitelline layer of
the ovum to gain access to its plasma membrane or oolemma (in birds the term egg
is reserved for the calcified hard-shelled product of oogenesis, whereas at the time
of ovulation, the follicular oocyte becomes an ovum; Wishart and Horrocks 2000 ).
Mammalian eggs are surrounded by the zona pellucida, and the spermatozoa must
digest a penetration slit in the zona matrix in order to reach the plasma membrane.
1.3.1 Electrophysiological Changes
After getting through the egg’s extracellular matrix, the fertilizing sperm adheres to,
and then fuses with, the oolemma. In many species, the first indication that fertiliza-
tion is under way is a change in the membrane potential of the egg’s plasma mem-
brane. Membrane potential changes during fertilization are well described in
invertebrates. The resting potential of the unfertilized sea urchin egg’s plasma mem-
brane is about −70 mV. The fertilizing sperm triggers an inward current (carried
mainly by an influx of extracellular Na+ and Ca2+ ions) that causes depolarization of
the membrane. Depolarization leads to an action potential, taking the membrane
potential to positive values (Hagiwara and Jaffe 1979 ). The initial depolarization
has been shown to coincide with gamete fusion, and the inward current that induces
the action potential is the consequence of ions flowing into the ooplasm through the
plasma membrane of the sperm. This was verified by measuring the electrical
capacitance of the egg’s plasma membrane (McCulloh and Chambers 1992 ). An
increase in membrane capacitance is due to the addition of the sperm surface area to
the oolemma (see below); the results of these experiments indicated that fusion is
indeed the first step of fertilization in these species and it takes place before any
other notable changes associated with egg activation.
Similar positive-going shift in the membrane potential was also reported in the
eggs of certain vertebrate species including medaka (Nuccitelli 1980 ), lamprey
(jawless fishes; Kobayashi and Yamamoto 1994 ), and frog (Ito 1972 ). The large
positive fertilization potential in lamprey eggs is mediated by the opening of Cl−
channels that reside mostly in the animal pole region. In frog eggs the Ca2+ wave
that induces activation also triggers a propagative opening of Cl− channels in the
oolemma, causing a positive shift in the membrane potential (Kline and Nuccitelli
1985 ). In many species, the resulting fertilization potential has an important
physiological role: it serves as a fast block to polyspermy. Voltage clamping of
the membrane potential of unfertilized Xenopus eggs blocks sperm entry, whereas
maintaining it below 0 mV under voltage-clamp conditions leads to polyspermy
(Charbonneau et al. 1983 ). The voltage that blocks sperm entry corresponds well
to the fertilization potential, indicating that the fertilization potential acts as a
fast, electrical block to polyspermy in these species (Jaffe and Cross 1986 ). The
electrical block to polyspermy at the plasma membrane operates in monospermic
urodele eggs as well (Iwao 2000 ).
1 Egg Activation at Fertilization