32
In most vertebrates, incorporation of the sperm into the ooplasm is followed by
the disassembly of the connecting piece of the sperm tail. The sperm generally con-
tributes two centrioles: a proximal centriole located within the connecting piece and
a distal centriole that is aligned with the axoneme. Only the proximal centriole has
microtubule-organizing capabilities; the distal centriole has undergone partial
degeneration during spermiogenesis (reviewed by Schatten and Sun 2009 ). Eggs, on
the other hand, lack centrioles and instead possess a variety of centrosomal proteins.
As the proximal sperm centriole becomes exposed to the egg cytoplasm, maternal
centrosomal components accumulate around it, and the sperm aster is formed. In
most animals the sperm aster is responsible for the migration and eventual union of
the male and female pronuclei (Schatten 1994 ). The sperm nucleus remains in the
center of the aster, and as the growth of the aster pushes it radially from the surface,
it moves from the egg cortex toward the center. The egg completes meiosis and the
second polar body is extruded. The egg chromosomes then form a female pronucleus
that migrates toward the sperm nucleus, guided by the sperm aster. The sperm
nucleus transforms into a male pronucleus; this transformation is brought upon by
disintegration of the nuclear envelope, decondensation of the chromatin, and refor-
mation of a new nuclear envelope. The male and female pronuclei become closely
apposed in the center of the egg (Schatten et al. 1985 ). DNA is replicated; the cen-
trioles also duplicate to form centrosomes on the opposite ends of the mitotic spin-
dle by the time the zygote enters the first cleavage division.
With the appearance of the pronuclei, the Ca2+ oscillations stop in mouse eggs; if
exit from meiosis is blocked, the oscillation goes on indefinitely (Marangos et al.
2003 ). In addition, the transfer of nuclei of 1- and 2-cell fertilized mouse embryos
to unfertilized eggs triggers Ca2+ oscillations that initiate at the time of nuclear
envelope breakdown (Kono et al. 1996 ). These findings led to the conclusion that
the train of Ca2+ spikes comes to an end when PLCζ, the messenger responsible for
the generation of the signal, is sequestered into the pronucleus (Larman et al. 2004 ).
The Ca2+ oscillations can be detected again during the first mitotic division of the
embryo, probably because PLCζ remains active, and after the nuclear envelopes
break down, it again generates IP 3 that causes Ca2+ release from the endoplasmic
reticulum (Kono et al. 1996 ). Interestingly, in nucleate and anucleate halves of
mouse zygotes obtained by bisection after fertilization, the Ca2+ oscillations cease at
about the same time irrespective of the presence of the pronuclei (Day et al. 2000 ).
In addition, fertilized bovine and rabbit eggs display oscillations that persist well
beyond pronucleus formation (Fissore et al. 1992 ; Fissore and Robl 1993 ). These
findings indicate species-specific differences and imply that additional factors may
also control the cessation of the Ca2+ oscillations.
1.4 Concluding Remarks
We have gone a long way toward the understanding of how a sperm activates an
egg. A major milestone was the realization that in all animals, an increase in the
egg’s intracellular free Ca2+ concentration is the stimulus that triggers embryo
Z. Machaty et al.