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blastocoel wall (Longo et al. 2004 ). Other forces involved with blastular morpho-
genesis remain obscure but certainly include localized regulation of cell–cell inter-
actions, as illustrated by the effective obliteration of the blastocoel following
interference with cadherin-dependent adhesion or Eph–ephrin-mediated cell–cell
repulsion (Heasman et al. 1994a, b; Winning et al. 1996 ).
4.4.3 Transition of Cell Division Factors from Oocytes
to Embryos
Egg activation, typically coincident with fertilization, is regarded as a natural
boundary between egg and zygote. Indeed, egg activation involves many processes
that turn a quiescent cell into an actively dividing one. A dramatic example of this
sharp transition is the cortical calcium wave associated with egg activation and/or
sperm entry, which triggers the exocytosis of cortical granules and remodeling of
surrounding egg membranes to act as a block against polyspermy (see Chap. 1 ).
Another dramatic example is the initiation of maternal transcript polyadenylation
upon egg activation, which results in the translation of their cognate protein prod-
ucts for use during embryonic development (see Chap. 2 ).
Besides the abrupt physiological transition brought about by egg activation and
the associated process of fertilization, several other profound alterations in cellular
processes are known to take place between the end of oogenesis and early embry-
onic development. This is exemplified by the transition between two different
mechanisms for spindle formation in mouse embryos (Courtois et al. 2012 ).
Vertebrates exhibit degeneration of centrioles during oogenesis; centrioles are pro-
vided solely by the sperm, an arrangement that ensures a constant centriole number
through generations and is an obstacle to parthenogenetic development (Symerly
et al. 1995 ; Delattre and Gönczy 2004 ). Thus, in vertebrates, spindle formation dur-
ing meiosis utilizes a centriole-independent pathway in which microtubules self-
organize into a pair of microtubule foci which, although wider than those organized
by centriole pairs, can nevertheless direct the formation of a barrel-shaped bipolar
spindle (Heald et al. 1996 , 1997 ; Gaglio et al. 1997 ; Walczak et al. 1998 ; Schuh and
Ellenberg 2007 ). During cleavage stages, embryonic cells typically use bipolar
spindles whose formation relies on centrioles inherited through the sperm. However,
in rodent lineages, not only oocyte centrioles but also sperm centrioles degenerate
(Woolley and Fawcett 1973 ; Schatten 1994 ; Manandhar et al. 1998 ), and the early
rodent embryo has to rely on a centriole-independent mechanism to generate
MTOCs and spindles. Courtois et al. ( 2012 ) found that, unlike most organisms, the
mechanism for MTOC formation in the early mouse embryo fails to undergo a sharp
transition at fertilization. Instead, early mouse embryos form MTOCs and spindles
that have morphological properties similar to those in oocytes during meiosis.
During the first eight embryonic cleavages, MTOC and spindle morphology change
gradually from the meiotic pattern to one that is typical of later embryonic stages,
A. Hasley et al.