153e.g., a centriole-dependent MTOC nucleation and the presence of conventional cen-
triole markers. These studies further revealed a similar requirement for the centriole-
independent formation of meiotic and early mitotic spindles on microtubule-dependent
motors, such as dynein and kinesin-5 (Schuh and Ellenberg 2007 ; Courtois et al.
2012 ), showing not only a continuing reliance on meiotic factors during the early
mitotic divisions but also the deployment of these factors in similar cellular
programs.
Another example of gradual transitioning from oocytes to embryos is the finding
in zebrafish of copies of housekeeping genes, which are specialized for maternal
expression and which function during both meiosis and early mitosis. This phenom-
enon has been observed in a maternally expressed form of the protein survivin,
which, as mentioned above, is a component of the CPC complex involved in furrow
induction and maturation. A mutation in the gene motley was found to affect one of
two survivin (a.k.a. birc) genes in the zebrafish genome, birc5b (Nair et al. 2013 ).
This gene exhibits predominantly maternal expression, whereas the related gene
birc5a is expressed both maternally and throughout zygotic development. Mutations
in motley/birc5b as well as Birc5b protein localization indicate a specialized role for
this gene copy in cytokinesis during both meiotic divisions and early zygotic mitotic
divisions. A similar dual function in meiotic and early embryonic mitotic divisions
is observed in the case of another maternal zebrafish gene, tmi (Nair and Pelegri,
unpublished). These examples highlight the continuation of cellular programs
across the generational boundary occurring at fertilization, possibly because, in the
absence of ongoing transcription, reutilization of programs involved in oocyte for-
mation is an effective way to implement processes subsequently required for early
embryonic development. Further studies will be required to understand the preva-
lence of such cellular program reutilization across the fertilization boundary as well
as the role of gene duplication in the generation of genes acting in such processes.
4.4.4 Mammalian Embryo Compaction
In mammals, the cleavage-stage embryo undergoes several mitotic divisions until
compaction, or intracellular adhesion, occurs to form a morula. Initially described
by Mulnard and Huygens ( 1978 ) in mouse embryos and further characterized by
others (Ziomek and Johnson 1980 ; Batten et al. 1987 ; Natale and Watson 2002 ), the
formation of a morula represents the first morphological disruption in embryo radial
symmetry (Fig. 4.11). It is thought that compaction is required for subsequent mor-
phogenetic events such as lineage specification, but how this process is regulated is
generally not well understood (Kidder and McLachlin 1985 ; Levy et al. 1986 ). In
the mouse, compaction is mediated by the formation of adherens junctions, or pro-
tein complexes between cells, of which epithelial cadherin (E-cadherin) is a major
component (Vestweber et al. 1987 ). As a type-1 transmembrane protein, E-cadherin
relies on calcium ions (Ca2+) and its intracellular binding partners, alpha-catenin
(α-catenin) and beta-catenin (β-catenin), to function. However, since E-cadherin
4 Vertebrate Embryonic Cleavage Pattern Determination