149depends on cytoplasmic volume and a limiting subunit such as free tubulin and
results in spindle scaling in smaller cells.
A limiting-component mechanism may be a simple and widely used property of
animal embryos, including those of vertebrates. Spindle size in early mammalian early
embryos is also consistent with a limiting cytoplasmic factor (Courtois et al. 2012 ), and
centrosome size in the early zebrafish embryo is significantly larger in early blastomeres
than in later ones (Lindeman and Pelegri 2012 ). Various components may also interact.
For example, centrosome size is known to influence spindle length (Greenan et al.
2010 ), and it will be interesting in the future to assess the role of centrosome apportion-
ing to spindle scaling. Mechanisms similar to those proposed to regulate spindle and
centrosome size, dependent on limiting components inherited in the egg, likely act in the
regulated generation of other subcellular organelles in the early embryo.
4.4.2 Specialization of the Cytoskeleton in the Early Embryo
The morphogenetic forces that produce each species’ unique embryonic cleavage
pattern must integrate with several other cellular activities during cell division to
construct the pregastrular embryo. These activities include the establishment of a
specialized basolateral membrane domain in each cleavage plane, the zippering
together of apical–basolateral margins along advancing furrows to produce a tight-
junctional osmotic barrier, and, ultimately, the osmotically driven inflation of inter-
stitial spaces such as the blastocoel. Where the egg begins with only a single outer
(apical) surface, the blastula must develop a functional, polarized epithelium to
physiologically isolate the interstitial space and/or blastocoel from the outside
world. During cleavage, blastomeres become adherent and distinct apical and baso-
lateral membrane domains develop, separated by apical tight junctions (Muller and
Hausen 1995 ; Merzdorf et al. 1998 ; Fesenko et al. 2000 ). This process, referred to
as compaction, occurs at different developmental times in different organisms. For
example, as discussed below (Sect. 4.4.4), in mammalian embryos, compaction is a
distinct phase beginning at about the eight-cell stage or later (Ducibella and
Anderson 1975 ; Fleming et al. 2000 ), while in holoblastically cleaving eggs of
amphibians and sturgeon (Zotin 1964 ; Bluemink 1970 ; Kalt 1971a, b; Bluemink
and deLaat 1973 ), it occurs contemporaneously with the earliest cleavages.
4.4.2.1 Basolateral Membrane Formation in Xenopus Cleavage
Amphibian early embryos are distinctive among dividing cells for the compara-
tively large amount of membrane added continuously during the cleavage process
(Bluemink and deLaat 1973 ). Cleavage furrowing is said to be unipolar because it
begins at one pole of the egg, and the contractile band then assembles and travels as
an arc extending progressively around the egg surface, eventually forming a com-
plete ring which then constricts inward while large amounts of new surface area are
4 Vertebrate Embryonic Cleavage Pattern Determination