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Early authors concluded that this must be the case, although these conclusions
were admittedly based on a few cases of poorly characterized abnormal embryos
generated following blastomere perturbations (Waddington 1956 ). However, in con-
trast to amphibians, the separation of early mammalian (rodent and rabbit) blasto-
meres does not result in complementary embryos either having or lacking dorsal
structures (Seidel 1956 ; Tarkowski 1959 ; Tarkowski and Wróblewska 1967 ).
Additionally, early mammalian blastomeres demonstrate a high degree of develop-
mental plasticity, with each of the four-to-eight cell blastomeres contributing to all
cell lineages in chimeric embryos (Tarkowski 1961 ; Mintz 1964 ; Kelly 1977 ).
Furthermore, cell fate specification with respect to epiblast/primitive endoderm/
trophectoderm is largely dependent on cell polarity related to inside or outside cell
position within the morula, as well as on the timing of asymmetric cell division in
generating inside cells (Hillman et al. 1972 ; Ziomek and Johnson 1980 ; Pedersen
et al. 1986 ; Morris et al. 2010 ) (see also Chap. 4 ). Ablation experiments have found
that removal of the animal or vegetal poles from fertilized eggs and early blasto-
meres is fully compatible with normal development (Zernicka-Goetz 1998 ;
Ciemerych et al. 2000 ), unlike the case in amphibians. It is therefore generally con-
cluded that segregation of maternal determinants in the egg is unlikely to direct axis
formation or cell fate patterning in mammals, or that if such a bias exists it can be
readily overridden by other cellular interactions.
Nevertheless, axis specification requires that symmetry breaking occur at some
point prior to gastrulation. When this asymmetry is established and to what extent it
depends on earlier developmental bias or is more or less random has been a recur-
ring debate. There have been various attempts to correlate cleavage patterns in the
early embryo with asymmetries in the blastocyst and conceptus and with the even-
tual anteroposterior axis of the embryo. A preponderance of the evidence however
suggests that much of this observed “bilateral symmetry” likely results from physi-
cal constraints imposed by the zona pellucida (vitelline membrane) or other exter-
nal constraints and is not connected to the orientation of the anteroposterior axis (for
detailed reviews of this literature, please see Takaoka and Hamada 2012 ; Zernicka-
Goetz 2013 ; Bedzhov et al. 2014 and references therein).
The most compelling evidence for an early cell fate bias is the observation that the
order and orientation of rotational cleavages in the mouse embryo can influence blas-
tomere fate in the blastocyst (Fig. 6.6). In particular, the vegetal blastomere (distal to
the polar body) that arises from a particular tetrahedral cleavage pattern (which
occurs in a subset of cases), will disproportionately contribute to the trophectoderm
in normal embryos (Piotrowska-Nitsche et al. 2005 ; Torres-Padilla et al. 2007a).
Furthermore, chimeras composed exclusively of vegetal blastomeres fail to survive
(Piotrowska-Nitsche et al. 2005 ), likely because these cells cannot generate sufficient
numbers of pluripotent epiblast cells to support development (Morris et al. 2012b).
Lineage labeling studies of individual or all four cells have found a similarly
biased contribution of four cell-stage blastomeres to either inner cell mass (ICM) or
trophectoderm (TE) fates in a subset of embryos (Fujimori et al. 2003 ; Tabansky
et al. 2013 ). Importantly, this developmental bias was reflected in cell lineage but
not in relative cell positioning toward the embryonic or abembryonic poles of the
D.W. Houston