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4.3.4.1 Meroblastic Cleavage in Aves and Reptiles
Most knowledge concerning the cleavage stages of avian embryos comes from stud-
ies in the domestic chicken (Gallus gallus). However, understanding of cellular and
molecular processes during cleavage stages is limited even in this well-known
developmental model organism (Lee et al. 2013 ; Sheng 2014 ; Nagai et al. 2015 ).
This is because, by the time the egg is laid, the embryo is long past the blastula
stage. Thus, studying cleavage-stage embryos in the chick requires the use of meth-
ods for obtaining eggs still developing in utero (Lee et al. 2013 ).
The chick embryo undergoes meroblastic cleavage, with blastomeres dividing
atop a large yolky mass (reviewed in Sheng 2014 ; Fig. 4.7). Fertilization is notable
because, while only a single female and single male pronucleus will give rise to the
zygotic genome, polyspermy is not uncommon (Lee et al. 2013 ). These supernu-
merary sperm, which can be few or many, are found in both yolk and blastoderm.
Their function, if any, is unclear, but they are capable of producing pseudo-furrows
that do not fully ingress.
Another notable feature of these embryos is that the point at which the first two
cleavage furrows meet is not centered at the middle of the embryo (Sheng 2014 ;
Nagai et al. 2015 ). There is a known correlation between asymmetric inheritance of
maternally deposited factors and establishment of PGCs, but whether this is associ-
ated with off-center early cleavages is unclear. It has also been proposed that the
off-center cleavages cause blastomeres to inherit heterogeneously deposited maternal
factors asymmetrically as they cellularize, leading to symmetry breaking and axis
induction. However, the ability to experimentally change the dorsal–ventral axis
after egg laying challenges this early-establishment hypothesis (reviewed in Sheng
2014 ).
The first two cleavages in the chick are stereotypical, with the second forming
perpendicular to the first in the plane of the blastoderm. The third furrow follows
this pattern, though evidence suggests that divisions at this point become asymmet-
ric, resulting in smaller cells in the interior and larger cells at the periphery (Lee
et al. 2013 ). Cell division becomes asynchronous at the fourth cleavage, and pre-
sumably some asynchronous division continues (Sheng 2014 ).
It is also roughly around this time that the embryo begins to form two layers of
blastomeres. As cleavage progresses, the embryo will reach 5–6 layers of cells in
thickness before thinning again during and after gastrulation (Sheng 2014 ). A study
found that approximately 75 % of surface cells (i.e., those in the uppermost layer)
divided in a direction parallel to the blastoderm plane, yielding two daughters in
the same layer, while deeper cells show more variation, with approximately 56 %
dividing in a direction 30–90° from that of the blastoderm plane (Nagai et al. 2015 ;
see below). These data suggest that cleavage orientation only partly explains
increasing layer number, but other mechanisms, such as rearrangement of already
cellularized blastomeres, may also be involved. The increase in layers also roughly
corresponds to the onset of formation of the subgerminal cavity, a space that sepa-
rates part of the blastoderm from the yolk. This structure may also influence blasto-
derm thickness.
A. Hasley et al.