1474.4 Cell Division Machinery During the Early Cleavage
Stage
Cell division in the early embryo is influenced by features characteristic of the egg-
to- embryo transition, such as the shift from meiotic to mitotic cycles, the inheri-
tance of a limited supply of cellular building blocks, and specializations for large
cellular size and unique embryo architecture. We address these topics in this
section.
4.4.1 Maternal Loads and Scaling of Spindle Size
During Early Cell Divisions
The early embryo develops with unique restrictions since, prior to zygotic gene
activation at the midblastula transition (see Chap. 9 ), all embryonic processes by
necessity are driven solely by maternal products. Thus, while cells at later stages of
the embryo and the adult produce on their own new products essential for cell
growth and division, cells in the early embryo are limited to the supply of cellular
building blocks originally stored in the mature egg. Species have evolved special-
ized systems for the storage in the egg and controlled use of maternal products dur-
ing early embryonic development. In addition to energy and essential building
blocks, the embryo must generate subcellular structures as it becomes multicellular.
It has been shown that the overall protein composition from the fertilized egg to the
midblastula transition only changes minimally (Lee et al. 1984 ; Peshkin et al. 2015 ).
Hence, the embryo must generate vital subcellular structures that can fulfill their
tasks under drastically different dimensional scales, while the cell size changes by
orders of magnitude from the fertilized egg to the midblastula transition. In particu-
lar, the cellular machinery must adapt to use a finite initial supply of building blocks
for vital subcellular structures as they are being used by the embryo and in the con-
text of a several-fold change in blastomere size. Interestingly, the limited supply of
histones and replication factors and their utilization by the exponentially increasing
DNA amount have been shown to trigger the onset of the midblastula transition
(Newport and Kirschner 1982a, b; Collart et al. 2013 ; Amodeo et al. 2015 ). In this
section, we use the scaling of spindle-associated structures as an example of mater-
nal product inheritance and adaptation to different length scales in the early cleav-
ing vertebrate embryo.
An important advance in the analysis of intracellular processes involved in early
embryonic cell division was the ability to reconstitute asters and bipolar spindles in
Xenopus extracts, first from oocytes to generate structures analogous to meiotic
spindles (Lohka and Maller 1985 ; Sawin and Mitchison 1991 ; Mitchison et al.
2013 ) and later from early embryos to generate sperm-asters and mitotic spindles
(Wühr et al. 2008 ). In both situations, remarkably normal spindles formed, but, in
spite of the absence of any cell boundaries in this in vitro system, the spindles exhib-
4 Vertebrate Embryonic Cleavage Pattern Determination