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including processes leading to blastomere cohesion. Finally, we discuss evolution-
ary conclusions beginning to emerge from the contemporary analysis of the phylo-
genetic distributions of cleavage patterns. In sum, this chapter seeks to summarize
our current understanding of vertebrate early embryonic cleavage patterns and their
control and evolution.
Keywords Blastomere • Spindle orientation • Cleavage plane determination
- Aster centering • Scaling • Cytoskeleton • Compaction • Cell cleavage type
- Evolution
4.1 Introduction
The pattern of early cell divisions in vertebrate embryos varies widely. It is important
to understand this patterning and its origin, since, in most organisms, the arrangement
of cells resulting from the early cleavages is responsible for generating the earliest
features of the embryo’s body plan. The cleavage pattern of the early embryo, and the
arrangement of cells that results, generates the early embryonic anatomy. During
activation of zygotic gene expression at the midblastula transition (MBT), new gene
expression initiates new patterns of cell behavior, generating morphogenetic move-
ments that further modify the embryo. However, early cleavage pattern constrains
subsequent developmental processes. Moreover, many cellular decisions, including
axis induction, germ layer specification, and germ cell formation, occur prior to
MBT. The early embryonic anatomy, which originates from the interaction between
the initial egg structure and dynamic processes driven by maternally inherited com-
ponents, must facilitate, or at least be compatible with, such inductive processes.
Various factors, such as embryo size, patterns of yolk deposition (in nonmam-
malian vertebrates), the symmetry of yolk deposition with respect to oocyte polar-
ity, localization of molecular cues, and cell shape, influence patterns of cell division.
The resulting cellular assembly in combination with inductive processes lays the
foundation for embryonic morphogenesis. This chapter addresses these cellular and
molecular processes, which generate this late blastula architecture, and their rela-
tion to the early pattern of the embryo. We explore some of the variety in vertebrate
embryonic cleavage patterns and discuss processes involved in their creation. We
first examine the two main classes of embryonic cleavage in vertebrates (meroblas-
tic and holoblastic cleavage). Later, we describe cellular mechanisms required for
furrow placement during early embryonic development, an essential factor in gen-
erating the early embryonic cleavage pattern. We also address additional cellular
and developmental mechanisms underlying morphological landmarks of the
embryo, such as the formation of specialized cytoskeletal structures in large embry-
onic cells and cellular compaction, as well as the regulated use of maternal building
blocks in cleaving embryonic cells. We also summarize current knowledge on
patterns and underlying molecular cues in other vertebrates, including mammals,
and in a well-studied proto-vertebrate system.
A. Hasley et al.