Vertebrate Embryonic Cleavage Pattern Determination

Abstract

The pattern of the earliest cell divisions in a vertebrate embryo lays the groundwork for later developmental events such as gastrulation, organogenesis, and overall body plan establishment. Understanding these early cleavage patterns and the mechanisms that create them is thus crucial for the study of vertebrate development. This chapter describes the early cleavage stages for species representing ray-finned fish, amphibians, birds, reptiles, mammals, and proto-vertebrate ascidians and summarizes current understanding of the mechanisms that govern these patterns. The nearly universal influence of cell shape on orientation and positioning of spindles and cleavage furrows and the mechanisms that mediate this influence are discussed. We discuss in particular models of aster and spindle centering and orientation in large embryonic blastomeres that rely on asymmetric internal pulling forces generated by the cleavage furrow for the previous cell cycle. Also explored are mechanisms that integrate cell division given the limited supply of cellular building blocks in the egg and several-fold changes of cell size during early development, as well as cytoskeletal specializations specific to early blastomeres including processes leading to blastomere cohesion. Finally, we discuss evolutionary conclusions beginning to emerge from the contemporary analysis of the phylogenetic 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.

Emergence of Embryo Shape During Cleavage Divisions

“…During early cleavage divisions, vegetal or yolk-rich blastomeres are created larger than animal blastomeres. Cephalopods, birds, reptiles and fish embryos illustrate the extreme case for the effect of yolk, whereby cell division occurs in a meroblastic fashion atop a mass of non-dividing yolk (Hasley et al, 2017). Hertwig tested the effect of yolk on cell division by centrifuging Xenopus eggs thus concentrating the yolk to one side of the egg.…”

Arabidopsis Class II Formins AtFH13 and AtFH14 Can Form Heterodimers but Exhibit Distinct Patterns of Cellular Localization

“…The eukaryotic cytoskeleton represents a dynamic network of protein filaments and tubules that has been extensively studied in a variety of model systems. In addition to basic cellular functions, including nuclear division, cytokinesis, organelle positioning, membrane trafficking and cell expansion, the cytoskeleton plays an important role in processes such as polar cell growth, cell division plane positioning, or cell to cell communication, which are essential for proper morphogenesis and thus the development of multicellular bodies in both metazoans [1] and plants [2]. This dynamic network relies on a large ensemble of cytoskeleton-associated proteins controlling organization, remodeling, and crosstalk of cytoskeletal systems, as well as coordination of the cytoskeleton with cell membranes and organelles.…”

Modeling Embryonic Cleavage Patterns

“…Recent experimental work in echinoderms, fish, and frog embryos has suggested that division plane specification in large eggs and blastomeres is instructed from interphase/anaphase microtubule (MT) asters which fill the large cellular volume, and probe cell shape through length-dependent MT forces [6,7]. This mechanism aligns the division axis with the cell shape long axis, and has been proposed to serve as a default cue guiding division plane orientation and position in early embryos [14,15]. Given the presence of asymmetric divisions in some blastomeres of many embryos, cell shape effect may be overridden or biased by additional cues such as vegetal polarity domains in echinoderms or by yolk layers in fish and amphibians, which alter the distribution of MT forces around cells.…”