The invariant cleavage pattern displayed by ascidian embryos depends on spindle positioning along the cell's longest axis in the apical plane and relies on asynchronous cell divisions.

The ascidian embryo is an ideal system to investigate how cell position is determined during embryogenesis. Using 3D timelapse imaging and computational methods we analyzed the planar cell divisions in ascidian early embryos and found that spindles in every cell tend to align at metaphase in the long length of the apical surface except in cells undergoing unequal cleavage. Furthermore, the invariant and conserved cleavage pattern of ascidian embryos was found to consist in alternate planar cell divisions between ectoderm and endomesoderm. In order to test the importance of alternate cell divisions we manipulated zygotic transcription induced by ß-catenin or downregulated wee1 activity, both of which abolish this cell cycle asynchrony. Crucially, abolishing cell cycle asynchrony consistently disrupted the spindle orienting mechanism underpinning the invariant cleavage pattern. Our results demonstrate how an evolutionary conserved cell cycle asynchrony maintains the invariant cleavage pattern driving morphogenesis of the ascidian blastula.

Centrosomes and spindles in ascidian embryos and eggs.

During embryonic development and maternal meiotic maturation, positioning of the mitotic/meiotic spindle is subject to control mechanisms that meet the needs of the particular cell type. Here we review the methods, molecular tools, and the ascidian model we use to study three different ways in which centrosomes or spindles are positioned in three different cellular contexts. First, we review unequal cleavage in the ascidian germ lineage. In the germ cell precursors, a large macromolecular structure termed the centrosome-attracting body causes three successive rounds of unequal cleavage from the 8- to the 64-cell stage. Next, we discuss spindle positioning underlying the invariant cleavage pattern. Ascidian embryos display an invariant cleavage pattern whereby the mitotic spindle aligns in a predetermined orientation in every blastomere up to the gastrula stage (composed of 112 cells). Finally, we review methods and approaches to study meiotic spindle positioning in eggs.