Shaping the embryo

We use the advantages of ascidian early embryogenesis to study the control of  1) Cell number,  2) Cell size,  and 3) Cell position and how this 3 aspects are integrated to generate embryo shape. While the time-dependancy of cell division will set cell number, the spatial aspects of cell division (size and position) rely on complex cell biomechanics and intracellular mechanisms positioning the mitotic spindle. We discovered recently that the timing of cell division informs cell position via cell cycle dependant changes in cell tension and cell adhesion.

• Cell Number

We found that cell number is precisely controlled such that all ascidian embryos display a 24-cell stage.  This asynchrony is maintained (at the 44-cell stage) and amplified giving rise to a 112-cell gastrula. We have shown in ascidians that cell cycle remodeling depends upon a gene-regulatory network controlled by b-Catenin (McDougall et al, 2012; Dumollard et al., 2013).

Dumollard et al., 2013

• Cell size

We studied how unequal cell division (UCD) is controlled in the ascidian embryo.  We had previously shown that UCD in Phallusia embryos occurred due to the attraction of one spindle pole towards a cortical structure termed the CAB (for centrosome-attracting body) during prometaphase through anaphase (Prodon et al., 2010, McDougall et al., 2015).  We then found that the microtubule depolymerase Kif2 is localized to the CAB and is involved in reducing the size of the proximal aster thereby facilitating the pulling of the spindle pole towards the CAB (Costache et al., 2017).  In terms of shaping the whole embryo, CAB ablation completely radializes the embryo indicating that UCD affects the shape of all blastomeres in the early embryo.

Costache et al. 2017

Dumollard et al., 2017

• Cell position

Recently we discovered that an apical shape sensing mechanism operates to align all the spindles parallel to the outside (apical) surface of the embryo, orienting them within each cell’s longest axis in the apical plane (with the exception of those oriented by the CAB) (Dumollard et al., 2017). Furthermore relaxation of apical cortical tension during mitosis allows tension oriented cell division reflecting the importance of embryo wide tension/forces in the ascidian embryo (Godard et al 2020). Intriguingly, tension-oriented cell divisions depend upon cell cycle asynchrony which creates differential cortical tension between soft mitotic and stiff interphasic cells (Godard et al., 2020) Such conserved cell cycle asynchrony provides a partial cell-biological explanation of the invariant cleavage pattern of ascidian embryos that was elegantly described by Conklin more than a century ago (Conklin, 1905).

Cell position

16-32 cell spindle rotation