Principles of Invariant Cleavage In a Compact-Type Blastula – InvBlastula

Although different classes of embryos display predictable patterns of early cleavage divisions up to the blastula stage, we still do not understand what generic rules lead to one or another embryonic shapes. Difficulty in answering this question is due to the lack of mechanistic understanding of how cell shape, cell tension, cell adhesion and cleavage pattern/timing are coordinated. Even though significant progress has been made in identifying the genes involved in driving cell fate decisions, this reductionist one gene-one function approach has led to comparatively less progress in understanding how cleavage patterns and overall embryonic shape emerges. Holoblastic embryos display either hollow (e.g echinoderm, jellyfish) or compact-type blastulae (spiralians, nematodes, ascidians…). In parallel, species-specific cleavage patterns can be either invariant (spiralians, nematodes, ascidians…) or more regulative/variable (echinoderm, jellyfish, mouse…). Strikingly, species that follow a more invariant cleavage pattern display also a compact-type blastula, while species that have a more variable cleavage pattern have a high tendency to develop a blastocoel. By studying the invariant cleavage pattern displayed by ascidian embryos we have furthermore learned that cell division orientation and maintenance of the compact-type embryonic shape depend on a small number of cell biological mechanisms that influence cellular biomechanics. Remarkably, when we perturb the invariant cleavage pattern by various ways, it systematically leads to the transformation of the compact-type ascidian embryo into a hollow-type blastula. Building on these observations and using the ascidian embryo as model system, our project aims to decipher the key cellular determinants of invariant cleavage patterning, and to characterize whether invariant cleavage patterning and compact-type blastulae formation may be causally linked. We will combine in toto imaging, precise perturbation, mechanical measurements and physical modeling to elucidate how a small number of cell biological processes affect the biomechanical properties and cleavage patterning of cells to control the overall cell position and embryonic shape (compact to hollow blastula transformation). From in toto imaging, we will segment cells, measure division planes and characterize the degree of shape and cleavage invariance in both perturbed and unperturbed conditions. Using force inference and micropipette aspiration, we will spatiotemporally map the forces shaping cells in embryos, and link them to cleavage patterns using modeling. By isolating cell singlets and doublets, we will disentangle cell-autonomous from non-autonomous biological processes (e.g. compaction, polarity,). Finally, by taking a MorphoEvoDevo approach, we will compare the hollow-type blastula of jellyfish with results from the ascidian to reveal generic design principles for embryo shape emergence. Overall, our project will form the basis for realistic 4D simulations recapitulating each embryonic trajectory in silico and where parameters can be altered to discover possible avenues for further investigation.