Abstract

Abstract: Embryos of many echinoderm species develop freely in sea water with a fertilisation envelope as the only protection from environmental insults. In the sea star Patiria miniata the fertilisation envelope not only protects, but also helps shape the early embryo, as the embryonic cells form a blastula by lining the inner surface of the envelope. This raises the possibility that the spatial constraints imposed by the envelope may influence epithelial morphogenesis, e.g. compaction, cell density and cell connectivity.

We used live-imaging of the sea star embryo coupled with deep learning-based segmentation, to dissect the relative contributions of cell density, tissue compaction, and cell proliferation on epithelial architecture. We found that the 3D connectivity of cells within the tissue changes over time as epithelial compaction and cell density increase. Importantly, these changes in 3D packing are due to the combined effects of cell division within the embryo and of the spatial constraints acting on the embryos.

These results raise interesting hypotheses concerning the role of protective structures, such as the fertilisation envelope, in key aspects of embryonic development and its evolution.

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