Epithelialization at Forming Somite Boundaries*

Mesenchymal to epithelial transition is an emergent mechanical process which results in mesenchymal cells attaining structure and polarity along with the formation of basal and apical protein domains. We study this transition in the context of somite formation which is an important part in the development of vertebrate embryos. Randomly distributed mesenchymal cells in the continuous presomitic mesoderm organize into strongly coupled epithelial cells forming a monolayer in a somite. Data from chicken embryos show epithelialization along the regions close to the ectoderm and endoderm first, followed by closing of the somite anteriorly and posteriorly. Here we explore what cell mechanics guide the emergent process of epithelialization at these boundaries. We model the transition from undifferentiated mesenchyme to elongated epithelia using filopodial processes and show that propagation of epithelial order and accretion \footnote{ H.Y.Kim,et al, \emph{On the role of mechanics in driving mesenchymal-to-epithelial transitions}, Semin Cell Dev Biol, (2016)} is sufficient to explain the observed monolayer. We further show how subsequent apical constriction of the formed layer explains the rounding of somite boundaries.