Sequential changes in cellular properties accompanying amniote somite formation

Somites are transient structures derived from the pre‐somitic mesoderm (PSM), involving mesenchyme‐to‐epithelial transition (MET) where the cells change their shape and polarize. Using Scanning electron microscopy (SEM), immunocytochemistry and confocal microscopy, we study the progression of these events along the tail‐to‐head axis of the embryo, which mirrors the progression of somitogenesis (younger cells located more caudally). SEM revealed that PSM epithelialization is a gradual process, which begins much earlier than previously thought, starting with the dorsalmost cells, then the medial ones, and then, simultaneously, the ventral and lateral cells, before a somite fully separates from the PSM. The core (internal) cells of the PSM and somites never epithelialize, which suggests that the core cells could be ‘trapped’ within the somitocoele after cells at the surfaces of the PSM undergo MET. Three‐dimensional imaging of the distribution of the cell polarity markers PKCζ, PAR3, ZO1, the Golgi marker GM130 and the apical marker N‐cadherin reveal that the pattern of polarization is distinctive for each marker and for each surface of the PSM, but the order of these events is not the same as the progression of cell elongation. These observations challenge some assumptions underlying existing models of somite formation. The process of epithelialization that culminates in the formation of somites begins very early in the pre‐somitic mesoderm, in defined order, starting dorsally, then medial and ventral, then lateral and finally the somite buds off – this challenges assumptions made by most models of somitogenesis.