Phased ERK-responsiveness and developmental robustness regulate teleost skin morphogenesis.

Elongation of the vertebrate embryonic axis necessitates rapid expansion of the epidermis to accommodate the growth of underlying tissues. Here, we generated a toolkit to visualize and quantify signaling in entire cell populations of periderm, the outermost layer of the epidermis, in live developing zebrafish. We find that oriented cell divisions facilitate growth of the early periderm during axial elongation rather than cell addition from the basal layer. Activity levels of ERK, a downstream effector of MAPK pathway, gauged by a live biosensor, predicts cell cycle entry, and optogenetic ERK activation controls proliferation dynamics. As development proceeds, rates of peridermal cell proliferation decrease, ERK activity becomes more pulsatile and functionally transitions to promote hypertrophic cell growth. Targeted genetic blockade of cell division generates animals with oversized periderm cells, yet, unexpectedly, development to adulthood is not impaired. Our findings reveal stage-dependent differential responsiveness to ERK signaling and marked developmental robustness in growing teleost skin.