A two-step mechanism generates the spacing pattern of the ciliated cells in the skin of Xenopus embryos

The skin of Xenopus embryos contains a population of specialized ciliated c ells that are distributed in an evenly spaced pattern, Here we describe two successive steps that govern the differentiation and the generation of the spacing pattern of these ciliated cells. The first step occurs in the inne r or sensorial layer of the non-neural ectoderm where a subset of cells are chosen to differentiate into ciliated-cell precursors. This choice is unde r the control of lateral inhibition mediated by a Suppressor of Hairless-de pendent Notch signaling pathway, in which X-Delta-l is the putative ligand driving the selection process, and a new Enhancer-of-Split-related gene is an epidermal target of Notch signaling. Because nascent ciliated-cell precu rsors prevent neighboring cells from taking on the same fate, a scattered p attern of these precursors is generated within the deep layer of the non-ne ural ectoderm, Ciliated-cell precursors then intercalate into the outer lay er of cells in the epidermis, We show that the intercalation event acts as a second step to regulate the spacing of the mature ciliated cells. We prop ose that the differentiation of the ciliated cells is not only regulated by Notch-mediated lateral inhibition, but is also an example where differenti ation is coupled to the movement of cells from one cell layer to another.