The functioning of certain tissues and organs relies on a directed flow of fluid that is produced by epithelial cells that bear motile cilia. Stubbs et al. have been investigating how ciliated cells insert into one such tissue -the skin of Xenopus embryos - in an evenly spaced pattern (see ). Ciliated cell precursors (CCPs) are produced in the inner ectodermal layer before radially intercalating into the outer ectodermal layer. Notch signalling determines the number of CCPs, but when it is inhibited, the epidermal pattern of ciliated cells is mostly unchanged even though more CCPs are produced. The researchers use confocal microscopy to show that CCPs can `wedge' anywhere between the outer ectodermal cells at their basal surfaces, but only insert apically and singly where at least three outer layer cells make contact. Thus,they suggest, the normal pattern of ciliated cells is maintained when CCPs are overproduced because access sites in the outer ectodermal layer are limited and only one CCP can fit into each site.
