Evolution of the capacity to form appendages (secondary outgrowths from the principal embryonic axes) potentiated the diversification of animal body plans. The basic mechanisms that underlie appendage growth in bilaterian model systems have been identified but little is known about appendage development in pre-bilaterians. Now, on , Matthew Gibson and colleagues report that three processes contribute to tentacle development in the sea anemone Nematostella vectensis. The initial step in tentacle development in this early-branching metazoan, they report, is the formation of a thickened ectodermal placode at the oral pole that progressively divides into four distinct tentacle domains, probably through the restricted expression of key effector genes. Next, Notch signalling triggers apicobasal thinning of the tentacular ectoderm, which drives the elongation of these primordia. Concomitantly, oriented cell rearrangements help to shape the elongating tentacles. By defining the mechanism of embryonic appendage development in an early-branching metazoan, these findings provide a foundation for understanding the evolutionary diversification of animal body plans.
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