Traction and the formation of mesenchymal condensations in vivo.

Although the segregation of mesenchyme into distinct aggregates is the first step in the development of a range of tissues that includes bones, somites, feathers and nephrons, we still know very little about the mechanisms by which this happens. There are two obvious types of explanation: first, that there are global pre-patterns within the mesenchyme whose molecular expression leads to tissue fragmentation and, second, that the condensations arise spontaneously through the local morphogenetic abilities of the cells. The only known mechanism for the latter possibility is cell traction and this paper suggests that current studies are compatible with traction playing a primary role in the formation of nephrogenic condensations in the developing kidney and the separation of somites, but not for the generation of feather rudiments where there is evidence of a prepattern of adhesivity.