Tissue elongation is a general feature of morphogenesis. In vertebrates,embryonic axis elongation involves cell intercalation and oriented cell division, processes controlled by the planar cell polarity (PCP) pathway(which ensures that neighbouring cells adopt the correct polarity in developing tissues by promoting cell-cell communication). But what about invertebrate embryos? On , Morais da Silva and Vincent report that both processes contribute to germband elongation (GBE) in Drosophila embryos. The germband, which forms the larva's trunk, doubles in length during early embryogenesis. Elongation of the anterior germband involves cell intercalation but not oriented cell division. By contrast, using time-lapse imaging of histone-GFP-labelled embryos, the researchers show that mitoses in the posterior germband are orientated along the axis of elongation during the fast phase of GBE and that cell division inhibition reduces GBE. This orientation of cell division, like cell intercalation in the anterior of the germband,requires segmental patterning, but neither process requires the PCP pathway. The authors propose, therefore, that an alternative means of planar polarisation must mediate tissue elongation in Drosophilaembryos.
