Bristling all over

They are runners-up in the Current Biology photomicrography competition. During normal development of the nervous system in Drosophila, a process called 'lateral inhibition' ensures that only a single neural precursor is selected from a group of initially equipotent cells. Where lateral inhibition fails, excess neural cells develop, as shown in the image above, where a 'tuft' of thorn-like bristles on an adult fly indicates the development of excess terminal sensory organs. In this case, the failure of lateral inhibition resulted from misexpression of the Xenopus homologue of the fly neuralized gene. The so-called 'proneural' genes encode transcriptional activators necessary for initiating neurogenesis in Drosophila. Transgenic flies that carry strong gain-of-function forms of the proneural gene achaete have many ectopic — though morphologically normal — bristles over much of the body surface. Occasionally, however, these mutant transgenes also cause the formation of bizarre structures such as the ones in the image below, which resemble the exploded remains of bristle structures. It's not known why this happens, but it seems that the failure to negatively regulate proneural activity can adversely affect the differentiation of terminal sensory organs.