The mechanosensory bristles of flies (Fig. 1A) are composed of
a bipolar sensory neuron with a specialized microtubular body at
the tip of its dendrite. The dendrite is surrounded by different types
of support cells, one of which produces the stiff shaft that amplifies
the touch sensitivity of the neuron. The axon is accompanied by a
glial cell. The entire set of cells is generated from a precursor cell
by a fixed lineage. Precursor cells arise within an epithelial field of
cells made competent through the expression of one or more
control genes of the bHLH type (proneural genes). The choice of
the precursor cell involves a process of lateral inhibition mediated
by the Notch-Delta interaction.