Role of the midline glia and neurons in the formation of the axon commissures in the central nervous system of the Drosophila embryo.

A row of midline precursor cells separates the two lateral neurogenic regions that give rise to most of the Drosophila CNS. From these midline precursors arises a discrete set of special glia and neurons. The growth cones of many CNS neurons initially head straight towards the midline and change their behavior after traversing it, leading to the hypothesis that these midline cells play a key role in the formation of the axon commissures. We have used a variety of cellular and molecular genetic techniques to elucidate the cells and interactions, including specific cell migrations, that are important for the normal formation of the two major commissures in each segment. This cellular analysis has led to a model that proposes a series of sequential cell interactions controlling the three stages in commissure development: (1) formation of the posterior commissure, (2) formation of the anterior commissure, and (3) separation of the two commissures. An initial genetic test of this model has used a number of mutations that, by either eliminating or altering the differentiation of various midline cells, perturb the development of the axon commissures in a predictable manner.