THE MIDLINE GLIAL-CELLS ARE REQUIRED FOR REGIONALIZATION OF COMMISSURAL AXONS IN THE EMBRYONIC CNS OF DROSOPHILA

The ventral nerve cord of arthropods is characterised by the organisat ion of major axon tracts in a ladder-like pattern. The individual neur omeres are connected by longitudinal connectives whereas the contra-la teral connections are brought about through segmental commissures. In each neuromere of the embryonic central nervous system (CNS) of Drosop hila an anterior and a posterior commissure is found. The development of these commissures requires a set of neurone-glia interactions at th e midline. Here we show that both the anterior as well as the posterio r commissures are subdivided into three axon-containing regions. Elect ron microscopy of the ventral nerve cord of mutations affecting CNS mi dline cells indicates that the midline glial cells are required for th is subdivision. In addition the midline glial cells appear required fo r a crossing of commissural growth cones perpendicular to the longitud inal tracts, since in mutants with defective midline glial cells commi ssural axons frequently cross the midline at aberrant angles.