DELAMINATION AND DIVISION IN THE DROSOPHILA NEURECTODERM - SPATIOTEMPORAL PATTERN, CYTOSKELETAL DYNAMICS, AND COMMON CONTROL BY NEUROGENIC AND SEGMENT POLARITY GENES

Cytoskeletal changes occurring during the delamination of precursors o f the peripheral (microchaete precursors in the pupal notum) and centr al nervous system (embryonic SI neuroblasts) were studied. The pattern of cell division in the ventral neurectoderm (VN) of wild-type embryo s was analyzed using BrdU incorporation and correlated to the pattern of neuroblast delamination. Finally, defects in the pattern of prolife ration of the VN and neuroblast delamination which occur in Notch and wingless mutant embryos were described. The results indicate that the patterns of delamination and mitosis are closely correlated: delaminat ion occurs either immediately after a cell has divided (in case of mic rochaete precursors) or shortly before the division (in case of the ne uroblasts). In addition, cytoskeletal changes similar to those occurri ng during mitosis can be seen in delaminating neuronal precursors. Thu s, during both mitosis and delamination, the discrete apicobasally ori ented microfilament-tubulin bundles break down. Microfilaments form a dense, diffuse cortical layer surrounding the entire cell body. Microt ubules are concentrated at the apically located centrosome. The relati onship between mitosis and delamination is supported by the finding th at the neurogenic gene Notch and segment polarity gene wingless (wg) a ffect both proliferation and delamination in the ventral neurectoderm. Thus, in embryos expressing the trunkated cytoplasmic domain of the n eurogenic gene Notch under heat-shock control (Struhl et al., 1993), a ll ventral neurectodermal cells go into mitosis prematurely, followed by the absence of neuroblast delamination. In wg loss-of-function muta nts, mitosis in the VN is irregular and generally postponed, accompani ed by irregularities in the timing of neuroblast delamination in gener al and the absence of a subset of neuroblasts. (C) 1994 Academic Press , Inc.