Larval Brain

I n this chapter we will start out by describing in more detail the progenitors of the nervous system, the neuroblasts and ganglion mother cells.Subsequentlywe will survey the generic cell types that make up the developing Drosophila brain, namely neurons, glial cells and tracheal cells.Finally,we will attempt a synopsis ofthe neuronal connectivity ofthe larval brain that can be deduced from the analysis ofneural lineages and their relationship to neuropile compartments. Synopsis ofthe Phases and Elements ofDrosophila Brain Development The Drosophila brain is shaped during three developmental phases that include the embryonic, larval and pupal phase. In the earlyembryo, a population ofneuroblasts (primary neuroblasts; Fig. lA, top) delaminates from the neurectoderm and generates, in a stem cell-like manner, the glia and neurons that differentiate into the fully functional larval brain (primary neurons and glia). Each neuroblast produces a highly invariant lineage ofcells that, at least temporarily, stay together and extend processes that fasciculate into a common bundle (primary axon tract; Fig. IB). After a phase ofmitotic dormancy that lasts from late embryogenesis to the end ofthe Ist larval instar, the same neuroblasts that had proliferated to form primary neurons during the embryonic period become active again and produce a stereotyped set ofsecondary lineages (Fig. lA, center). Neurons of the secondary lineages are delayed in regard to morphological and functional differentiation. They form short, unbranched axons that fasciculate in secondary axon tracts (Fig. IC). During the pupal phase (metamorphosis) secondary neurons mature and, together with restructured primary neurons, form the adult brain (Fig. lA, bottom). The mature Drosophila brain ofthe larva and adult is ofthe ganglionic type (Fig. IB, C). Cell bodies of neurons and glial cells form an outer layer, or cortex, around an inner neuropile that consists of highly branched axons and dendrites, as well as synapses formed in between these processes. Because the neuropile is virtually free ofcell bodies, it is extraordinarily compact. The typical insect neuron has a neurite that projects throughout a large part ofthe neuropile (Fig. IB; see also section 'The Generic Cell Types ofthe Drosophila Brain' below) . Tufts ofterminal arbors (dendritic and axonal) branch off the neurite close to the cell body (proximal branches) and at its tip (terminal branches; Fig. IB). Dendritic and axonal branches are assembled into neuropile compartments. Long axons are bundled into tracts that interconnect these compartments (Fig. IB, C). Glial sheaths envelop the cortex surface (surface glia), groups ofneuronal cell bodies (cortex glia) and the neuropile (neuropile glia). Neuropile glial cells also form septa that subdivide the neuropile into several distinct compartments. *Corresponding Author: Volker Hartenstein-Department of Molecular Cell and Developmental Biology, University of California LosAngeles, LosAngeles, California 90095, USA. Email: volkerhwrncdb.ucla.edu Brain Developmentin Drosophila melanogaster, edited by Gerhard M. Technau. ©2008 Landes Bioscience and Springer Science+ Business Media. 2 Brain Development in Drosophila melanogaster