Ultrastructural elements of CNS development in the stage 15 Drosophila (Diptera: Drosophilidae) embryo

The ultrastructural neuroanatomy of the wild-type Drosophila melanogaster (Diptera: Drosophilidae) embryo at Stage 15 was examined with the transmission electron microscope. This particular embryonic stage is an approximate midpoint of neurogenesis. No blood-brain barrier has yet formed in the CNS as illustrated by lanthanum tracer infiltration into the neuropil. First structural signs of axe-axonal synapses, in the embryonic neuropil are seen as electron-dense plaques on the cytoplasmic sides of apposing pre- and postsynaptic membranes. Very few clear synaptic vesicles (30-40 nm in diameter) are present, and none of these is clustering, although some are docking on the presynaptic membrane. In the perikaryal rind of the ventral ganglion, several glial somata are shown surrounding a single neuronal soma. Finger-like processes of the glial somata extend into extracellular spaces and contact the surface of the neuron. The functional significance of these findings is discussed. (C) 1997 Elsevier Science Ltd.