Metamorphosis of the central nervous system of Drosophila melanogaster Meigen (Diptera: Drosophilidae) during pupation

We studied the metamorphosis of the central nervous system (CNS) and neighbouring muscles of Drosophila melanogaster (Diptera: Drosophilidae) during pupation by transmission electron microscopy (TEM). The age of white pupa was assumed to be 0 h and the process of metamorphosis was monitored, onward between 6 and 96 h at 25°C. The profiles in the neuropil showed degeneration at 6 h and its extent increased by 12 h. The presence of glycogen in some of these profiles indicated their larval character. Between 12-18 h, the neuronal profiles became separated from one another, the intervening space was filled with extracellular fluid, and some of the larval synapses degenerated. Synaptic vesicles started reappearing around 18 h and synapses were detectable by 24 h. Neuronal processes compactly filled the neuropil by 65 h and the maturation of synapses continued until 86 h. The degeneration of profiles in the neuropil was found to be bimodal, peaking at 12 and 42 h, and that of cortical cells was unimodal with a peak at 42 h. The number of neuronal profiles increased with the development time, indicating that more branching of neuronal profiles occurs in neuropils as the metamorphosis progresses. Average number of synapses per unit area (or volume) is minimum at 18 h and maximum at 72 h, when the average number of synapse per axon profile is 0·54. Because 2 axon profiles share one synapse, a value close to 0·5 for monad synapses shows that, on an average, each axon profile at least makes one synapse at this stage of development. Subsequently, there is more than 75% of reduction in the number of synapses during 73 and 78 h.