We studied the metamorphosis of the central nervous system (CNS) and neighbouring muscles ofDrosophila 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.
In muscles, vacuoles suggesting histolysis appeared by 6 h. Their ultrastructure became deranged between 12–18 h and myoblasts were found to be present since 8 h. Except for a few muscles in the thorax, such as larval oblique muscles and pharyngeal muscles, most of the muscles in the head and thorax lost all the ultrastructural details and histolyzed by 18 h. Around 38 h, imaginai muscles were detectable, and well-developed muscles were found by 55 h. However, myofibrils continued to be added laterally to the preformed muscles even at 96 h.
Electron-dense mitochondria (EDMITs) were found in the neuropil, cortex and muscles of pupa, along with mitochondria of characteristic shape and normal appearance. These EDMITs often occurred in large clusters of more than 100, at times near the surface of the tissue. A few of these were enclosed in vacuoles and were darker than the rest of the EDMITs and normal looking mitochondria. Histochemistry with diaminobenzidine showed the presence of cytochrome c and marker enzyme cytochrome oxidase, both in EDMITs and normal mitochondria. EDMITs were not found to be present in any tissue of the adultDrosophila.