Fine-structural changes in a lepidopteran nervous system during metamorphosis.

The fine structure of the metamorphosing abdominal nerve cord of Manduca sexta has been studied. In fifth instar larvae, the connectives are ensheathed by a complex, thickened neural lamella. The underlying perineurium at this stage consists of 2 layers. The outer layer consists of interdigitating type I cells which are attached to the overlying neural lamella by hemidesmosomes, and to each other by occasional gap and tight junctions which persist throughout development. They are attached by desmosomes to a thin underlying type II cell layer, which is joined by gap and tight junctions and which has desmosomal attachments with the underlying glial membranes. The larval axons are surrounded by multiple glial wrappings containing bundles of microtubules. During the first week after larval-pupal ecdysis, the neural lamella degenerates and is phagocytosed by invading haemocytes. The underlying perineurial I cells gradually become hypertrophied and vacuolated. At the same time the type II layer, which does not increase in size, appears to be composed of either one or two cells which form a continuous 'bracelet' around each connective. The cellular bracelet is joined at one or two places by extensive gap, tight and septate junctions, and gap junctions are also seen along its perineurial I and glial borders. The underlying axons are embedded in vast amounts of glial cytoplasm containing relatively few microtubules. During the second week after larval-pupal ecdysis, the neural lamella is reformed and the perineurium flattens again. Type I and II cell junctions remain as described in earlier stages. Before adult emergence, the axons are again wrapped by glial cells rich in microtubules.