Insect neurometamorphosis: V. Fine structure of axons and neuroglia in the transforming interganglionic connectives of Galleria mellonella (L.) (Lepidoptera)

Abstract

Connectives, some of which shorten 1.4 mm, show a striking reorganization of axons and glia. Axons lose structural uniformity as they come to resemble those which degenerate after being cut from their somata. Some axons contain many close-packed, spindle-shaped vesicles, seemingly originating from neurotubules. Others lack these organelles, and their cytoplasm appears rarefied, or filled with dense bodies, myeloid bodies, autophagic vacuoles, and altered mitochondria. Some axons seem to regress entirely, for their limiting membranes become discontinuous. Larval axons are isolated by glial sheaths, a configuration soon disrupted as extracellular spaces enlarge. During glial metamorphosis microtubules diminish, and rarefaction gives the cytoplasm a polymorphic appearance. Autophagic vacuoles, interrupted cytomembranes, altered mitochondria, and vacuoles also suggest extensive glial degeneration. This may contribute to the adult condition, where numerous unsheathed, contiguous axons occur in irregular bundles delineated by scanty glial cytoplasm. Abundance and size distribution of axons at different stages of development were determined.