Invertebrate Neuroglia-Junctional Structure and Development

Abstract

The morphological characteristics of the neuroglial cells of invertebrates are reviewed, including the ultrastructural and enzyme cytochemical features of their cell bodies and attenuated cytoplasmic processes, the various ways they ensheath the nerve cells, including the loosely myelinated condition, their modifications due to intraganglionic localization and their interactions with other glial cells in the form of homocellular junctions. The spectrum of heterocellular axo-glial associations that occur in invertebrates is considered with particular reference to the different kinds of intramembranous organization they exhibit as revealed by freeze-fracture. Recent studies on glial cell development in a range of arthropods, during embryonic and pupal stages, reveal the importance of glial cell tight junctions in forming the tracer-excluding blood-brain barrier. These occluding junctions are now shown to be, in some cases, vertebrate-like in their complexity. The stages in their assembly, which may be concurrent with those of gap junction formation, reveal a number of differences from vertebrate glia. During metamorphosis, glial cells dissociate and the dynamics of the concomitant interglial junctional disruption and their intramembranous particle dispersal without apparent internalization, as well as their subsequent reassembly, are examined. The stimuli triggering these glial events and the physiological significance of the various glial modifications are considered.