Transganglionic Degeneration in Vibrissae Innervating Primary Sensory Neurons of the Rat: a Light and Electron Microscopic Study

Previous studies have shown that transection of peripheral branches of primary sensory neurons leads to light microscopical degeneration argyrophilia and ultrastructural changes in the central termination areas of these neurons. This type of degeneration has been termed transganglionic degeneration (TGD). In the present experiments TGD has been studied specifically in neurons innervating the rat vibrissae at the light and electron microscopic levels. Light microscopically, small amounts of degeneration argyrophilia are observed in the magnocellular zone of the trigeminal subnucleus caudalis at 8-14 days survival. At longer survival times there are substantial amounts of degeneration in this area. At the ultrastructural level the first signs of TGD are observed at 6 days survival, when some terminals show a small increase in electron density, loss of synaptic vesicles, and mitochondrial disintegration. Terminals showing a more advanced increase in electron density become common at 8 days survival, but few of them are still left at 14 days survival. Neurofilamentous terminals appear in small numbers 8-14 days postoperatively. Various forms of degeneration in myelinated axons are observed from 8 days survival and are common also at 80 days survival. Electron-dense axons are rather unfrequent, but more or less disrupted myelin sheaths containing disintegrated axoplasmic remnants and empty areas are common as well as extremely expanded myelin sheaths. Glial cells containing axonal and myelin debris are seen from 8 days survival and become a more common finding at longer survivals. A most striking finding 8-10 days postoperatively is a complex relationship between glial cells and less darkened terminals, indicating phagocytosis before reaching an entirely darkened state. The findings clearly show that peripheral nerve transection leads to severe central alterations in a population of mechanoreceptor neurons innervating the vibrissae of the adult rat.