Long Term Survival of Severed Distal Axonal Stumps in Vertebrates and Invertebrates

Abstract

SYNOPSIS. Severed distal stumps of nerve axons have now been reported to survive for months to years in both vertebrate and invertebrate nervous systems While low (>15°C) temperatures may increase survival times in some preparations such as unmyelinated garfish olfactory axons, temperature between 15 and 25°C is not the only significant factor determining the time course of survival in goldfish Mauthner axons and for many invertebrate axons For example when different axons in a crayfish are all studied at the same temperature, long term survival differs in different axons In some cases these differences appear to be due to differences in the nature of the ghal reaction or the presence of synaptic contacts. The possible cellular mechanisms for long term survival fall into three general cate gories slow degradation of axonal proteins de novo axoplasmic protein synthesis, and transfer of proteins from adjacent cells to severed axonal stumps In crayfish and squid giant axons, there is evidence that proteins are indeed transferred intact from glia to axons or from axon to axon, possibly via exocytotic/endocytotic processes However cellular mechanisms for long term survival may well differ in different axons of the same organism, much less between axons in organisms from different phyla In particular the ghal sheaths of myehnated vertebrate axons which demonstrate long term survival might be expected to impede ghal/axonal or axonal/axonal protein transfer. The study of long term survival of severed distal stumps is important for studies of axonal regeneration because axons in organisms having long survival times often show functional reconnection much more rapidly and with higher specificity than do axons in organisms lacking long survival times The study of long term survival is also important to cell biologists for an understanding of the molecular mechanisms which allow a piece of cytoplasm separated from direct cytoplasmic contact with any nucleus to remain morphologically intact and functionally competent for months to years.