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

SYNOPSIS. Severed distal stumps of nerve axons have now beenreported to survive for months to years in both vertebrate andinvertebrate nervous systems While low (>15°C) temperaturesmay increase survival times in some preparations such as unmyelinatedgarfish olfactory axons, temperature between 15 and 25°Cis not the only significant factor determining the time courseof survival in goldfish Mauthner axons and for many invertebrateaxons For example when different axons in a crayfish are allstudied at the same temperature, long term survival differsin different axons In some cases these differences appear tobe due to differences in the nature of the ghal reaction orthe presence of synaptic contacts. The possible cellular mechanisms for long term survival fallinto three general cate gories slow degradation of axonal proteinsde novo axoplasmic protein synthesis, and transfer of proteinsfrom adjacent cells to severed axonal stumps In crayfish andsquid giant axons, there is evidence that proteins are indeedtransferred intact from glia to axons or from axon to axon,possibly via exocytotic/endocytotic processes However cellularmechanisms for long term survival may well differ in differentaxons of the same organism, much less between axons in organismsfrom different phyla In particular the ghal sheaths of myehnatedvertebrate axons which demonstrate long term survival mightbe expected to impede ghal/axonal or axonal/axonal protein transfer. The study of long term survival of severed distal stumps isimportant for studies of axonal regeneration because axons inorganisms having long survival times often show functional reconnectionmuch more rapidly and with higher specificity than do axonsin organisms lacking long survival times The study of long termsurvival is also important to cell biologists for an understandingof the molecular mechanisms which allow a piece of cytoplasmseparated from direct cytoplasmic contact with any nucleus toremain morphologically intact and functionally competent formonths to years.