GLIAL-CELL TYPES, LINEAGES, AND RESPONSE TO INJURY IN RAT AND FISH - IMPLICATIONS FOR REGENERATION

Axons of the mammalian central nervous system do not regenerate sponta neously after axonal injury, unlike the central nervous system axons o f fish and amphibians and the peripheral nervous system of mammals, wh ich possess a good regenerative ability (Grafstein: The Retina: A Mode l for Cell Biology Studies, Part II, 1986; Kiernan: Biol Rev 54:155-19 7, 1979; Murray: J Comp Neurol 168:175-196, 1976; Ramon y Cajal: Degen eration and Regeneration of the Nervous System, 1928; Reier and Webste r: J Neurocytol 3:591-618, 1974; Sperry: Physiol Zool 23:351-361, 1948 ). It was previously believed that intrinsic differences between the c entral nervous system neurons of mammals and fish account for their di fferences in regenerative ability. The past decade, however, has seen an accumulation of evidence, indicating that mammalian central nervous system neurons are able to regenerate injured axons, at least to some extent. This was first demonstrated by Aguayo and colleagues (David a nd Aguayo: Science 214:931-933, 1981; Kierstead et al: Science 246:255 -257, 1989), who showed that injured mammalian central nervous system axons can grow for a considerable distance into an autograft of a peri pheral nerve. It was also demonstrated that injured rabbit optic axons can regenerate into their own environment (i.e., into the distal part of the injured optic nerve), if the injured nerve is treated so as to make it conducive for growth (Lavie et al: J Comp Neurol 298:293-314, 1990; Eitan et al: Science 264:1764-1768, 1994). It is generally beli eved today that the glial cells surrounding the axons play a crucial r ole in determining the regenerative capacity of any central nervous sy stem. In this review, we discuss various aspects of glial cells in the context of regeneration, with special emphasis on similarities and di fferences between the glial cells of rat and fish optic nerves, repres enting a nonregenerative and a regenerative system, respectively. (C) 1995 Wiley-Liss, Inc.