Presence of unmyelinated axons in the lumbar ventral roots of the 129 mouse strain

The 129 mouse strain has become of increasing interest to neurobiologists d ue to its importance in gene targeting studies. However it has been pointed out that 129 mice suffer from a number of neuroanatomical idiosyncrasies t hat may make them less attractive as animal models in neurobiology. Here we show that 129 mice also differ from other commonly used strains in possess ing large numbers of unmyelinated axons in their lumbar motor roots. By con trast in all other strains of mice (C57BL/6, C3H, Swiss-Webster) that we st udied the axons in the L5 roots are all myelinated. Additionally we show th at 129 mice have smaller myelinated axons than other mouse strains and perf orm poorly in the rotorod test. These characteristics must be kept in mind in studies of mutant mice that are frequently performed on a mixed genetic background containing a129 contribution. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.