EXTENSIVE MOTOR-NEURON SURVIVAL IN THE ABSENCE OF SECONDARY SKELETAL-MUSCLE FIBER FORMATION

Mice with a null mutation in the myogenic basic helix-loop-helix regul atory gene myogenin have severe developmental muscle defects resulting in loss of secondary muscle fibers and perinatal death, In this study , we used the myogenin mutant mouse as a model to study the effects of the loss of secondary muscle fibers and the contribution of primary m uscle fibers on the survival of motor neurons during programmed cell d eath, We demonstrate that in the absence of secondary skeletal muscle fibers there is complete survival of facial motor nucleus motor neuron s and approximately 60% survival of spinal lumbar motor neurons in the myogenin mutant mouse, The surviving spinal motor neurons maintain ax onal projections into the hindlimb and display aspects of synaptic con tact. into the remaining rudimentary fibers, These findings suggest th at primary muscle fibers, representing approximately 10% of normal mus cle mass, contribute significantly to the control of motor neuron cell survival in mammals. (C) 1996 Wiley-Liss, Inc.