‘Sprawling’: A New Mutant Mouse With Failure of Myelination of Sensory Axons and a Deficiency of Muscle Spindles

A new neurological mutant mouse ‘Sprawling’ was studied and this paper presents some of its clinical and pathological features. The disorder is transmitted by a dominant gene (symbol Swl) and when fully developed is characterized by an ataxia and defective position sense chiefly affecting the hind limbs. The main pathological abnormalities were found in both central and peripheral branches of the axons of sensory ganglion cells. The sensory roots, dorsal columns of the spinal cord and the peripheral nerves showed a deficiency of myelinated axons. This abnormality was more severe caudally than rostrally. No evidence for axonal degeneration or regeneration was present yet sensory ganglion cells showed marked chromatolysis which persisted throughout the life of the animal. There was a severe deficiency of sensory receptors and spindles were virtually absent from the hind limb and paw muscles. It seems likely that this inherited disorder of the nervous system is due to a failure in the maturation and myelination of sensory axons with a secondary failure in the development of muscle spindles and other receptors which are dependent on their sensory innervation for normal differentiation.