NEUROTROPHIN-3 AND TRKC IN MUSCLE ARE NONESSENTIAL FOR THE DEVELOPMENT OF MOUSE MUSCLE-SPINDLES

NEUROTROPHIN-3 (NT3) or TrkC null mutant mice were examined for the pr esence of muscle spindles. Muscles of mastication, but not limbs, cont ained spindles in newborn and adolescent mutants. The intramuscular di stribution and morphological properties of spindles in mutant masticat ory muscles were indistinguishable from those of wild-type spindles. I ntrafusal fibers of NT3- or trkC-deficient spindles expressed the slow -tonic isoform of myosin heavy chains, characteristic of wild-type spi ndles. Sensory nerve endings were observed in spindles of mutants by e lectron microscopy. Thus, NT3 or trkC, which is expressed in wild-type spindles, may serve functions other than those related to spindle ass embly. Presumably, proprioceptive neurons innervating jaw muscles are dependent on factors other than NT3 for survival and maintenance. (C) 1998 Rapid Science Ltd.