Development and regeneration of muscle spindles in mammals and birds.

The morphological and physiological development of avian and mammalian muscle spindles is reviewed, with emphasis on the recent literature. Subjects covered include the effect of sensory innervation and growth factors on the induction of muscle spindles, genesis of intrafusal fiber types as defined by isoforms of myosin heavy chains, and the establishment of the outer spindle capsule. Because of its relevance to normal development, degeneration and regeneration are also treated. Similarities and differences between mammalian and avian muscle spindle formation are discussed.