Rat muscle spindles contain one nuclear bag2, one nuclear bag1, and two nuclear chain fibers. The three different types of intrafusal fiber in spindles may be a reflection of concomitant changes in proportions of slow primary, slow/fast secondary, and fast secondary myotubes during the period of spindle development. We examined whether experimentally altering the available muscle substrates would impact the intrafusal fiber type composition of spindles. De novo formation of spindles in muscles devoid of primary myotubes was induced by crushing the nerve to the medial gastrocnemius muscle in newborn rats and administering nerve growth factor for ten days afterwards. Encapsulated fibers of the reinnervated muscles examined one month after nerve crush had myofibrillar adenosine triphosphatase and myosin heavy chain profiles similar to normal bag2, bag1, or chain intrafusal fibers. However, spindles in reinnervated muscles contained fewer fibers than controls. Most experimental spindles contained chain and/or bag1 fibers, the two fiber types which ordinarily arise during secondary myogenesis. In contrast, bag2 fibers, fibers that normally form concomitant with primary myogenesis, were absent from nearly 90% of spindles in reinnervated muscles. The paucity of bag2 fibers may reflect the absence of primary myotubes, whereas the prevalence of chain and/or bag1 fibers may reflect that secondary myotubes or myofibers that descended from the secondary myotubes were the principal muscle substrates available for spindle formation in the nerve-crushed muscles.(ABSTRACT TRUNCATED AT 250 WORDS)