NEOMYOGENESIS IN NEONATALLY DE-EFFERENTED AND POSTNATALLY DENERVATED RAT MUSCLE-SPINDLES

The ultrastructure of muscle spindles de-efferented by the extirpation of the lumbosacral spinal cord at the age of 2 days and subsequently deprived of their sensory innervation by the section of the sciatic ne rve at 3-4 weeks of age was studied in serial sections of 2-month-old rat hindlimb muscles. De-efferentation leaves the primary sensory neur ons and their peripheral axons intact and capable of inducing the musc le spindle morphogenesis during the critical period of their developme nt. In de-efferented and subsequently denervated muscle spindles, new supernumerary intrafusal muscle profiles (SIPs) appeared in the muscle spindle A region. They were formed in intimate spatial relation with the original intrafusal muscle fibres (IMFs) predominantly from activa ted satellite cells derived from both nuclear bag (larger diameter) an d nuclear chain fibres. SIPs, however, lacked the typical nuclear accu mulations, as well as other ultrastructural distinctions present in co ntrol IMFs. The majority of differentiated SIPs separated from origina l IMFs, whereas the less differentiated STPs were usually closely appo sed to the surface of the parent IMFs and both were covered by the com mon basal lamina. In some spindles, the original IMFs and/or new SIPs at different stages of their differentiation were found together and t hey formed clusters of variable shape and composition. In the majority of clusters, all profiles seemed to be isolated along their entire le ngth, although in few clusters, occasional cytoplasmic connections of variable length between intrafusal profiles were found. This result is important for the interpretation of the forthcoming study of expressi on of muscle spindle-specific myosin heavy chain isoforms in denervate d SIPs in rat muscle spindles gradually deprived of their motor and se nsory innervation.