DIFFERENTIATION OF SUPERNUMERARY FIBERS IN NEONATALLY DE-EFFERENTED RAT MUSCLE-SPINDLES

The myofibrillar ATPase (mATPase) activity and the pattern of expression of several myosin heavy chain (MHC) isoforms and of M-protein (Mr 165000) were studied in serial cross sections of neonatally deefferented 5- to 8-week-old rat hindlimb muscle spindles with supernumerary intrafusal fibres. In a sample of 5- to 6-week-old neonatally deefferented muscle spindles cut through the A region, the average number of intrafusal fibres per spindle was 8.4 in comparison to 4.2 in control spindles. Parent fibres extended throughout the whole encapsulated portion of the spindle, whereas supernumerary fibres were found only in the A region. The diameters of the supernumerary intrafusal fibres varied from less than 1 mum up to 10 mum approximately. On the basis of the mATPase activity and the pattern of expression of MHC isoforms and of M-protein, the vast majority of the supernumerary fibres could be classified as nuclear bag2, bag1 or chain fibres. However, some supernumerary fibres with small diameters exhibited features that did not fit any of the three known intrafusal fibre types. Two major processes, namely fibre splitting versus activation and fusion of satellite cells, might account for the formation of supernumerary fibres. The data presented suggest the existence of at least two types of intrafusal satellite cells. One type of satellite cell is related to the nuclear bag fibres and gives rise to myotubes which, if they have sensory innervation, can express slow tonic MHC and, therefore, differentiate into a phenotype similar to that seen in nuclear bag fibres. The other type of satellite cells form myotubes which attain a fast phenotype similar to that seen in nuclear chain fibres irrespective of the presence or absence of sensory innervation.