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

The myofibrillar ATPase (mATPase) activity and the pattern of expressi on of several myosin heavy chain (MHC) isoforms and of M-protein (Mr 1 65000) were studied in serial cross sections of neonatally deefferente d 5- to 8-week-old rat hindlimb muscle spindles with supernumerary int rafusal fibres. In a sample of 5- to 6-week-old neonatally deefferente d muscle spindles cut through the A region, the average number of intr afusal fibres per spindle was 8.4 in comparison to 4.2 in control spin dles. 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 f rom less than 1 mum up to 10 mum approximately. On the basis of the mA TPase activity and the pattern of expression of MHC isoforms and of M- protein, the vast majority of the supernumerary fibres could be classi fied as nuclear bag2, bag1 or chain fibres. However, some supernumerar y 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, migh t account for the formation of supernumerary fibres. The data presente d 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, c an express slow tonic MHC and, therefore, differentiate into a phenoty pe similar to that seen in nuclear bag fibres. The other type of satel lite 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.