“Type grouping” in skeletal muscles after experimental reinnervation

APPLICATION OF histochemical techniques to the study of human muscle biopsies has helped to eludicate a number of pathologic reactions of skeletal muscle that are not evident in routinely stained paraffin-embedded tissue.1-4 One of these abnormalities has been termed “type grouping” and defined as “groups of fibers of the same histochemical type much larger than usual . . . consisting of 50-100 fibers or more.”Z Normally, in histochemically mixed muscles, the two basic histochemical fiber types (types 1 and 2) are intermingled in a mosaic pattern when viewed in cross section. In normal human muscle the number of fibers of similar histochemical type in tight contiguity does not exceed 50 and is usually less than 15. Type grouping is commonly observed in muscle biopsies of patients with chronic peripheral neuropathies (Fig. 1) and somewhat less often in the advanced stage of motor neuron disease. It has been postulated to be a sign of reinnervation by “collateral sprouting.”Z In this paper we present an experimental model in which typical type grouping was produced in histochemically mixed muscles of the guinea pig by experimental reinnervation. The results support the suggestion that collateral sprouting is at least one pathogenic mechanism which may be responsible for type grouping in disease in human beings.