SUMMARY

Data on the myosin heavy chain (MyHC) composition of human muscle spin-dles are scarce in spite of the well-known correlation between MyHC composition and functional properties of skeletal muscle fibers. The MyHC composition of intrafusal fibers from 36 spindles of human biceps brachii muscle was studied in detail by immunocy-tochemistry with a large battery of antibodies. The MyHC content of isolated muscle spin-dles was assessed with SDS-PAGE and immunoblots. Four major MyHC isoforms (MyHCI, IIa, embryonic, and intrafusal) were detected with SDS-PAGE. Immunocytochemistry revealed very complex staining patterns for each intrafusal fiber type. The bag 1 fibers contained slow tonic MyHC along their entire fiber length and MyHCI,-cardiac, embryonic, and fetal isoforms along a variable part of their length. The bag