EXPRESSION OF MYOSIN HEAVY-CHAIN ISOFORMS IN DEVELOPING HUMAN MUSCLE-SPINDLES

We studied serial sections of human fetal limb muscles (10-25 weeks of gestation) by light microscopic (LM) immunocytochemistry, using speci fic antibodies against slow-tonic, slow-twitch, fetal, embryonic, and alpha-cardiac myosin heavy chain (MHC) isoforms, neurofilament protein , laminin, and myomesin. One set of the first-generation myotubes expr essed slow-tonic MHCs, and slow-twitch, fetal, and embryonic MHCs from the tenth week of gestation. These primary myotubes were identified a s developing nuclear bag fibers. Second-generation myotubes in close a pposition to the primary nuclear bag myotubes initially expressed only fetal and embryonic MHCs. One or more of these secondary myotubes acq uired expression of slow-tonic and slow-twitch MHCs and gave rise to n uclear bag fibers. Most of the nuclear bag precursors expressed alpha- cardiac MHC. The secondary myotubes that expressed fetal and embryonic MHC but not slow-tonic, slow-twitch, or alpha-cardiac MHCs gave rise to the nuclear chain fibers. This study shows that different populatio ns of fiber precursors, each with a unique sequence of MHC expression, gave rise to the nuclear bag and chain fibers, despite the presence o f a common afferent nerve, from the early stages.