INNERVATION REGULATES MYOSIN HEAVY-CHAIN ISOFORM EXPRESSION IN DEVELOPING SKELETAL-MUSCLE FIBERS

The influence of innervation on primary and secondary myogenesis and i ts relation to fiber type diversity were investigated in two specific wing muscles of quail embryo, the posterior (PLD) and anterior latissi mus dorsi (ALD). In the adult, these muscles are composed almost exclu sively of pure populations of fast and slow fibers, respectively. When slow ALD and fast PLD muscles developed in ovo in an aneurogenic envi ronment induced after neural tube ablation, the cardiac ventricular my osin heavy chain (MHC) isoform was not expressed. The adult slow MHC i soform, SM2, appeared by embryonic day 7 (ED 7) in normal innervated s low ALD but was not expressed in denervated muscle. Analysis of in vit ro differentiation of myoblasts from fast PLD and slow ALD muscles iso lated from ED 7 control and neuralectomized quail embryos showed no fu ndamental differences in the pattern of MHC isoform expression. Newly differentiated fibers accumulated cardiac ventricular, embryonic fast, slow SM1 and SM3 MHC isoforms. Nevertheless, the expression of slow S M2 isoform in myotubes formed from slow ALD myoblasts only occurred wh en myoblasts were cultured in the presence of embryonic spinal cord. O ur studies demonstrate that the neural tube influences primary as well as secondary myotube differentiation in avian forelimb and facilitate s the expression of different MHC, particularly slow SM2 MHC gene expr ession in slow myoblasts.