Evidence that reserve cells are a source of regenerated adult newt muscle in vitro

Abstract

Skeletal muscle of adult, metamorphosed urodele amphibians regenerates despite the absence of satellite cells¹, the myogenic reserve cells present beneath the external lamina of muscle fibres in larval urodeles and in other vertebrates^2–9. Adult-stage muscles of several urodeles examined (Notophthalmus viridescens^1,10, Ambystoma maculatum¹⁰, Triturus vulgaris, Triturus cristatus¹¹, Hynobius tokyoensis¹² and Ambystoma mexicanum¹³) have instead a unique cell type enveloped in its own external lamina. Several studies suggest that these cells are larval satellite cells that have become completely surrounded by a segment of external lamina during metamorphosis^10–12. Although Popiela¹⁰ termed these cells ‘pericytes’, he suggested that they might function as a myogenic reserve population. Cherkasova, who was the first to recognize these cells as distinct from vascular pericytes, termed them post-satellite cells and also proposed that they might be myogenic¹¹. Here, we present evidence that post-satellite cells in adult N. viridescens give rise to regenerated myotubes in vitro. The myotubes could be stained with MF20, a monoclonal antibody to purified skeletal muscle myosin¹⁴, and also with 22/18, a monoclonal antibody that reacts with an antigen present in newt-limb blastema myogenic cells¹⁵. The presence of myogenic post-satellite cells in adult newt muscle provides an alternative to the hypothesis that myoblasts in regenerating adult urodele limbs arise through dedifferentiation of mature myofibres^15–17.