Ectopic skeletal muscles derived from myoblasts implanted under the skin

We investigated the potential of cultured myoblasts to generate skeletal mu scle in an ectopic site. Myoblasts from a clonal cell line or from expanded primary cultures were injected under the skin of the lumbar region of adul t syngenic Balb/c mice. One to 7 weeks after injection, distinct muscles, o f greater mass in mice injected with clonal myoblasts (6-78 mg, n = 37) tha n in mice injected with primary myoblasts (1-7 mg, n = 26), had formed betw een the subcutaneous panniculus carnosus muscle and the trunk muscles of ho st animals, These ectopic muscles exhibited spontaneous and/or electrically -evoked contractions after the second week and, when stimulated directly in vitro, isometric contractile properties similar to those of normal muscles . Histological, electron microscopical and tissue culture examination of th ese muscles revealed their largely mature morphology and phenotype. The fib res, most of which were branched, were contiguous, aligned and capillarised , exhibited normal sarcormeric protein banding patterns, and expressed musc le-specific proteins, including desmin, dystrophin, and isoforms of develop mental and adult myosin heavy chain, Enveloping each fibre was a basal lami na, beneath which lay quiescent satellite cells, which could be stimulated to produce new muscle in culture. Presence of endplates (revealed by cc-bun garotoxin and neurofilament staining), and the eventual loss of expression of neural cell adhesion molecule and extrasynaptic acetylcholine receptors, indicated that some fibres were innervated, That these muscle fibres were of implanted-cell origin was supported by the finding of Y-chromosome and a lack of dystrophin in ectopic muscles formed after subcutaneous injection of, respectively, male myoblasts into female mice and dystrophin-deficient (mdx) myoblasts into normal C57B1/10 muscle, Our results demonstrate that a n organised, functional muscle can be generated de novo from a disorganised mass of myoblasts implanted in an extramuscular subcutaneous site, whereby the host contributes significantly in providing support tissues and innerv ation, Our observations are also consistent with the idea that myogenic cel ls behave like tissue-specific stem cells, generating new muscle precursor (satellite) cells as well as mature muscle. Subcutaneous implantation of my oblasts may have a range of useful applications, from the study of myogenes is to the delivery of gene products.