The developing human muscle: ultrastructural differences between myoblasts and fibroblasts.

Material from foetuses of nine weeks to nine months development was examined by electron microscopy to obtain some information about the various stages in the development of skeletal muscle and structures which are intimately related to it, such as blood vessels, nerves, neuromuscular spindles, fibroblasts and their product, i.e., collagen. Regarding the myoblasts and fibroblasts it was interesting to compare their ultrastructural features since there has been some difficulty in the identification of these two types of mononucleated cells by the authors which deal with light microscopy in the study of developing skeletal muscle. It was possible to note that between nine and eighteen weeks of development the muscle is different from mature skeletal muscle and is composed predominantly of two populations of muscle cells. These are the myoblasts which are mononucleated cells and the myotubes which are multinucleated muscle cells. After eighteen weeks of development there occur only muscle fibres similar to those of adult muscle. Fibroblasts also occur frequently in the developing muscle: they too are mononucleated cells. Fibroblasts and collagen fibrils are seen between the muscle cells and structures related to them as early as nine weeks of development. Bundles of collagen were also seen in the cytoplasm of fibroblasts.