Muscle development in the human fetus as exemplified by m. sartorius: a quantitative study.

M. sartorius was removed from 21 human fetuses ranging from 7.0 to 35.0 cm crown-rump length (CR). Various gross amd cellular changes (as seen in a transverse section) which take place in developing human skeletal muscle were quantified. The weights, lengths and cross sectional areas (at mid-length level) of m. sartorius were found to exhibit allometric relationship with CR and body weight (BW). Initially (7.5 cm CR) myotubes were more numerous and larger (40 micrometer 2 cross sectional area) than the myofibres (about 26 micrometer 2). This situation was soon reversed, however, so that at about 19 cm CR myotubes were only a very small proportion of the total muscle cell population and somewhat smaller than the myofibres in cross sectional area. At about 21 cm CR all myotube appearance was lost, whilst the total number of myofibres increased rapidly up to about 22.5 cm CR, and thereafter the rate slowed down. This stage (22.5 cm CR) seemed, in fact, to be about the time when hypertrophy of myofibres started to markedly replace hyperplasia as the main factor contributing to total muscle cross sectional area increase, although there was still a 6% contribution from hyperplasia at 35 cm CR. At 18 to 22 cm CR there were no more myofibres in the smallest size class (2-3 micrometer diameter). This may be an indication that real hyperplasia had ceased at this point so that beyond this the hyperplasia seen was only apparent and represented longitudinal growth of existing myofibres. Throughout the period studied the amount of intercellular space decreased (at a declining rate) from about 62% at 7 cm CR to about 21% at 35 cm CR. Results on counts of nuclei suggested that total muscle nuclear proliferation slowed down in later gestation. Myofibril number was not related to myotube size but increased, though at a declining rate, with myofibre size. All the muscle parameters mentioned were plotted against CR, and sometimes BW, and regression equations given wherever possible.