Musculature of the limbs
The extremity buds that form out of mesenchyma cells of the somatopleura have of themselves no myogenetic potency. The myoblasts of the extremities immigrate as still division-capable premyoblasts out of the dermatomyotome of the associated segment or somite along extracellular matrix structures. The upper extremity buds (stage 12) contain an inflow of premyoblasts of the somite pairs 9 - 13 and the buds of the lower limbs (stage 13) from the somite pairs 26 - 32. The myoblasts are subject to an active cell proliferation and fuse to become a syncytium, the primary myotube. In this way two masses of premuscle arise in the forming extremities. The one lies on the stretching side (dorsal) while the other lies on the bending side (ventral) of the extremity. The newly formed extremity sections are again and again newly settled by myoblasts according to the juvenility principle. Only the distal rims of the hand and foot anlagen remain free of myoblasts. These premuscle masses soon begin to arrange themselves into layers and then into individual muscle blastemas. In the further development individual blastemas again fuse (polymerization), which is very likely due to the local influence of the forming skeleton. The definitive muscle anlagen are formed. Other muscle blastemas are destroyed (sarcolysis). The definitive extension of muscles can still change (e.g., thoracohumeral muscles: major pectoral muscle; spinohumeral muscles: latissimus dorsi muscles, spinocostal muscles: rhomboid muscles). Only at ca. 9 weeks of age have the anlagen of most of the muscle groups attained their definitive form.
The tendons to the individual muscles develop independently of them. Thus, for a time, tendons can also form there where no muscles are present. The myoblasts that have immigrated into the extremities also contain no information related to the muscle pattern, as experiments on chicken embryos have shown. Probably, an interaction takes place between the forming skeletal structures and the arising muscle blastemas.
Like the premyoblasts the innervation follows the mesenchymal guiding structures. Nevertheless, this takes place initially independent of the forming musculature. Thus it can happen that the motoneurons also innervate various muscles at a time. For the further development and the growth of the muscles, though, the innervation is very important. Via the impulses that the muscle fibers receive from the motoneurons, the muscle fibrils move into the center of the myotubes, while the nuclei move to the periphery (compare Fig. 6).