[Growth of spinal muscles in the human fetus]

The spinal muscles, located in the paravertebral region, derive embryologically from the medial part of the somites. It has been shown in different animals that their differentiation occurs within the somite itself following the action of diffusible factors of chordal, neural and epiblastic origin. In these animal species, it thus appears that several factors determine the potential of migration as well as the muscular specification of the somitic cells. In 13 human foetuses, aged from 12 to 40 weeks of pregnancy, without any neuro-muscular disorder, transversal sections of both the vertebral and the paravertebral regions have been made at the level of the thorax and of the abdomen. Following rapid fixation, decalcification and paraffin embedding, semi-serial histological sections of 10 microns have been stained with H&E or Masson's trichrome and examined under light microscopy. Our results confirm that the primordia of the spinal muscles are present before the end of the embryonic period proper. The main modifications observed during foetal life concern the overall growth of the muscular mass, with a neat preeminence of the lombar region after 18 weeks. The differentiation of the individual muscle fibers is similar to that observed in other territories in the developing organism, with a craniocaudal gradient of maturation. Thus, if myogenic specificities really exist in the medial part of the somites in humans, it is likely that they concern the initial mechanisms involved in the activation of the myogenic program and not the mechanisms leading to the subsequent differentiation and growth of the fibres.