Expression of intermediate filaments at muscle insertions in human fetuses.

Desmin and vimentin are intermediate filaments that play crucial roles in the maturation, maintenance and recovery of muscle fibers and mesenchymal cells. The expression of these proteins has not been investigated extensively in human fetuses. In the present study, we examined the immunohistochemical expression of intermediate filaments in skeletal muscles of the head, neck and thorax in 12 mid-term human fetuses at 9-18 weeks of gestation. We also used immunohistochemistry to localize the expression of the myosin heavy chain and silver impregnation to identify the fetal endomysium. Expression of desmin and vimentin was already detectable in intercostal muscle at 9 weeks, especially at sites of muscle attachment to the perichondrium. At this stage, myosin heavy chain was expressed throughout the muscle fibers and the endomysium had already developed. Beginning with punctate expression, the positive areas became diffusely distributed in the muscle fibers. At 15-18 weeks, intermediate filament proteins were extensively expressed in all of the muscles examined. Expression at the bone-muscle interface was continuous with expression along the intramuscular tendon fibres. These results suggest that the development of intermediate filaments begins in areas of mechanical stress due to early muscle contraction. Their initially punctate distribution, as observed here, probably corresponds to the earliest stage of fetal enthesis formation.