The development of muscle spindles in human fetuses.
Investigations were performed on the gastrocnemius muscle in 27 human fetuses aged 9-32 weeks, 2 fetuses of 74 and 220 C.-R. length were examined electron microscopically. The first muscle spindles were observed at 11 and 12 weeks and were covered by a capsule composed of single layer of cells. The intrafusal fibers, being in myotube stage, contained well developed regular actin as well as myosin filaments. Among the myotube cells interdigitating myoblasts are observed. Well developed sensory nerve endings covered by a basal lamina were found in the sockets on the surface of the intrafusal fibers. During the further stages of development an increase in diameter and number of intrafusal fibers have been noted. The capsule was composed of several layers. However, the inner and outer layers of the capsule consisted of cells resembling those of the perineurium. Within the muscle spindles satellite cells were observed. The results of these investigations have shown that the muscle spindles are established early in the fetal period. Furthermore, it was found that the development of muscle proprioceptors was not finished before birth.
D005333:Fetus / Q000166:cytology*; D006801:Humans; D008465:Mechanoreceptors / Q000648:ultrastructure*; D008854:Microscopy, Electron; D009132:Muscles / Q000294:innervation* / Q000648:ultrastructure; D009210:Myofibrils / Q000648:ultrastructure