Early fetal development of hard tissue pulleys for the human superior oblique and tensor veli palatini muscles

The trochlea for the superior oblique muscle as well as the hamulus for the tensor veli palatini muscle is well known as a fibrocartilage-associated, hard tissue pulley that changes the direction of the tendon. However, details of the fetal development of these structures remain obscure. We carried out a histological study of hematoxylin-eosin-stained preparations from 20 human fetuses (7-15 weeks of gestation) and clarified a common rule for the formation of these pulleys: changing in the location of a structure for the muscle insertion. At the early stage, the muscle and insertion exhibit an almost straight course alongside the primitive pulley, but because the structure for insertion later moves away from a straight line along which the muscle acts, the tendon begins to turn around the cartilage by 12 weeks. The posterior shift of the soft palate is clearly evident, but rotation of the sclera or eyeball is difficult to identify in sections. To some degree, the trochlea may originate from a common anlage with the sclera. We hypothesize that, from the evolutionary point of view, the hamulus or trochlea do not form for the pulley itself but as a structure independent of the related muscle function. The fetal topographical anatomy around the tensor veli palatini, as well as its relationship to the tensor tympani, is also described.