Early fetal development of the intermediate tendon of the human digastricus and omohyoideus muscles: a critical difference in histogenesis.

The digastricus and omohyoideus muscles are known to carry two muscle bellies connected by an intermediate tendon. However, according to our histological observations of 22 fetuses (7-20 weeks of gestation), the mode of formation of the intermediate tendon was critically different between these two muscles. At 7-9 weeks, the posterior belly of the digastricus carried a definite intramuscular tendon continuous with a long descending tendon. The stylohyoideus, external carotid artery and hypoglossal nerve appeared to impede attachment of the tendon to the Reichert or hyoid cartilage. The digastricus anterior belly did not contain any intramuscular tendon, but desmin-positive muscle fibers consistently surrounded a bulb-like mesenchymal condensation at the caudal free end of the digastricus posterior tendon. Thus, most parts of the digastricus tendon were apparently derived from the posterior belly. In contrast, the omohyoideus always possessed a single long muscle belly until 15 weeks. The intermediate tendon first appeared at 15 weeks as a short plate-like connective structure along the medial margin of the muscle. Vimentin immunoreactivity suggested the presence of mechanical stress along the plate-like tissue, possibly due to bending of the long muscle. Muscle fibers were replaced by collagen fibers to form an intermediate tendon by 20 weeks.