Comparative anatomy of the subclavius muscle and clavicle: a histological study using human, swine and mouse fetuses

Background: Some mammals including the swine carry a fibrous vestigial clavicle, but a subclavius muscle (SBM) extends between the first rib and the supraspinatus muscle surface fascia. We aimed to examine development of the SBM and clavicle for finding a specific factor to provide the curious morphology.

Materials and methods: Histological sections of early- and midterm fetuses of the swine, human and mouse were observed and compared at the almost same morphological stage.

Results: In all three species, the initial SBM was seen extending between the cartilaginous first rib and a mesenchymal clavicle. At the early stage, the human and mouse fetuses carried the mesenchymal manubrium sterni above the heart bulbus as well as the acromion above the humeral head. However, in the swine fetus, the manubrium remained far caudal to the first rib, while the acromion was in the laterocaudal side of the glenohumeral joint. In place of the acromion, the swine supraspinatus muscle was large and covered the humeral head. At midterm, the human and mouse SBM attached to the membranous bone of the clavicle. Endochondral ossification occurred at the lateral and medial ends of the human clavicle, while it was seen in the medial half of the mouse clavicle anlage with a homogenous eosinophilic matrix.

Conclusions: The swine clavicle seemed to lose the endochondral parts due to the caudally-shifted manubrium sterni and acromion. The medial or clavicular attachment of the swine SBM might migrate to a nearby fascia of the supraspinatus muscle in the later development.