Comparative anatomy of subclavius muscle and clavicle: a histological study using human, pig and mouse foetuses

BACKGROUND: Some mammals including pigs 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 the development of the SBM and clavicle in order to find a specific factor that might explain this curious morphology. MATERIALS AND METHODS: Histological sections of pig, human and mouse early- and midterm foetuses were observed and compared at the 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 foetuses carried the mesenchymal manubrium sterni above the heart bulbus, as well as the acromion above the humeral head. However, in the pig foetuses, 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 pig 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 pig clavicle seems to lose the endochondral parts due to the caudally-shifted manubrium sterni and acromion. The medial or clavicular attachment of the pig SBM might migrate to a nearby fascia of the supraspinatus muscle in later development.