After fusion of the two primary ossification centres, secondary cartilage develops directly from mesenchymal tissue at the sternal and acromial ends.
Based on their histological organization, each secondary cartilage end displays the configuration of a physis with its five areas of histological differentiation: reserve zone I, proliferation II, hypertrophy III, calcification IV, and vascular invasion V .
Overall, there were distinct histological differences between the acromial and sternal secondary cartilages regarding cellularity, secondary ossification centre, and kind of cartilage.
Dominguez, Comparative Osteogenesis of the clavicle secondary cartilages and epiphyseal cartilages of long bones.
The role of movement and tissue interactions in the development and growth of bone and secondary cartilage in the clavicle of the embryonic chick.
Growth of the clavicle and development of clavicular secondary cartilage in the embryonic mouse.
SUMMARY: The clavicle arises by intramembranous ossification, but it develops secondary cartilages as growth centers.
The secondary cartilages of the clavicle present important morphologic differences with the primary cartilages of long bones, but similar to that observed in the condylar cartilage of the mandible.