SHEARMAN, R. M.; University of Chicago: Endochondral bone growth of the pectoral girdle in Rana pipiens

Our knowledge of endochondral bone development is largely derived from histological analyses of long bones in mammals and birds. This study explores bone growth in the anuran pectoral girdle, a structure built from dermal, endochondral, and cartilaginous elements. To determine the pattern of chondrogenesis in the shoulder girdle, a developmental series of Rana pipiens was stained with Alcian Blue and antibodies to Type II Collagen. Specimens were also sectioned and stained with Hematoxylin and Eosin. Cartilage formation in the shoulder girdle, as visualized by Alcian Blue staining of proteoglycans, occurs first in the scapula followed by the coracoid. Collagen II is not present in the scapula and appears in the coracoid after both elements are visible with Alcian Blue. Differences in staining for Collagen II and proteoglycans may reflect a complex process of extra cellular matrix (ECM) production and modification during girdle development. Despite variation in ECM composition, cartilage condensations of the scapula and coracoid are indistinguishable in histological sections. In fact, thin sections of the girdle indicate that the process of endochondral bone formation, from cartilage condensation to the differentiation of resting, proliferating, and hypertrophic chondrocytes, is similar to that of the limb. However, at later stages of shoulder development, the cellular organization of the scapula and coracoid differs from that of a long bone. Rather than forming caps of articular cartilage at the epiphyses, endochondral bones of the shoulder girdle merge seamlessly into adjacent skeletal elements and develop complex joint surfaces with other bones. The unique epiphysial designs in the scapula and coracoid contribute to the complex ontogenesis and distinctive morphology of the anuran pectoral girdle.