SHEARMAN, R.M.; University of Chicago: The frog pectoral girdle: a non-amniote model of bone formation

Histogenesis of vertebrate skeletal tissue is highly diverse, particularly among lower vertebrates. The process of bone formation varies at several levels of biological organization; e.g., the role of cells (osteocytes) within calcified matrix, and the timing and patterning of ossification of homologous bones. Despite this diversity, our understanding of bone development is predominantly limited to that of a few mammals and birds. The frog pectoral girdle provides a unique non-amniote model. The shoulder girdle is a composite structure built from dermal and endochondral bones, and it displays a wide range of morphological diversity within frogs. Coracoids can be fused or overlapping and clavicles vary from robust to entirely absent. The shape of the scapula ranges from short and thick to elongate and slender. A growth series of Rana pipiens was cleared and stained with Alizarin Red and Alcian Blue. The pectoral girdle develops from separate cartilaginous halves that extend medially until they fuse at the midline. The endochondral bones of the pectoral girdle begin to ossify before the dermal elements, however, lateral structures (scapula and cleithrum) begin to calcify prior to medial structures (coracoid and clavicle). The clavicle grows along the anterior portion of the procoracoid cartilage (a unique element in the frog shoulder girdle) in a lateral to medial direction. This pattern of development differs from that of the mammalian clavicle, which has two discrete centers of ossification. All of the ossified structures of the frog pectoral girdle are closely associated with a transient cartilage model. Unlike endochondral bones that assume the shape of their cartilaginous precursor, dermal bones do not develop the same morphology as the structures on which they ossify.