Histology of the bone-tendon interfaces of limb muscles in lizards

Lizards exhibit continual bone growth at the epiphysis, and their limb muscles are distributed differently from those of mammals because of differences in weight bearing. We therefore characterized the bone-tendon (B-T) interface (also termed the enthesis) in lizards. Using the forelimbs of five monitor lizards and three iguanas, we performed histological investigations on 57 B-T interfaces. Most reptilian tendons were very short and were often composed of wavy fiber bundles. Fibrocartilage (FC)-mediated direct insertion was observed at all epiphyses, whereas periosteum-mediated indirect insertions, including fleshy attachments, were often located on the flat surfaces of the pectoral girdles and at the diaphyses of the limb bones. The reptilian B-T interface was characterized by variability in the morphology of the FC-mediated insertions, especially by morphologies intermediate between those of FC- and periosteum-mediated interfaces; i.e., 1) various degrees of absence of the clear FC zonation seen in mammals, including the tidemark; 2) involvement of the periosteum in the FC; 3) the presence of various types of FC cells in the tendon near the interface, to reinforce the tendon against compression or shear stress; and 4) both FC and hyaline cartilage (lateral articular cartilage) receiving the tendon at the epiphysis. Overall, variations in the connective tissue, especially the FC tissue, were very evident in the reptilian B-T interface. The specific structures of the interfaces probably represent adaptations to the continuous growth and loose joint structures of lizards.