The repair of fractured membrane bones in the newly hatched chick.

Repair of a fractured membrane bone, the quadratojugal (QJ), has been studied in the newly hatched chick. Complete open fractures never united by bony fusion, even in birds maintained for six months post-fracture. Extraperiosteal connective tissue invaded the fracture gap and formed thick fibrous bundles which stabilised the fracture. Cartilage of two types formed on these bones. One was derived from periosteal cells and the other from osteoblasts or osteocytes. Considerably more cartilage formed in bones partially fractured than in those completely fractured. The "periosteal" cartilage did not form if the periosteum was removed at the time the bone was fractured. This was because, although the fibrous layer of the periosteum regenerated, the cambial layer did not. Metaplastic cartilage did form in the absence of the periosteum. Isolating fractured bones within polyethlene or glass tubes prevented accumulation of a blastema between the bony fragments. Cartilage did not form inside the tubes but did form where the ends of the tubes abutted onto the bones. Large defects in the bones (4 mm gaps, 4 mm of bone in the place of the QJ) healed via fibrous union with minimal osteogenesis and no chondrogenesis. Severing M. depressor mandibulae at the time the bone was fractured inhibited chondrogenesis, favoured osteogenesis and resulted in development of a pseudarthrosis. The potential for differentiation of the cells of the QJ and the role of the adjacent tissues as they related to repair of the fracture was discussed, and the ability of cells from membrane bones to become chondrogenic emphasized.