Defective association between collagen fibrils and proteoglycans in fragile bone of osteogenesis imperfecta.

The relationship between collagen fibrils and proteoglycans in the bone matrices of three osteogenesis imperfecta (OI) patients was observed ultrastructurally to clarify the mechanism responsible for pronounced bony fragility. Collagen fibrils and noncollagenous components were prepared from the bone matrix of patients with osteogenesis imperfecta and a control sample, respectively. Compared with the control, a 95.2% decrease was found in the number of proteoglycan granules periodically associated with the cross-banding of collagen fibrils in one OI sample from a patient with severe bony fragility. Extractability of collagen fibrils and proteoglycans was high in this sample. However, proteoglycans and collagen fibrils could be reattached in vitro to reproduce the condition observed in the controls. There was no decrease in the number of proteoglycan granules attached to prepared collagen fibrils in a sample from a patient with predominantly bowing deformity of bones. Hypothetically, the separation of proteoglycans from collagen fibrils in OI may be associated with increased bony fragility. In osteogenesis imperfecta patients, the mechanism inducing bowing deformity appears to be different from that inducing fragility.