Accelerated endochondral osteoinduction in the absence of bone matrix particles in a rat model system

Ethanol-precipitated proteins obtained from demineralized rat bone powder (DBP) by 4M guanidine-HCl extraction have been shown to reproducibly induce ectopic endochondral bone formation when subcutaneously implanted in rats in the absence of bone matrix particles. Histologic and biochemical analysis revealed a temporal sequence of chondrocyte differentiation, calcified cartilage formation, neovascularization, osteoblast differentiation, bone formation, osteoclastic bone remodeling, and hematopoietic marrow development that is complete by 21 days. In contrast to previous reports, these results clearly show an osteoinductive response independent of the presence of insoluble extracellular bone matrix. Compared with conventional DBP implants, the guanidine-extractable protein (GE) produces an accelerated and more robust osteoinductive response. Histologically, the initial chondrogenic response at days 6 to 9 is greatly amplified. Alkaline phosphatase specific activity peaks at day 9, several days earlier than for DBP, and is sixfold higher. Calcium accumulation in GE implants at day 12 is fivefold greater than with DBP, and all mineral is localized within the matrix of newly calcified cartilage and new bone. Osteoclasts are up to ninefold more abundant in the rapidly remodeling GE ossicle, making space for hematopoietic marrow. Delivery of GE coprecipitated with inert bone matrix particles was also more effective than DBP, although the response was somewhat attenuated compared with GE alone. Bony filling of 4-mm defects in rat mandibular rami was elicited by 10 mg of GE and followed an endochondral process with increased neovascularization compared with DBP and unimplanted controls. This guanidine-extractable protein fraction should prove useful for inducing quantities of chondrocytes and osteoclasts for in vitro study, and for analysis of osteoinductive requirements.(ABSTRACT TRUNCATED AT 250 WORDS)