Histological appearance of the graft in AOC 14 + sheep. (A) Solid union between the allograft and the host bone depicted by well-organized and well-connected thick bone trabeculae. (B) Inner portion of the allograft filled mostly by mature bone trabeculae with sites of active new bone deposition represented by the presence of osteoid seams stained in red and covering portions of the trabecular surface. (C) Osteogenesis in the connective tissue that fills the allograft. Note the similarity of the loose connective tissue associated with the newly formed bone trabeculae and the supporting stroma of normal bone marrow. This aspect is histologically distinct from the clearly non-osteogenic fibro-vascular connective tissue that surrounds the site of osteogenesis. (D) Restoration of the bone marrow microenvironment with the colonization of the supporting marrow stroma by hematopoietic cells. Masson-Goldner trichrome stain; 5- μm -thick undecalcified sections. Magnification (A-C) 4× and (D) 20×. Histological appearance of the graft in AOC 14 - sheep. (A) Despite the osseous union achieved between the host bone and the allograft, the inner portion of the graft is devoid of osseous filling. (B) Inner portion of the cortical graft occupied by connective tissue with little or no osteogenic potential. (C) Most of the osteoforming response in this graft was represented by woven bone in an initial state of formation. (D) Remnants of cancellous graft are being actively resorbed by multinucleated osteoclasts without signs of restoration of the bone marrow microenvironment. Masson-Goldner trichrome stain; 5- μm -thick undecalcified sections. Magnification (A-C) 4× and (D) 10×.