Bone matrix proteins in affected proteoglycan-induced spondylitis (PGISp) mouse vertebrae. In an unaffected sacral joint, collagen type I (A) (×40) and osteocalcin (B) (×40) immunohistochemical staining clearly delineate the bone compartment, only staining the vertebral bone. Collagen type I staining is evident at the fringes of the excess matrix in an affected joint however (D) (×40), (boxed area in E) (×200). This collagen matrix is only osteocalcin-positive at the outermost periphery (F) (×40), (boxed area in G) (×200,). Rabbit immunoglobulin (IgG)-negative control shows no background staining (C) (×40).