IMMUNOLOCALIZATION OF TYPE-IX COLLAGEN IN NORMAL AND SPONTANEOUSLY OSTEOARTHRITIC CANINE TIBIAL CARTILAGE AND ISOLATED CHONDRONS

Objective: The pericellular localization of type IX collagen in avian and mammalian hyaline cartilages remains controversial, while its dist ribution during osteoarthritic degeneration is poorly understood. This study aimed to compare and contrast the immunohistochemical distribut ion of type IX collagen in normal mature and spontaneously osteoarthri tic canine tibial cartilage. Design: Thick vibratome sectioning techni ques were evaluated and compared with isolated chondrons using a range of streptavidin-linked probes in combination with light, confocal and transmission electron microscopy. Results: In normal intact samples, type IX collagen was concentrated in the pericellular microenvironment , while a weaker extracellular reaction around each chondron separated the territorial matrix from the unstained interterritorial matrix. Fu rther differentiation was evident in isolated chondrons where the fibr ous pericellular capsule stained more intensely than the tail and inte rconnecting segments between columnated chondrons. Two regions of type IX reactivity were identified in osteoarthritic tissue: an intensely stained superficial reactive region below the eroding margins, and nor mal deep layer cartilage where pericellular staining persists. The sup erficial reactive region was characterized by chondron swelling and ch ondrocyte cluster formation, a loss of pericellular type IX staining, and a significant increase in matrix staining between clusters. Disint egration and loss of fibrillar collagens was evident in both the swoll en microenvironment and adjacent territorial matrices. Conclusions: Th e results suggest that changes in type IX distribution, expansion of t he pericellular microenvironment and chondrocyte proliferation represe nt key elements in the chondron remodeling and chondrocyte cluster for mation associated with osteoarthritic degeneration.