The major basement membrane components localize to the chondrocyte pericellular matrix--a cartilage basement membrane equivalent?

In this study, we demonstrate that articular cartilage chondrocytes are surrounded by the defining basement membrane proteins laminin, collagen type IV, nidogen and perlecan, and suggest that these form the functional equivalent of a basement membrane. We found by real-time PCR that mouse chondrocytes express these four cardinal components of basement membranes and demonstrated by immunohistochemistry that the proteins are present in bovine and mouse cartilage tissues and are deposited in a thin pericellular structure. Immunoelectron microscopy confirmed high laminin concentration in the pericellular matrix. In cartilage from newborn mice, basement membrane components are widespread in the territorial and interterritorial matrix, while in mature cartilage of adult mice the basement membrane components are localized mainly to a narrow pericellular zone. With progression into old age, this layer becomes less distinct, especially in areas of obvious mechanical attrition. Interestingly, individual laminin subunits were located in different zones of the cartilage, with laminin alpha1 showing preferential localization around a select population of superficial layer chondrocytes. We propose that the chondrocyte, like several other cell types of mesenchymal origin, is surrounded by the functional equivalent of a basement membrane. This structure is presumably involved in maintaining chondrocyte phenotype and viability and may well allow a new understanding of cartilage development and provide clues to the progression of degenerative joint disorders.