Localization of the Potential Zonal Marker Clusterin in Native Cartilage and in Tissue-Engineered Constructs

An increasing number of studies aim to engineer cartilage tissue that more closely mimic the zonal organization of native articular tissue. Further understanding of zonal differences is crucial for successful development and evaluation of such grafts. We therefore aimed to characterize the secretion of the potential zonal marker clusterin by zonal articular chondrocytes in osteoarthritic and healthy articular cartilage and in tissue-engineered constructs. Clusterin secreted by superficial, middle, and deep zones equine chondrocytes was immunolocalized in cytospins of alginate cultured superficial, middle, and deep zones equine chondrocytes. Clusterin was present within the superficial zone of native cartilage; after isolation of the cells from healthy articular cartilage, staining for clusterin was limited to cells derived from the superficial zone. Staining disappeared after expansion, but reappeared during (re)differentiation and was more pronounced within the cultures derived from the superficial zones of the cartilage. The presence of clusterin was associated with clusters of differentiating chondrocytes, rather than highly proliferative cells and did not specifically colocalize with proteoglycan-4. Because staining for clusterin was more abundant in cultures of superficial chondrocytes compared to those of cells of the deeper layers, it may be used to further characterize zonal cartilage constructs.