Cathepsin D in cartilage: the immunohistochemical demonstration of extracellular enzyme in normal and pathological conditions.

Extracellular cathepsin D has been demonstrated immunohistochemically in cultured chicken and rabbit cartilage. Enzyme released from viable cells was captured by specific anti-cathepsin D antibody to form extracellular immunoprecipitates which were demonstrated with fluoresceinlabelled antibodies. In unfixed frozen sections these immunoprecipitates could be dissolved in antigen excess, and closely resembled those observed in immunoprecipitin lines. Extracellular cathepsin D was detected in the perichondria of normal chick cartilaginous limb-bones and, to a lesser degree, beneath them in the epiphyseal cartilage. In cultured rabbit ear cartilage, cathepsin D was detected around chondroblasts and some peripherally situated chondrocytes. In chicken and rabbit cartilage exposed to supra-physiological levels of retinol, much extracellular enzyme was detected which had been released from chondrocytes of hypertrophic and epiphyseal chick cartilage and rabbit ear cartilage.