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 fluorescein-labelled 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.

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