Quantitation of autoradiographic grains in different zones of articular cartilage with image analyzer

A novel method is introduced for the estimation of grain numbers in autoradiographic sections of articular cartilage with an image analyzer. It is based on separation of grains from the underlying structures by gray level thresholding and determination of the percentage of total area occupied by grains in a relatively large measuring field. The mean grain size is used as a reference to calculate grain numbers per cell profile and per unit area of tissue in various zones of bovine articular cartilage labelled with 35S-sulphate in tissue culture. The results demonstrate considerable zonal differences as well as site related topographic variation in the rate of 35S-sulphate incorporation. The largest site-related variation in the grain counts was observed in the superficial zone, suggesting a delicate control of proteoglycan synthesis in this zone.