Tibiae and humeri were removed from suckling rats at intervals of time after intraperitoneal injection of C14-L-phenylalanine, C14-L-leucine, S35-sulfate, or Ca45 Cl2. Autoradiograms of sections of the bones were prepared. Ca45 was removed from sections treated with dilute acetic acid; neither the concentration of S35 nor that of C14 was thereby markedly decreased. The S35 was removed from the demineralized sections on incubation in a solution of testicular hyaluronidase; the C14 was not. These results are interpreted as indicating that most of the S35 was present in the bones as chondroitin sulfate and that most of the C14 in the bones was present as protein. In the epiphyses, the C14 was initially concentrated in the proliferaing and hypertrophic chondrocytes, as was the S35. Secretion of S35- and C14-labeled materials into the matrix followed. Thereafter, however, although the S35-labeled material (chondroitin sulfate) persisted in the matrix, albeit at a diminished concentration, and was incorporated into metaphyseal bone, the C14-labeled material (protein) was almost completely removed from the matrix. When rats were given repeated doses of 17-ß-estradiol benzoate so as to inhibit resorption of their metaphyses, repeated doses of S35-sulfate were discerned as strata of S35 in their metaphyses. This was not the case if the rats received repeated doses of C14-L-phenylalanine or C14-L-leucine. On the basis of the results in these experiments it is suggested that although a portion of the chondroitin sulfate produced by the chondrocytes of the epiphyseal plate is retained and becomes part of the cores of metaphyseal spicules of bone, the protein of the proteinpolysaccharide is somehow removed before calcification of the cartilage ensues.

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