The distribution of a single, subcutaneous tracer dose of sodium bicarbonate labeled with C14 has been visualized in bone and epiphyseal cartilage from a series of newborn rats by the technique of coated autoradiography.
Contrary to expectations, the deposition of radioactive carbon occurred primarily in the formation of the organic components of bone and epiphyseal cartilage, rather than in their mineral portions. Evidence is presented to indicate that the greatest share of labeled organic matrix of both bone and cartilage takes the form of newly synthesized protein, especially collagen. In cartilage, some labeling of chondroitin sulphate may also occur. Glycogen, however, plays no role in the reactions observed.
Observation of the reactions of epiphyseal cartilage over a three-day period after injection has yielded information which is interpreted as being indicative of the formation of labeled matrix by the chondrocytes. Following its elaboration, this matrix is passed from the cell into the surrounding stroma. The rate at which the chondrocytes synthesize and extrude labeled matrix varies in different regions of the epiphyseal cartilage. This variation gives some insight not only into the rapidity of the interstitial growth of cartilage, but also into the relative rapidity with which renewal of the matrix occurs. It is suggested that a reduced rate of matrix renewal in the zone of hypertrophic chondrocytes may be instrumental in causing the accrual of mineral salts in the zone of provisional calcification.
Autoradiographic reactions of the bone matrix are in general similar in position to those already described for mineral-seeking isotopes. However, because they are not obscured by diffuse, exchange-type reactions commonly encountered in mineral studies, they offer a high degree of precision for microscopic study. In some areas, it is possible to visualize the appearance of labeled bone matrix before bone-salt deposition has begun.