Turnover of the organic matrix of cartilage and bone as visualized by autoradiography

Tibiae and humeri were removed from suckling rats at intervals of time after intraperitoneal injection of C(14)-L-phenylalanine, C(14)-L-leucine, S(35)-sulfate, or Ca(45) Cl(2). Autoradiograms of sections of the bones were prepared. Ca(45) was removed from sections treated with dilute acetic acid; neither the concentration of S(35) nor that of C(14) was thereby markedly decreased. The S(35) was removed from the demineralized sections on incubation in a solution of testicular hyaluronidase; the C(14) was not. These results are interpreted as indicating that most of the S(35) was present in the bones as chondroitin sulfate and that most of the C(14) in the bones was present as protein. In the epiphyses, the C(14) was initially concentrated in the proliferaing and hypertrophic chondrocytes, as was the S(35). Secretion of S(35)- and C(14)-labeled materials into the matrix followed. Thereafter, however, although the S(35)-labeled material (chondroitin sulfate) persisted in the matrix, albeit at a diminished concentration, and was incorporated into metaphyseal bone, the C(14)-labeled material (protein) was almost completely removed from the matrix. When rats were given repeated doses of 17-beta-estradiol benzoate so as to inhibit resorption of their metaphyses, repeated doses of S(35)-sulfate were discerned as strata of S(35) in their metaphyses. This was not the case if the rats received repeated doses of C(14)-L-phenylalanine or C(14)-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.

A CELL SYSTEM IN WHICH RATE AND AMOUNT OF PROTEIN SYNTHESIS ARE SEPARATELY CONTROLLED

The mean cross-sectional area of Haversian systems in adult human ribs tends to be constant in the face of sevenfold changes in the rates at which these systems are made. This implies that different mechanisms control the total amount of cellular work in making Haversian systems and the rate at which this work is performed.

THE EFFECT OF ANTISERUM, ALONE AND WITH HYDROCORTISONE, ON FOETAL MOUSE BONES IN CULTURE

1. The effects of normal rabbit serum and of rabbit antiserum to whole foetal mouse tissues, on the isolated limb bones of late foetal mice were studied in organ culture, and the influence of hydrocortisone on these effects was investigated. 2. Unheated normal serum caused slight loss of metachromatic material from the cartilage matrix, and some resorption of both cartilage and bone. 3. In unheated antiserum to foetal mouse tissues, the terminal cartilage was smaller and less metachromatic than in paired controls in normal serum, while osteoclasis was so intense that in many explants the bone had almost disappeared. The amount of necrosis varied with different batches of antiserum. 4. The changes produced by normal serum and antiserum could be largely prevented by heating the sera to 57 degrees C for 45 minutes. 5. The effects could also be inhibited by the addition of hydrocortisone to the unheated sera; as little as 0.1 microg hydrocortisone per ml of medium had a well marked protective action. 6. It is suggested that (a) unheated antiserum causes a release of lysosomal enzymes with consequent breakdown of intercellular material, (b) this release is due to an indirect action on the lysosome via an increased permeability of the cell membrane, (c) hydrocortisone does not affect the antigen-antibody reaction, but inhibits the autolytic changes that normally follow this reaction, possibly by stabilising both the lysosomal and cell membranes.

COLLAGENOLYTIC ACTIVITY IN MAMMALIAN BONE

Collagen from bone was incubated with bone-cell homogenate. At the end of incubation the collagen had been partially broken down to ultrafiltrable units indicating that ferments with collagenolytic activity, which can be released by homogenization, exist in bone cells.

MICROSCOPIC DETERMINATION OF BONE PHOSPHORUS BY QUANTITATIVE AUTORADIOGRAPHY OF NEUTRON-ACTIVATED SECTIONS

Longitudinal sections of human cortical bone were submitted to thermal neutrons. γ-ray spectra were recorded repeatedly during 15 days following irradiation. They showed that Na²⁴ is predominant as early as 3 hours after activation and that all the γ-emitters have decayed on the 15th day. When the γ-rays have disappeared, β-rays are still produced by the sections. It was proved by the absorption curve in aluminium that all these β-rays are issued from the P³² induced in the sections by activation of P³¹. Therefore autoradiograms registered 15 days after activation reveal the distribution of P³² in the sections. γ-ray spectra and β-ray absorption curves of neutron activated sections of ivory demonstrated a mineral composition similar to that of bone. Autoradiograms of ivory sections activated for various times were used to establish the relation between the optical density of the autoradiograms and the radioactivity in P³². When the bone autoradiograms are compared with the ivory standards of known radioactivity, the optical densities of single osteons (Haversian systems), can be related to their phosphorus contents. Autoradiograms and microradiograms of the same sections were examined side by side. The least calcified osteons, that contain 80 per cent of the calcium of the fully calcified osteons, also contain about 80 per cent of the phosphorus of the fully mineralized osteons. It is concluded that the Ca:P ratio remains constant while mineralization of bone tissue is being completed.

TISSUE DISTRIBUTION OF ELECTROLYTES, CA47, AND MG28 IN ACUTE HYPERCALCEMIA

The in vivo effects of acute hypercalcemia on plasma and tissue distribution of electrolytes, radiocalcium, and radiomagnesium were studied in acutely renal-ligated dogs. Hypercalcemia abruptly increased plasma potassium and phosphorus concentrations but had no significant effect on plasma concentrations of other electrolytes. Skeletal muscle and myocardial sodium decreased 15%, skeletal muscle chloride decreased 22%, and pancreatic and aortic potassium increased 12%. Tissue phosphorus concentrations were unchanged. Total cellular calcium increased 40–70% in liver, myocardium, nerve, and aorta, was suggestively increased in skeletal muscle, and was unchanged in pancreas, brain, and bone. Exchangeable cellular calcium increased 35–75% in liver, skeletal muscle, myocardium, nerve, and aorta and was unchanged in pancreas and brain. Bone exchangeable calcium increased 260%. Exchangeable cellular magnesium of skeletal muscle and pancreas decreased 28% but total cellular magnesium concentrations were unaltered. The changes noted in individual tissues were reflected by 50% increases in exchanged able body and over-all tissue calcium pools and in unchanged exchangeable body and over-all tissue magnesium pools. No significant changes in fractional rates of calcium and magnesium turnover occurred. These data indicate that hypercalcemia causes 1) cellular accumulation of calcium by many but not all tissues, 2) inhibition of magnesium uptake by certain cells, and 3) changes in tissue sodium and potassium distribution which are compatible with known in vitro inhibitory effects of calcium on cellular electrolyte transport.

BONE CELLS: BIOCHEMICAL AND BIOLOGICAL STUDIES AFTER ENZYMATIC ISOLATION

Short-term incubation of rat calvaria in buffered crude collagenase permitted the isolation of morphologically intact cells that absorb vital dyes, contain alkaline phosphatase, and multiply in tissue culture. Freshly harvested cells were similar to whole bone segments in aerobic glucose metabolism.