4 tetlarBt et te te 126 Distally, interposed between this newly formed endo- chondral bone and the heavily labeled trabeculae in the secondary spongiosa, there was a transitional zone characterized by trabeculae having unlabeled cores of cartilage and diffusely labeled bony lamellae. This pattern clearly reflected events accompanying linear bone growth (134 »/day) subsequent to labeling. The transitional zone represented the matrix around the jJuxtametaphyseal chondrocytes which had acquired the least radioproline 4 hr after injection. The grain counts in osteoblasts lining the trabeculae in the sccondary spongiosa were essentially normal. The rates of appositional lamellar bone formation on the periosteum, endosteum, and epiphyseal bone were also normal, and the OAI approximated values obtained for the pair-fed control group. Rachitic Rats Treated with Phosphorus While the epiphyseal cartilages from rats fed the basal rachitogenic ration supplemented with inor- ganic phosphate were thinner than the rachitie plates and their cytoarchitecture was more nearly normal, the 4-hr pattern of labeling resembled that of the rachitic cartilage. In the upper and middle layers, radioprolinc was detected in the nucleus and cytoplasm of the eclls and in their extracellular matrix. Only the first 1-2 hypertrophic cells per column in the lower layer were heavily labeled; matrical tracer deposition was very slight. No unusual patterns of labeling were detected in the metaphysis or in the periosteum and endosteum, but the eells and surfaces of the epiphyseal bone were only lightly marked. Labeling of the cartilage and metaphyseal trabeculae 3 days after injection also resembled the pattern deseribed for rats maintained on the rachito- genie dict. Linear growth was 80 »/day. Compared to the normal and rachitie rats, little appositional bone growth (1-2 g/day) was registered on perios- teal and epiphyseal bone surfaces. While dense bands of silver grains over labeled collagen lamellae were thinner than normal at these states, OAI values were nearly equivalent to those caleulated for the bones of control rats fed the basal rachitogenic ration which had been supplemented with phosphorus and vitamin D. These eriteria also indicated that endosteal apposition rates were normal (= pair-fed controls). and unchanged by the dict. The grain counts in osteoblasts were normal. Rachitic Rats Treated with Vitamin Do The cartilage from vitamin D-treated rachitic rats was more normal than the rachitic in appearance, but the pattern of labeling resembled that described for the phosphate-supplemented group. Here, too, the grain counts in periosteal osteoblasts were abnormally low 4 hr after injection. Vitamin D treatment of the rachitic rats did not improve the rate of appositional bone growth on the periosteal and epiphyseal bone surfaces (1.0-1.5 p/ dav). In fact, the measurements were significantly less than those recorded for the phosphate-supplemented rats, but there was less difference between these two groups in terms of the thickness of the dense lines of silver grains developed in the emulsion above radioproline-rich collagen lamellae, OAI values, and the numbers of grains over osteoblasts. The bone growth provided by endosteal cells was equal to that of the pair-fed controls and rachitic rats. Whereas the bulk of the periosteal and epiphyseal bone ac- tivity remained in the surface lamellae, the endosteal hot lines were buried. Linear bone growth was approximately 44 »/day. Rachitic Rats Treated with Phosphorus and Vitamin Ds The 4-hr and 3-day patterns of labeling in the bones of rachitic rats treated with both phosphate and vitamin D were not appreciably different from the effect of either partially enriched dietary supplement. Incomplete labeling of the lower layer of the cartilages produced, after 3 days’ growth, the characteristic transitional zone in the metaphyseal trabeculae. Linear growth of the tibias was estimated to be 81 »/day. Only the periosteal osteoblasts con- tained fewer than normal numbers of grains. However, appositional growth on periosteal and epiphyseal bone surfaces was more rapid than that observed for the animals supplemented with vitamin D or phos- phorus alone. The ratio on the epiphyseal bone perse was normal (= pair-fed controls). Endosteal bone growth was particularly enhanced; the rate and OAT index aproximated values achieved by controls fed the commercial chow ad libitum. The matrical band of silver grains was thinner than normal, but equivalent to that produced in the bones of the animals fed each of the supplements alone, DISCUSSION The results from this study confirm previous observations that rats maintained on a diet deficient in phosphorus and vitamin D are stunted in growth and develop rickets within 7 days.'®) There appears to be some aspect of the artificial diet other than its Ca/P ratio and vitamin D content that prevents full restitution of growth potential when both these nutrients are replaced in amounts which in commercial feeds provide for good growth. This problem has also been encountered by other investigators