’ . beat at eee ee eet ee ee ad ee SMRD eek Fre. 96.—Autoradiographs of the proximal tibial cartilage of rats sacrificed 4 hr after a single intraperitoneal injection of 7H proline. Left, control; right, rachitic. Note that the silver grains in the emulsion are distributed over the cells and extracellula matrix throughout the control cartilage, whereas the thickened lower zone of hypertrophic cells in the rachitic cartilage is unlabeled Hematoxylin and eosin. Original magnification 250 x. physeal bone was 6.7 », 4.7 », and 3.9 », respectively. In the pair-fed group, somewhat less bone growth was registered on the epiphyseal bone (2.8 »/day), but otherwise there was no obvious effect due to dietary restriction. The thicknesses of the bands of labeled collagen were in fairly good agreement with these daily appositional growth rates, and the OAI ratios approximated a value of 1. In addition to the dense band of silver grains which moved away from the cells as new matrix was deposited (Figure 95), there was a diffuse distribution of grains in the matrix deposited during the second and third days. This has been described by Tonna™” as a “trail” due presumably to reutilization of metabolized radio- proline by the osteoblasts. Rats fed the fully supplemental basal rachitogenic ratio for 2 weeks showed essentially the same pattern of radioproline uptake and retention. However, the rates of linear growth (97 »/day) and lamellar bone apposition on periosteal and epiphyseal bone surfaces were substantially less than in the other control groups, and their OAI was approximately 2. Only the rate of endosteal apposition was within normal limits (= pair-fed controls). The poor growth observed for this group may be due to voluntary reduction in food intake as the rats did not favor the diet and ate less. Rachitic Rats At 4 hr, the autoradiographs showed *H-proline retention in and around the cells in the upper and middle layers of the cartilage. In the lower layer, the label was found predominantly in the youngest hypertrophie cells—a distinct difference from the nor- mal pattern. Little proline was detected in the older juxtametaphyseal chondrocytes (Figure 96}. In the metaphysis, cortical and transverse epiphyseal bone, the early distribution of the isotope was similar to the controls, and the grain counts suggested that rachitic osteoblastic vigor was normal. After 3 days, the middle and lower layers of the cartilage were uniformly labeled by radioproline, as were the remnants of the cartilage left by endochondral ossification in the primary spongiosa.