Scanning electron microscopy of rachitic rat bone

The SEM appearances of lateral endosteal surfaces of anorganic tibial diaphyses from rachitic, vitamin D-treated and normal rats are described. Different types of surface could be recognised in all three groups. These included irregular, incompletely mineralized surfaces corresponding to mineralization fronts (I) and partially mineralized collagen fibers (II), smooth resting surfaces (III) and scalloped surfaces which were undergoing, or had previously undergone, osteoclastic erosion. Percentage areas occupied by each surface type was measured using a computer-linked planimeter. In normal bones 38% of the sampled surface was in a resting phase. The bulk of the remaining surface consisted of mineralization fronts (47.3%) and partially mineralized collagen fibers (10%). There was little resorbed surface (4.7%) on normal bones and all of this had the appearance of active rather than non-active resorption. Compared with normal bone, rachitic and D-treated bones had a significantly smaller (P < 0.05) proportion (< 10% in each case) of surface occupied by resting bone matrix. In the rachitic bones 84.7% of the endosteal surface was incompletely mineralized matrix. Of this, 39.7% was mineralization fronts. The amount of partially mineralized collagen fibers in both rachitic (45.0%) and D-treated bones (38.0%) was greater (P < 0.05) than that present on normal bones. The amount of resorbed surface on D-treated bones (35.3%) was greater (P < 0.05) than on both rachitic (10.6%) and normal bones. The results are interpreted as indicating a slowing in the rate of mineralization between stages II and III in the rachitic and D-treated groups. This may be a hypophosphataemic effect. The D-treated rats may show an accelerated transition between stages I and II. The increased bone resorption is probably an indirect effect of low phosphate, an effect blocked in the rachitic animals by the deficiency of vitamin D.