Abstract
1. The sensitivity and reliability of three representative histochemical tests for calcium (von Kóssa, ferrocyanide, and alizarin) have been compared by applying them to vertebral ossification centers in the rat fetus.
2. The histochemical results were controlled by concurrent study of calcium45 autoradiographs and of soft x-ray microradiographs of the same or adjacent sections.
3. Microradiographs and von Kóssa-positive reactions produced essentially identical representations of mineralized bone and cartilage.
4. The von Kóssa reaction coincided with high grain density regions of calcium45 deposition in stripping-film autoradiographs but did not reflect the sparser densities.
5. The ferrocyanide reaction showed gradients of staining density corresponding in many sites with the varying degrees of radiocalcium density. Faint ferrocyanide reactions extended beyond the limits of microradiographic mineral opacity.
6. The resolution obtained with contact autoradiographs and alizarin dye-lake histological preparations was inadequate for study at high magnification.