Project III seeks to develop a noninvasive, three-dimensional microscope and apply it to the characterization of the mineral density variations and tubule structure in dentin. The proposed imaging technique is x- ray tomographic microscopy (XTM), a high spatial resolution variant of computed tomography. Specifically, the project will utilize synchrotron radiation and dynamically reflecting x-ray magnifying optics to achieve 1 micrometer spatial resolution in three-dimensions over a field of 1 cubic millimeter. The sensitivity to variations in the apatite (mineral)/collagen ratio will be 1%. This will be sufficient to accurately map the tubule density, intertubular separation and perhaps the tubular diameter as a function of position within the tooth. Because x-ray tomographic microscopy is truly three-dimensional, data equivalent to high magnification images from 1,000 contiguous thin sections will be obtained from a single measurement on a dentin sample. The hypothesis that will be tested using XTM is that the tubule density and separation as a function of position within the tooth is the same for all teeth of the same type and developmental state. Attempts to bond to dentin will require adhesion to either the collagen or mineral phases. In either approach, it will be important to understand the morphology of the dentin, and how it changes within the tooth from the pulpal cavity to the dentino-enamel junction. This project will address the characterization of the morphology and mineral distributions within dentin in the hydrated state. This is possible because the technique of x-ray tomographic microscopy is noninvasive, and does not require any special atmospheres or vacuum. As part of the project, x-ray microscopy will be used to examine hydrated dentin, and then the same material after desiccation. Changes in the morphology due to tissue drying, such as shrinkage of the tubules, will be recorded and used to correct scanning electron microscopy images. Development of a high resolution, non-invasive technique for studying mineralized tissues in the hydrated state should provide a significant new tool for the evaluation of important problems in dentistry and medicine.