A correlated electron microscopic and microradiographic study of human enamel

Abstract

The origin of radiolucent line and honeycomb patterns seen in microradiographs from hypocalcified human enamel (amelogenesis imperfecta) was investigated using a combined microradiographic and elect ron microscopic approach. The technique involved the preparation of suitable ground sections and of corresponding microradiographs. Subsequently, enamel pieces displaying the desired X-ray absorption images were sectioned with a diamond knife for electron microscopy in such a way that the microstructural organization in the pathway of incident X-rays was depicted in the thin sections. It was found that the radiolucent zones were not related to the interrod regions as had been suggested in connection with previous observations of similar line patterns. Judging from the size and packing of the crystals, both rod and interrod regions appeared mineralized to the same degree, but even if mineralization differences had existed, the geometry of the interrod regions relative to the direction of the X-rays excluded their participation in forming the radiolucent line patterns. The rod sheaths, on the other hand, were virtually devoid of mineral matter and were aligned in such a way that they formed pathways of lower mineral content parallel to the X-rays in all enamel volumes which displayed radiolucent patterns microradiographically. Even though the width of a single rod sheath (1000–2000Å) was well below that of the radiolucent zones (1–2μ), the width of the sheath pathways, when projected onto a plane perpendicular to the direction of incident X-rays, approached the latter figure due to the curvature and lateral deviations of the rod sheaths. On the basis of these observations, it is concluded that the radiolucent line pattern in the microradiographs of the hypocalcified human enamel represent summation images of the rod sheath spaces. This conclusion is discussed in the light of previous microradiographic and electron microscopic studies on normal developing and mature human enamel and it is suggested that even in normal enamel the line patterns are summation images of the rod sheaths.