Human Enamel Prisms

The enamel of modern humans and their ancestors varies in microanatomical structure in ways that are thought to resist the propagation of cracks. To examine this problem it is necessary to image and observe the orientations of units of enamel structure called prisms that course outward from the junction with underlying dentine to the outer surface of the tooth. In this example of modern human enamel deep to the surface of a cut and polished tooth and imaged by backscattered electron imaging in the scanning electron microscope, we see that the prisms have divergent courses. Some prisms are seen to course longitudinally and wander lengthwise in the plane of the image while others course in and out of the plane of the image and appear semicircular. This heterogeneity provides crack propagating resistance to a tooth, enabling it to withstand the mechanical forces of chewing. Some early hominins with large robust teeth have more anti-crack propagating heterogeneous enamel than other species. Color was imparted to the image by an image analysis program for measuring prism orientation.