Since fossil material cannot be decalcified, it is also one of the few methods of observing growth layers in fossil teeth. The difference in refractive indexes between these two beams is known as birefringence and has a positive value for collagen fibrils and the opposite sign for apatite. Early observations you may come across state that cement consists of alternate layers of birefringent and isotropic tissue under polarised light. Recent observations note that annuli are anisotropic and alternate with growth zones, which are isotropic. In the annuli, collagen fibrils are oriented in a more regular fashion than in growth zones.
LAGs are more birefringent than either annuli or growth zones.
Transmitted Ordinary Light Images of intact dental thin-sections under transmitted ordinary light also show a layering in cement. Under transmitted ordinary light, differences in the orientation of collagen fibrils will appear as alternating bright and dark layers. Growth zones, which are opaque bodies, appear as relatively dark layers while annuli appear relatively bright. This banding is often visible only with difficulty in fossil teeth, however. Stained, Histological Thin Sections.