Some useful references . What follows is a summary of the more common and generally accessible methods of examining growth structures in dental tissues. A correspondence between the growth structures observed in recent bony tissues and analogous structures observed in fossil specimens has been suggested. Prudence is advisable, however, since the biology of extinct populations can only be imperfectly understood. Fossils undergo diagenetic processes resulting in loss of collagen, but the mineral component of the tissues and their crystallographic alignment may remain intact, preserving incremental structures.
LAGs appear bright under ordinary transmitted light and highly birefringent under polarised light.
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.