Evolutionary origins of dentine in the fossil record of early vertebrates: diversity, development and function.

5.1 Introduction Dentine has a central role in the support and function of the tooth. It is formed at the interface of the dental epithelium and dental mesenchyme (see Chapter 4), and is the vital link or selective barrier between the sensory and vascular supply of the pulp and the functional surface of the tooth, usually enamel of the crown (see Chapter 6). How do the formative cells mediate between the environment of the tooth surface and the tissue they maintain, either to allow a sensory function, or to effect repair of the dentine to maintain a supportive but wear-resistant material but still with vital properties? Curiously these are questions that still remain largely unsolved. As Lumsden (1987) emphasised, the initial appearance of dentine in the exoskeleton, largely as a protective cover to the skin, probably allowed the Ordovician agnathans to detect tactile, temperature and osmotic changes. This view that dentine allowed the earliest vertebrates to engage in electroreception, was advanced by Northcutt and Gans (1983), Gans and Northcutt (1983) and Gans (1987) (see section 5.5.1). Neural crest-derived cells, initially with a sensory function, became one population of skeletogenic cells, possibly providing a sensory function for dentine as one of the first vertebrate hard tissues. Although Tarlo (1965) drew attention to the striking similarity between human orthodentine and the dentine of the dermal armour of an early jawless (agnathan) heterostracan fish, little insight has been gained into how this histological similarity might allow a comparable sensory mechanism. In the heterostracan example the dentine tubercles are not covered by enamel. [ABSTRACT FROM AUTHOR]