Growth of Enamel and Dentine
Ameloblasts and odontoblasts each secrete an organic matrix that mineralizes to become mature enamel or dentine respectively. Growth of enamel and dentine is appositional and not interstitial as, for example, is cartilage growth. Enamel and dentine never remodel during life and a permanent record of their growth remains, literally embodied within the microstructure of a tooth. A circadian rhythm influences ameloblast and odontoblast matrix secretion. Daly increments of growth (enamel cross striations) are visible in histological preparations of teeth along the lengths of enamel prisms and in dentine (von Ebners lines) crossing the dentine tubules at right angles. In both thin-enamelled African great apes and thick-enamelled modern human teeth, rates of enamel secretion vary between ~2.5 to 6.0 micrometers per day. However, rates of secretion stay slower for longer in inner forming human enamel, which contributes to the greater overall time it takes to form tooth crowns. There is little good evidence for differences in rates of dentine formation between great apes and humans in either crowns or roots but tooth shape is largely defined by differing dentine formation rates, which must, therefore, exist. Coarser increments of enamel and dentine growth also exist. Striae of Retzius in enamel and Andresen lines in dentine are spaced several days apart. Perikymata on the enamel surface (alternating horizontal troughs and ridges) are associated with striae of Retzius. The modal periodicity of these long-period growth increments is 8 days in humans and 7 days in Pan but is variable in both. The rate at which tooth crowns and roots grow, or extend, in length is determined by rates of odontoblast and ameloblast differentiation in the developing tooth germ. Primates, carnivora, proboscidea, rhinoceros, and hippos all have enamel patterns that consist of incomplete cylindrical surfaces that are arranged in horizontal rows, while most perissodactyla and artiodactyla have enamel formed in a pattern of incomplete cylindrical surfaces that are arranged in vertical rows (Hillson, 2005). Extension rates are faster in taller crowned or longer rooted teeth that form in a shorter time (e.g. Gorilla versus Pan and Homo).
Primates, carnivora, proboscidea, rhinoceros, and hippos all have enamel patterns that consist of incomplete cylindrical surfaces that are arranged in horizontal rows.