b'YOUR GUIDE TO THE 2017 AAEP CONVENTION SPONSORED BYwith age until they expire. Dixon de-EnamelThe cells that produce enamel, the hardest body tissue, die as the tooth The Evolutionscribed dental and related tissues:cant heal if damaged. An erupted tooths complete enamel content is laid down of Equineerupts, meaning enamel is dead and and wears throughout a horses lifetime. over 25 might have little enamel left and a reserve crown (everything lying below it) Dentistry Therefore, the crowns of some horses composed entirely of cementum and den-MICHELLE ANDERSON tin. This results in the smooth-mouthed horse, said Dixon, with little to no cheek digestive system result fromCementumThis is the softest of the T he modern horses dentition andteeth ridges left to chew properly. millions of years of evolution incalcified dental tissues, covering the response to changing food sources androot and anchoring it to the periodontal climates. Likewise, how todays equineligaments. In horses, cementum plays a veterinarians treat those teeth muststructural and protective role by sur-evolve and improve constantly, said Pa- rounding the tooth. It forms under the draic Paddy Martin Dixon, MVB, PhD,gingiva (gums) as the tooth erupts and, FRCVS, Dipl. EVCD (Equine). so, later becomes part of the clinical (vis-Dixon presented the Frank J. MilneBIANCA MCCARTY ible) crown. Cementum is also deposited State-of-the-Art Lecture on the evolutionon the reserve crown roots once it erupts.of horses and equine dentistry. Dixon is asubstance on earth, but not a single livingDentinAs the main tooth structure, this professor of equine surgery at the Univer- mammal can break it down, Dixon said.living calcified tissue without blood sup-sity of Edinburghs Royal (Dick) School ofRather, they rely on the complex com- ply supports enamel. Cells called odon-Veterinary Studies, in Scotland, where hemunity of microbes that live in the guttoblasts build predentin, which becomes studies dental and upper airway disorders.for help. As mastication (chewing) anddentin. In healthy human teeth, dentin is digestion adapted to changing vegetation,very sensitive and never exposed. But in The Evolution of Horses & Their Teeth the gut microbiome evolved to help equidthe equine tooth, enamel and cementum Hyracotherium (aka eohippus, orancestors digest cellulose, Dixon said.wear away, leaving dentin unprotected on the Dawn Horse) evolved in NorthAt the same time, early equine ances- the occlusal surface. For unknown rea-America circa 55 million years ago. Thetors transitioned from consuming large,sons, exposed equine dentin doesnt cause rabbit-sized animal lived in forest land,infrequent, nutrient-rich meals to smaller,pain like humans experience, Dixon said. browsed on soft, leafy vegetation, and hadfrequent, less nutritious meals, more likePulpTooth pulp is soft, sensitive, and brachydont (short crowns, well-developedthe grazing we see in horses today.vascular and contains the blood ves-roots, and narrow canals in the roots)Horse have a low-efficiency gutsels necessary to deliver nutrients to the teeth similar to those of humans or dogs.and eat a low-energy diet, Dixon said.dentin-building odontoblasts that give These were unlike the hypsodont teethBecause of this, they spend a lot of timehorses teeth sensation.(high-crowned with cementum extendinggrazing16 to 18 hours a dayand thisPeriodontal tissueThese tissues past the gum line toward the occlusal, orcauses a lot of wear on the teeth. (including the gingiva and periodontalchewing, surface, providing extra materialMany grasses and plants contain silicaligament), alveolar bone (socket), and ce-for wear and tear) of todays horses.and other phytoliths (minute mineralmentum attach the teeth to the jawbones, The climatic event that extinguishedparticles), which also cause tooth wear.allowing for flexibility and tooth eruption. dinosaurs preceded by some millions ofSo, the short-toothed prehistoric equineAll these tissues and the complete tooth years an increase in atmospheric carbonancestors evolved to have more durable,structure have evolved to grind cellulose dioxide, which favored growth of fibrousconstantly erupting, longer teeth.down to 2-millimeter sizes so the gut mi-plants, such as tundra grasses. Thesecrobes can break it down, Dixon said. replaced the lusher, more nutritious veg- A Look at Dental Tissuesetation Hyracotherium consumed. TheseHorses have incisors (used for cuttingDomestication and Dentistryplants cell walls contained more structur- and tearing), cheek teeth (premolars andAs humans started using horses for al carbohydrates, including cellulose. Inmolars used for grinding), wolf teeth (thattransportation, warfare, and work, and response to these changes, Hyracotheriumwere originally premolars), and canineentertainment, they developed early and its descendants adapted their diet.teeth (used for defense in other species).veterinary theories. Written evidence Cellulose is the most common foodTeeth erupt slowly above the gum linedocuments ancient treatments, but few of March 2018THE HORSE AAEP Wrap-Up TheHorse.com/AAEP2017 A9'