Many mammals are born with two sets of teeth – where ‘milk’ or ‘baby’ teeth are lost as the mammal grows and are later replaced with ‘adult’ teeth. In some groups of mammals, like rodents, teeth have open-rooted dentition, so teeth are not lost early in life, but rather continue to grow. Rats have 16 teeth in total, including four incisors near the front of the mouth for gnawing and 12 molars (three molars in each quadrant) for chewing food. The molars experience wear when meeting a tooth on the opposite jaw (occluding) while eating, preventing them from growing too long. As a rodent grows from juvenile to adult, the jaws also grow longer and the distance between the top and the bottom jaws increases too. The teeth, therefore, must continue to grow to maintain contact while eating. But if rodents are fed a soft-food diet and the teeth do not experience forces during chewing, will the teeth continue to grow? To better understand what factors influence tooth growth, Ourania Stergiopulos and colleagues from the Universities of Geneva and Bern in Switzerland investigated how diet and age impact adult tooth growth in juvenile and adult rats.
The researchers compared two groups of juvenile and adult rats over 3 months – one that was fed hard pellets and another fed soft water-soaked pellets. Each group included 24 rats, with 12 juveniles and 12 adults. The researchers X-rayed the rats’ jaws throughout the experiment (at the beginning of the experiment, at the 2 month mark and at the 3 month conclusion). They analyzed the scans by measuring tooth length on the lower jaw in each rat. As the teeth grew, the team also measured the positions of the adult teeth.
The group discovered that the teeth continued to grow regardless of the rats’ age or whether they were fed soft or hard pellets. However, the teeth of the young rats fed softened pellets grew longer compared to those of the young rats fed hard pellets. In the adult rats, the teeth grew in the same way despite the differences between the hard and soft diets. This suggests that lack of contact between teeth especially impacts the growth of teeth in young rats, encouraging additional growth so they eventually contact a tooth on the opposite jaw and experience force from chewing.
Changes to an animal's diet may cause a range of downstream effects, such as alterations in tooth position and shape, muscle development in the jaw and, ultimately, the ability to chew. In juvenile rats, teeth grow more when they aren't chewing hard food and the teeth are not experiencing chewing forces to restrict growth. Interestingly, we might be able to use this knowledge in human dentistry. If children receive a dental implant, any surrounding adult tooth growth may be affected by contact with, or the position, of the implant, especially as the child's jaw and face continues to grow. It is imperative that we continue to study the factors that affect tooth growth and tooth position in an effort to improve human dentistry and to better understand the chewing movements and developmental patterns of animals.
