Tooth ontogeny in the upper jaw and tongue of the rainbow trout (Salmo gairdneri).

Tooth development has been studied in the upper jaw and tongue of a series of rainbow trout embryos (Salmo gairdneri). The sequence of tooth ontogeny in the maxillary bone differs significantly from that seen in the premaxillary bone. In the maxillary bone the teeth developed in an almost perfect alternating sequence from front to back, the teeth occupying the odd-numbered tooth positions developing ahead of those occupying the even-numbered tooth positions. However an alternating sequence was not encountered in the premaxillary bone of the rainbow trout where, from tooth position 4, teeth developed in sequence, and at approximately the same rate, in both directions. The significance of this difference was discussed. The teeth on the tongue developed in an alternating sequence, the fourth tooth developing first followed by the second tooth. The teeth in positions one, three and five later developed simultaneously. Evidence was presented which indicated that resorption of a tooth may commence from its pulpal surface in association with the appearance of multinucleated giant cells and not from the external surface in relation to a successional tooth. This was particularly so in the case of lingual teeth where the successor was laterally positioned or in midline teeth which lacked successional teeth.