Primate Odontogenesis

¹ Department of Anatomy, Medical College of South Carolina, Charleston, South Carolina

Oblique-jaw X-ray films of 25 rhesus monkeys were examined to ascertain the sequence of initial calcification of the permanent mandibular second molar and premolar teeth. The first permanent molars were removed from 6 rhesus fetuses in order to study the mode of tooth formation exhibited by these teeth. The (M₂P₂) sequence was observed in 52.0 per cent, the P₂M₂ sequence in 12.0 per cent, and the M₂P₂ sequence in 36.0 per cent of the monkeys. It was suggested that there has been a gradual shift in percentages from the P₂M₂ sequence in man to a M₂P₂ order in some of the New World monkeys, paralleling the changes which have occurred in the order of eruption. Calcification of the permanent first molars was shown to begin as early as approximately 120 days' gestation. The major pattern of cusp appearance was mesiobuccal, mesiolingual, distobuccal, and distolingual. As in man, the instability of the sequence revolved around the formation of the distobuccal cusp.

We attempted to delineate as succinctly as possible some of the major issues confronting students of non-human primate odontology today. Studies of the growth and development of the dentition, especially longitudinal investigations, have received almost studied avoidance in the past. There are no standards of normality for either the time or the sequence of calcification of the teeth in the non-human primates. This is true for both the permanent and deciduous dentitions. There is much more to learn about the timing of tooth eruption in colony-reared monkeys, and, once there is adequate information on tooth formation and eruption, the various sequences must be explained.

Two reasons come to mind which are, we believe, very much responsible for the present situation. First, long-term studies cost money, require a great deal of time, and offer the researcher few immediate rewards. Some of these difficulties can be obviated today because of the numerous primate colonies being constructed in this country by the National Institutes of Health. These colonies should provide ideal conditions for attacking longitudinal growth problems. Second, most infrahuman primates are considered to be experimental animals; ergo, one experiments on them. We believe it is necessary, however, to obtain information relating to normative growth before tampering with the basic mechanisms underlying growth and development, for we must know the normal before we can properly evaluate the results of experimental trauma.