Development of the dentition in Alligator mississippiensis. Later development in the lower jaws of embryos, hatchlings and young juveniles

Development of the lower dentition from stage 19 (day 26) embryos to 11 days after hatching (day 76) was studied in a close series of accurately aged (and staged) specimens of Alligator mississippiensis using macroscopy, light microscopy, scanning electron microscopy, radiography and detailed reconstructions. This study complements our earlier investigations of lower dentition development in alligator embryos from initiation to stage 18, day 26 (Westergaard & Ferguson, (1986) and so provides the first careful documentation of dentition development in any non-mammalian tetrapod from the first initiated tooth to the first erupted and functional dentition. In this way, it is possible to test previous models of dental development, e.g. Woerdeman (1919) suggested that four Odontostichi would be resorbed from initiation to eruption, and Edmund's (1962) Zahnreihe theory predicts that 84 embryonic teeth would be resorbed in this time period. Both are wrong: approximately 19 early teeth (resorptive group) are resorbed or shed without becoming functional; seven teeth (transitional group) function for a short period (less than two weeks) or are sometimes resorbed or shed without becoming functional and 36 teeth (functional group), initiated during embryonic life, function for longer periods. All teeth produce dentine, but a differentiated enamel organ is absent in teeth 1–8, 9b, 10, 12 and 14: these teeth never produce enamel. Teeth 9a and 11 have a poorly differentiated enamel organ and form only a thin layer of enamel; the remaining teeth develop a typical mammalian enamel organ and produce enamel. As several resorptive teeth produce enamel, there is no simple relationship between dental competence and function. Tooth families are rather arbitrarily defined before separation by connective tissue, thus supporting the Tooth Position theory (Westergaard, 1980, 1983). Differential jaw growth enables the establishment of up to five intervening tooth families between families of the first tooth row. Growth is most pronounced in the middle of each jaw half. Except for teeth 1 and 3, the teeth from odd- and even-numbered families can be fitted into fairly smooth initiation curves. The most active initiation phase occurs between embryonic days 21 and 33. By hatching, families founded by teeth 1 to 5 (3, 6, 12, 2 and 4) have developed four generations of teeth, families 17, 19 and 20, two generations and all other families, three generations of teeth. The interval of time between initiation of successive teeth from the same tooth family ranges from 6–26 days. One clutch of embryos showed a disappearance of tooth family 6 from one jaw side and another clutch exhibited an extra tooth family 0: both these clutch-related anomalies were associated with differences in jaw growth. The embryonic dental system seems to be evolutionarily very plastic, changes of jaw growth enabling the creation or removal of tooth positions anywhere in the odontogenic area. A new progress zone model of dentition initiation is proposed based primarily on a positional information system in the jaw epithelium.