Polyphyodont

Abstract: Edmund (1960) has shown that in the dentitions of almost all non-mammalian vertebrates, teeth are replaced in waves which regularly sweep through alternate tooth positions. He explained the ontogeny of these patterns of tooth replacement in terms of biological units called Zahnreihen whose existence has been accepted by nearly all workers studying tooth replacement. In the present paper it is argued that there is no unequivocal evidence, either during development or in adult animals, that Zahnreihen have any biological significance. Reconstructions were made from serial sections of the developing dentitions in the lower jaws of 15 embryos of Lacerta vivipara. It was evident that Zahnreihen have no significance in this animal. Rudimentary teeth were produced with varying frequency in positions 3, 5, 6, 8, 10 and 13. Contrary to the predictions of all previous theories explaining the ontogeny of tooth development in reptiles it was in these apparently random positions that the first teeth were produced. Furthermore, apart from during the first few days of embryonic dental development, it was clear that the development of a row of alternating teeth was initiated in sequence from the back to the front of the jaw to be followed by a similar sequence of development of the intervening teeth. On the basis of this evidence a new model to explain the sequence of tooth initiation in reptiles is proposed. The following assumptions have been made. (A) Ectomesenchymal cells migrate anteriorly through the developing jaws initiating a reaction from the oral ectoderm. (B) The oral ectoderm develops competence to react to the ectomesenchyme in three stages. First it generates abortive clumps of ectodermal cells; second it becomes capable of inducing the adjacent ectomesenchymal cells to form dentine and third it becomes capable of laying down enamel. (C) At all times the dental lamina has the potential of taking part in tooth development according to the regional competence achieved. (D) Developing tooth germs produce a condition which inhibits tooth development around them. Using these assumptions it is possible to explain all stages in the development of the wave replacement of alternate teeth in L. vivipara. It is also possible to explain previous observations on the ontogeny of reptilian dentitions. The sphere of inhibition which surrounds developing teeth is particularly important because it ensures that developing teeth are evenly spaced through the jaw. It is argued that the wave replacement of alternate teeth is an automatic sequel to this and is of only secondary functional significance.

Abstract: Most dentate vertebrates, including humans, replace their teeth and yet the process is poorly understood. Here, we investigate whether dental epithelial stem cells exist in a polyphyodont species, the leopard gecko (Eublepharis macularius). Since the gecko dental epithelium lacks a histologically distinct site for stem cells analogous to the mammalian hair follicle bulge, we performed a pulse-chase experiment on juvenile geckos to identify label-retaining cells (LRCs). We detected LRCs exclusively on the lingual side of the dental lamina, which exhibits low proliferation rates and is not involved in tooth morphogenesis. Lingual LRCs were organized into pockets of high density close to the successional lamina. A subset of the LRCs expresses Lgr5 and other genes that are markers of adult stem cells in mammals. Also similar to mammalian stem cells, the LRCs appear to proliferate in response to gain of function of the canonical Wnt pathway. We suggest that the LRCs in the lingual dental lamina represent a population of stem cells, the immediate descendents of which form the successional lamina and, ultimately, the replacement teeth in the gecko. Furthermore, their location on the non-tooth-forming side of the dental lamina implies that dental stem cells are sequestered from signals that might otherwise induce them to differentiate.

Abstract: 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.