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The developmental cycle of the teeth in Plethodon cinereus is analyzed on morphological grounds using alizarin preparations. All the stages in development do not occupy the same proportion of the life cycle time. Functional teeth and germs at an early stage in development occupy a large proportion of the life cycle time, whereas the processes of tooth shedding and ankylosis occur very quickly. The time during which any locus does not bear a functional tooth, and is therefore a non-functional locus, is reduced to a minimum. P. cinereus has a basic pattern of tooth replacement which is consistent with Zahnreihen which are 2.0 tooth spaces apart. Variations in the replacement pattern are common and these are produced by relatively small fluctuations in the spacing of the Zahnreihen around the „mean„ of 2.0. Localized disturbances which produce breaks in the replacement pattern and cause waves to cross also occur. These may be due to the failure of tooth germs to develop, the fusion of tooth germs, or may be the result of the inherent variability in a complex biological system. This variability causes individual tooth germs to develop too slowly or too quickly and hence assume an „abnormal” position thus causing breaks in the replacement pattern. Tooth replacement may be controlled by an intra-local mechanism(s) rather than by stimuli which travel along the jaw.

Sections of undemineralized tooth germs ofAmbystoma andTriturus were examined. The ultrastructure of early germs, both larval and adult, and of dentinogenesis, resembled that of mammals. In adult bicuspid teeth, once the dentine of the cusps was mineralized, mineral crystals of a similar size to early mammalian enamel crystals, appeared between the dentine and the inner dental epithelium (i.d.e). Concomitantly, the i.d.e showed features of mammalian secreting ameloblasts. This new layer, regarded as true enamel, lacked collagen, possessed an ordered arrangement of crystals and reached a maximum thickness of 6 μm.

When tooth germs derived from 14-day mouse embryos were cultured on gelatin sponges in vitro for 37 days, they lost their characteristic morphology, appearing as a layer of undifferentiated epithelium on the sponge surface, with the mesenchymal cells scattered throughout the interstices. These cultures were then transplanted subcutaneously into isologous, newborn recipients and, over a period of 56 days, developed into incisor teeth that were almost perfect in shape and structure.

Electron microscope studies were made of polyoma virus infected and uninfected organ cultures of tooth germs taken from 13- or 14-day C3H mouse embryos. Cultures were examined at 18 days and at 38 or more days of in vitro age to include both morphologically transformed and untransformed odontogenic epithelium. Epithelial and mesenchymal cells were readily identified. Virus was seen in all infected cultures and in no controls. In nuclei the virus was found in both spherical and filamentous forms, while intracytoplasmic virus was only spherical in type. No virus was seen in nuclei of cells in mitosis. No difference was detected in the pattern of virus distribution between transformed and untransformed epithelial cells.

1. 1. All dentine found in transplants of tooth germs ultimately changed into a structure that simulated bone. 2. 2. The rate and extent of this change varied but the end result always appeared to be the same. 3. 3. No osteoid tissue was induced at the site of implantation of tooth germs in intracerebral transfers as it was in the anterior eye chamber. 4. 4. Odontoblasts did not seem to take part in this change; but other cells among the odontoblasts and in the pulpal area demonstrated an osteogenic capacity.

Vertebrate dentitions arise at various places within the oropharyngeal cavity including the jaws, the palate, or the pharynx. These dentitions develop in a highly organized way, where new tooth germs are progressively added adjacent to the initiator center, the first tooth. At the same time, the places where dentitions develop house the contact zones between the outer ectoderm and the inner endoderm, and this colocalization has instigated various suggestions on the roles of germ layers for tooth initiation and development. Here, we study development of the axolotl dentition, which is a complex of five pairs of tooth fields arranged into the typically tetrapod outer and inner dental arcades. By tracking the expression patterns of odontogenic genes, we reason that teeth of both dental arcades originate from common tooth-competent zones, one present on the mouth roof and one on the mouth floor. Progressive compartmentalization of these zones and a simultaneous addition of new tooth germs distinct for each prospective tooth field subsequently control the final shape and composition of the axolotl dentition. Interestingly, by following the fate of the GFP-labeled oral ectoderm, we further show that, in three out of five tooth field pairs, the first tooth develops right at the ecto-endodermal boundary. Our results thus indicate that a single tooth-competent zone gives rise to both dental arcades of a complex tetrapod dentition. Further, we propose that the ecto-endodermal boundary running through this zone should be accounted for as a potential source of instruction factors instigating the onset of the odontogenic program.

Dentitions are formed by the successive initiation of individual tooth germs, but there are periods of overlap, each new germ being initiated before the previous germ has reached its full size. It might be expected that dental growth as a whole proceeds at a particularly high rate during these overlap periods. However, measurement of mouse lower first and second molars at daily intervals from 14 to 23 days after fertilization showed that there were compensatory changes in growth rate of the two germs during their overlap period, and that the rate of growth of the two teeth taken together remained fairly constant.

Histologic investigation of experimentally moved deciduous and permanent teeth and their surrounding tissues reveal: Orthodontic movement of teeth of the deciduous or mixed dentition produces migration of underlying and adjoining tooth germs. This is made possible by bone resorption and deposition. Hence orthodontic treatment in the deciduous dentition can affect the development of the permanent successors. The direction in which tooth germs move corresponds to the direction taken by the roots of the adjacent erupted teeth. Therefore movement of the roots of these teeth in the proper direction is essential. Displacement of the roots of teeth in the same direction as their crowns are shifted is assured only if tipping movements are avoided. Where tipping appliances must be used observation of the following rule may help to keep the center of rotation near or at the apex: “Use weak forces, applied near the gingival margin, in a pulling manner and in a direction never pointing toward the root.”

Tooth formation and replacement were analyzed in 41 specimens of Scomberomorus cavalla. Teeth of this species are laterally compressed blades adapted for cutting action on prey. Development of each tooth begins at the tip and proceeds toward the base. New tooth buds form in a resorption cavity in the base of an older tooth. Resorption occurs at two points on each tooth: in the base and in the anterior-posterior edges just above the jawline. Teeth pass through seven stages of develop- ment and resorption, but remain longest in the preeruptive and primary functional stages. Replace- ment occurs in overlapping waves in alternate tooth loci. Replacement waves may be flat, progress from the rear to front, or progress front to rear. Most replacement data can be adapted to either the Zahnreihen concept of Edmund (1960, 1969) or the tooth family concept of Osborn (1971, 1972, 1974). One tooth germ failure, causing the crossing of replacement waves, is noted. Polyphyodont tooth replacement in lower vertebrates was summarized by Edmund (1960, 1969), who primarily considered reptilian dentitions, but made a few observations on other groups. He proposed that tooth replacement in all polyphyodont dentitions was regulated by a series of stimuli passing caudad along the dental lamina producing tooth germs at each locus. He demonstrated that the interval between successive stimuli determined the direction in which replacement waves, operating in alternate tooth loci, would progress. Edmund used the term Zahnreihen to indicate the teeth produced by a single stimulus. When depicted graphically, Zahnreihen appear as obliquely running lines connecting teeth in adjacent loci. Edmund demonstrated that if Zahnreihen producing stimuli pass down the dental lamina at intervals of 2.0 tooth posi- tions, simple alternate replacement will result. If stimuli are produced at inter- vals greater than two tooth positions, rear to front waves are produced. When stimuli occur at intervals of less than two tooth positions, fron to rear waves are produced. Lawson, et al. (1971) presented a theoretical analysis of variations in replacement waves produced by different Zahnreihen spacings. Osborn (1971, 1972, 1974) rejected the Zahnreihen concept in favor of the tooth family concept. His findings, based primarily on studies of Lacerta vivipara, suggest that tooth replacement is regulated by a sphere of inhibition surround- ing developing teeth and extending into adjacent loci. Since a developing tooth temporarily inhibits tooth development in adjacent loci, adjacent teeth are always in different stages of development. Gillette (1955) first suggested such a sphere of inhibition around the developing teeth of Rana pipiens. Osborn (1971, 1974) suggested that tooth initiation, and initiation of rear to front replacement, is caused by the invasion of neural crest cells from the caudal end of the jaw bone. Some previous studies of tooth replacement in bony fishes include: Norris and Prescott (1959) studied replacement in Girella nigricans; Roberts (1967) surveyed many of the characoid genera; Lawson and Manly (1973) studied an additional characoid species Ctenolucius hujeta; Miller and Radnor (1973) investigated the holostean Amia calva; and Wakita, et al. (1977) reported on tooth replacement in

The larval dentition differs from the adult dentition, which is complete at an early stage of the juvenile period. The minor row (row B) is not formed during the larval period. The tooth germs always appear medial to the functioning tooth row, and the multi-rowed dentition is formed by the rows of the younger replacement teeth and the older functioning teeth in the larval dentition. The sequence of the attachment of the earliest teeth, which are formed at Stage 3, to the pharyngeal bone of larva was 4-5-3 (these numbers correspond to tooth positions of the major row in the adult phase). Subsequently, tooth germs appeared in the order 4-2-5-3-1-4-2. It is maintained in the major row (row A) of the adult dentition. In the row, the teeth were replaced in a cyclic order 5-3-1-4-2. This replacement wave progresses alternately and cephalad, and the pattern is similar to that of the jaw tooth replacement in many reptiles. ONY fishes usually have five pairs of branchial arches, four of which bear gill rakers medially and gill lamellae laterally, and some

Rat tooth germs were used as a model to study variations on mineral content after culture.

The effectiveness of glycerol and ethylene glycol in preserving the viability of tooth germs subjected to various freezing temperatures was investigated by explanting the tissues, after thawing, to tissue cultures, and observing the extent of cellular outgrowth and the degree of internal differentiation. Tooth germs were pretreated with various concentrations of the preservative agents, frozen indirectly in dry ice-alcohol solutions (−78 °C), isopentane cooled in liquid nitrogen (about −120 °C), or liquid nitrogen (−198 °C), and cultured in Carrel flasks. Glycerol in 30% solution appeared to be the most effective pretreating agent, and the slowest rate of freezing (dry ice-alcohol at −78 °C) was the least damaging of the freezing rates employed.

Pairs of first molar tooth germs from 17-day rat embryos were cultured in vitro , one member of the pair having additional vitamin A added to the medium on which it was grown. The germs cultured on hyper-A medium were smaller than the control teeth, and lagged in cusp development. The histodifferentiation of the enamel organs was severely affected, but the dental papillae showed less differences. The changes were almost completely reversible up to 7 days of culture.

Rapidly developing 19-day foetal rat maxillary molars were studied in vivo and as xenografts on chick chorio-allantoic membrane for 10 days. Animals were sacrificed at 19, 21, 24, 27, and 29 days post-copulation. The xenografts were sacrificed at 2, 5, 8, and 10 days following transplantation to the CAM. Several morphological and cyto-differentiation aspects of odontogenesis in vivo and in xenografts were examined histologically. The chronological comparison between in vivo and xenografts revealed the xenografts to have delayed chemical and morphologic development. Negligible collagen synthesis was detected in xenografts; however, significant collagen concentrations were observed in vivo. Collagen and hexosamine concentrations were found to reflect differentiation and morphologic changes in mammalian tooth germs developing in situ. These biochemical parameters were found to be useful tools to correlate in vivo and xenograft odontogenesis.