Development of the embryonic dentition in Xenopus laevis (Daudin): Descriptive and experimental studies between Stages 54 and 61

wenty-two tadpoles from late Stage 54 to Stage 61 (Nieuwkoop and Faber, 1967) were anesthetized daily and observed under a dissecting microscope to identify the developing tooth germs and to measure the distances between them. The development of the dentition began during late Stage 54 in the medial two-thirds of each upper jaw quadrant with the abrupt appearance of 4-8 tooth germs over 24 days to form the Initial Dental Row (IDR). At Stage 55 the Distal Dental Row (DDR) began slowly extending back from the IDR over the next 2 weeks. In almost every case 1-3 tooth germs developed in the interdental space between the first tooth germs in both the IDR and DDR. Each jaw quadrant contained 17-23 tooth germs (mean: 20.5) by Stage 61. Employing the criteria of tooth germ position and developmental time, 9 developmental tooth germ types and 4 developmental configurations of tooth germs were identified. Two relationships emerged in the developing dental row. First, the average distance between a pair of tooth germs varied inversely with the time it took for an interdental tooth germ to develop. Second, the average distance between the first tooth germs to appear in the dental row related directly to the number of tooth germs that later developed between them.;Five experimental procedures were performed: (1) A tooth germ was extirpated at the beginning of the development of the dentition in 35 animals. The resultant dental initiation patterns were not simply the expected dental configuration less the removed tooth germ. Rather, tooth germ number, type, position and development time were modified around the surgical site, suggesting an epigenetic or regulative element in the developing dental row. (2) The entire middle third of the upper jaw quadrant, including the suprarostral cartilage and associated intraoral and extraoral tissues, was extirpated from 6 tadpoles at Stage 45. The surgical defect was maintained during the observation period. Dental development was normal in the remaining thirds of the jaw quadrant. (3) Sections (250-350...