Chimaerische Zahnanlagen aus Triton-Schmelzorgan und Bombinator-Papille: Mit Beobachtungen über die Entwicklung von Kiemenzähnchen und Mundsinnesknospen in den Triton-Larven1
Authors: Wagner von Gerhart 
Published: April 2021

ABSTRACT

1. The experiment (pp. 161–2)
A piece of the neural crest containing the presumptive trabecular, mandibular, and hyoid arches was exchanged between early neurulae of Triton alpestris and Bombinator pachypus. As a consequence, cartilaginous visceral arches and tooth papillae of Bombinator mesectoderm were formed in the Triton hosts (only these are considered).

2. Comparison of normal tooth formation in Triton and Bombinator (pp. 162–6)

(a) The larvae
Triton larvae possess real teeth, which are attached to true dental bones: in the upper jaw to the premaxillary, maxillary, vomerine, and palatine (the last two forming the vomero-palatine); in the lower jaw to the dentary and the splenial. The enamel organs of these teeth arise, as Adams showed in 1924 and de Beer in 1947, partly from the ectodermal area (dentary, premaxillary, maxillary, vomerine), partly from the endodermal area (splenial, palatine) of the mouth epithelium.
On the other hand, Bombinator larvae do not possess any true teeth up to metamorphosis. Only during metamorphosis are real teeth formed in premaxil-lary, maxillary, and vomerine. The lower jaw remains entirely without teeth.

(b) The adult teeth
The problem of enamel and dentine, i.e. the problem of the origin of the hard superficial layer of the teeth (the classical ‘enamel’), was settled by the researches of Schmidt (1938 and subsequently) for fishes. This group of animals completely lacks real enamel, i.e. enamel originating in the enamel organ. The hard exterior layer of their teeth consists of transformed dentine and is called ‘durodentine’.
Reptiles and mammals certainly possess real enamel. For amphibians the situation is not entirely clear. A critical comparison between the adult teeth of Triton and Bombinator shows that Triton possesses a cap of real enamel, whereas Bombinator only possesses a covering of durodentine.

3. The mixed tooth germs (pp. 166–74)

(a) Formation, shape, and degeneration of the individual tooth-germ
The mixed tooth germs appear as early as the first row of teeth of the Triton host. No more mixed tooth formation takes place during the formation of the second and third row of the host. The mixed tooth germs consist of an ectodermal or endodermal enamel organ of the Triton host and of a papilla of Bombinator mesectoderm. A thin layer of predentine (the ‘Zahnscherbchen’) ending in a long point is formed between the two tissues. Arrangement and number of the Bombinator odontoblasts are to a great extent donor-like, whereas the shape of the point of the ‘Zahnscherbchen’ is host-like. The mixed tooth-germs never break through, but begin to degenerate about 19 days after opera-tion: the odontoblasts loosen and become mesenchymal cells before their necrosis, the enamel-organ becomes flat and the ‘Zahnscherbchen’ is dissolved. Some particularly big tooth-papillae fuse with the Bombinator cartilage and are transformed into cartilage instead of mesenchyme during the degeneration of the tooth-germs.

(b) The dissimilar behaviour of the different tooth areas
In the operated Triton larvae the dental bones are lacking completely. As is known, they too originate in the mesectoderm (cf. Sellman, 1946, p. 122; Wagner, 1949, pp. 542 f.). Instead, the Bombinator mesectoderm forms, in the Triton hosts, two cartilaginous elements typical of anurans but lacking in the urodeles: an infra-rostral cartilage instead of a dentary bone, and a less perfect suprarostral carti-lage instead of the premaxillary bone. Mixed tooth germs were formed most regularly near these two cartilaginous areas. Such mixed tooth germs are also formed, but less frequently, in the area of the absent splenial and palatine. Closer analysis shows that all mixed tooth papillae originate in the mesecto-derm of the mandibular arch. That is to say that the suprarostral cartilage origi-nates in the most rostral part of the mandibular arch (Wagner, 1949, pp. 543 f.), and that also the tooth germs in the area of the palatine are very probably not formed from the trabecula, but from the commissura quadrato-cranialis anterior, belonging to the mandibular arch.
So it is seen that only the mandibular arch of Bombinator, which in normal development always remains toothless, induces tooth germs in Triton. This paradoxical fact is discussed on p. 182.

4. The tooth germs on the branchial arches of the Triton larvae (pp. 174–7)
In agreement with Credner (1886) and Stadtmüller (1924) we are convinced that the ‘ß lterfortsätze’ on the branchial arches of the Triton larvae are real tooth germs. They are of theoretical interest here, because they remain in a similarly low grade of realization (‘Realisationsstufe’, Lehmann, 1945) as the mixed tooth germs and, like these, degenerate after a certain time.

5. Taste buds (pp. 177–9)
Reisinger (1933) maintains that the taste buds in the endodermal part of the mouth epithelium are formed by immigration of mesectoderm into the epithe-lium. This opinion is disproved by our chimeras. Taste buds, as well as enamel organs, can be formed by the endodermal epithelium (in confirmation of Land-acre, 1907; Holtfreter, 1933; de Beer, 1947).

6. The inductive factors of tooth formation (pp. 179–84)
The two formative elements of the tooth germs, mesectoderm and mouth epithelium, influence each other during the formation of the individual tooth germ in a ‘reciprocal inductive influence’ (Woerdeman, 1946, pp. 14–15). Each tissue remains inactive without the other, but together they achieve a real ‘kombinative Einheitsleistung’ (Lehmann, 1945). The role of the endoderm seems restricted to its general topographically arranging influence (formation of the branchial ß ssures and of the mouth opening), without which the normal migration and differentiation of the mesectoderm—and, as a consequence, tooth formation—do not take place (Mangold, 1936, 1950; Balinsky, 1947). A first determinative influence must be ascribed to the archenteron roof, because differences in tooth-forming potency seem to be already present in the neural crest (Sellman, 1946).