A microscopic and microanalytical study (Fe, Ca) of the teeth of the larval and juvenile Ambystoma mexicanum (Amphibia: Urodela: Ambystomatidae)*

> Abstract We studied the teeth of larvae and one juvenile of the axolotl Ambystoma mexicanum, a urodele species that undergoes partial metamorphosis, by light microscopy of ground sections, backscattered electron imaging and semi-quantitative microanalysis in the scanning electron microscope. By applying these techniques it was possible to identify enamel, enameloid and dentin in the teeth. Iron was found to be present in enamel and enameloid, the concentrations being highest in the enamel. A staining indicative of the presence of iron was observed in the inner dental epithelium of tooth germs. Dentinal tubules mostly followed a straight course, but some recurved over a short distance distally. In larval teeth and teeth of "larval type" in the juvenile individual the dentinal tubules ended in the basal portion of the enameloid. Our results show that in the axolotl, monocuspid teeth of the "larval type" that developed after partial transformation still possess an enameloid layer beneath a thin enamel cap. The fi ndings of the present study are consistent with the view that enameloid matrix is secreted by odontoblasts, while enameloid maturation is (largely) controlled by ameloblasts. > Zusammenfassun

A microscopic and microanalytical study (Fe, Ca) of the teeth of the larval and juvenile Ambystoma mexicanum (Amphibia: Urodela: Ambystomatidae)*

> Abstract We studied the teeth of larvae and one juvenile of the axolotl Ambystoma mexicanum, a urodele species that undergoes partial metamorphosis, by light microscopy of ground sections, backscattered electron imaging and semi-quantitative microanalysis in the scanning electron microscope. By applying these techniques it was possible to identify enamel, enameloid and dentin in the teeth. Iron was found to be present in enamel and enameloid, the concentrations being highest in the enamel. A staining indicative of the presence of iron was observed in the inner dental epithelium of tooth germs. Dentinal tubules mostly followed a straight course, but some recurved over a short distance distally. In larval teeth and teeth of "larval type" in the juvenile individual the dentinal tubules ended in the basal portion of the enameloid. Our results show that in the axolotl, monocuspid teeth of the "larval type" that developed after partial transformation still possess an enameloid layer beneath a thin enamel cap. The fi ndings of the present study are consistent with the view that enameloid matrix is secreted by odontoblasts, while enameloid maturation is (largely) controlled by ameloblasts. > Zusammenfassun

Sex dimorphic dentition and notes on the skull and hypbranchium in the hynobiid salamander Pachyhynobius shangchengensis FEI, QU & WU, 1983 (Urodela: Amphibia)

A noticeable sex-dimorphic dentition is described in the hynobiid salamander Pachyhynobius shangchengensis. In the upper and lower jaw, the male possesses pedicellate teeth with a chisel- or spearhead-like crown, i.e. secondary (labial) cusps of teeth are largely reduced, whereas the primary (lingual) cusps are fl attened antero-posteriorly and exhibit sharp edges. In contrast, females have pedicellate, somewhat fl attened teeth more variable in shape, but with small bladed labial and large bladed lingual cusps. Vomeres of both sexes bear typical bicuspid, pedicellate teeth. Skulls including the hyobranchium are largely ossifi ed with some minor differences between males and females. However, females appear to lack the pars hypohyalis in the hyobranchial apparatus

Long-term effects of arrested metamorphosis on dental systems in Salamandra salamandra (Salamandridae: Urodela)

We describe the dental systems of six larvae of Salamandra salamandra hypophysectomized in 1976 under an earlier project to arrest metamorphosis. Larvae cover roughly three developmental stages from early (intrauterine) larval stage to early metamorphosis. Animals survived surgery and lived up to 16 months until fixation. One specimen was studied by histological serial sections. In the period until fixation larvae grew from approximately 3 cm to 7 cm in length (dependent on the developmental stage before surgery) retaining their larval appearance. Changes in the dental systems depended on the stage the larvae had reached before surgery. Generally, after surgery some traits had started or continued development, such as appearance of maxillae as well as resorption of the palatinal tooth patches and the coronoids in the youngest larva examined, whereas other traits had been largely retained and even continued to grow (e. g. larval dentate vomers). In three larvae, the anterior part of the palatines was covered by the posterior margin of the vomer giving the impression of a ‘vomeropterygopalatinum’, known from some newts as temporary fusion of vomer and palatine in consequence of a delayed metamorphosis. However, in the Salamandra-larvae both elements appeared to be connected syndesmotically rather than to be fused by bone tissue The oldest larva (metamorphic stage IV, approximately at the onset of metamorphosis), had lost the palatinal portion of pterygopalatina and had reduced the tooth patches of the vomers to a single row, but outgrowth of the vomerine bar had not taken place

Early tooth transformation in the paedomorphic Hellbender Cryptobranchus alleganiensis (DAUDIN, 1803) (Amphibia: Urodela)

Dentition of the upper and lower jaws and the palate of three larvae of different sizes (29 mm, 47 mm, 53 mm long) and one adult specimen of the paedomorphic Cryptobranchus alleganiensis is described. The 29 mm larvae had ossifi ed, but not fully developed premaxillae and vomeres, partly ossifi ed incomplete palatines, dentaries and still developing coronoids. Teeth were present only on the premaxillae, dentaries and vomeres, but were still not ankylosed to the bones; they were monocuspid and non-pedicellate. However, in the 47 mm larva teeth on all dentigerous bones were bicuspid and pedicellate as typical for urodele teeth after metamorphosis. Thus, paedomorphosis infl uences dentition already in a relatively early time in ontogeny in C. alleganiensis. Such heterochronous effects on dentition obviously occur in different degrees within paedomorphic Urodela and deserve closer attention. Presently, however, knowledge of them is still fragmentary. With the exception of the palatines and the coronoids, which were toothless, praemaxillae, vomeres and dentaries bore a single row of established teeth. Course and number of dental laminae was consistent with the conditions found in other Urodela (a continuous dental lamina in the upper jaw arcade; a discontinuous in the lower jaw arcade and two vomerine dental laminae). However, in contrast to other Urodela we found no signs of dental laminae accompanying the coronoids and the palatines, which explains the total absence of teeth on these bones

Remodelling of the palate: an additional tool to classify larval salamandrids through metamorphosis

Schematic drawings as well as some cleared and stained preparations of the upper jaw and the palate (mouth roof) of larval (before and during metamorphosis) and transformed specimens of the larviparous Salamandra salamandra are presented to illustrate changes especially of the palate through metamorphosis. We distinguished seven stages ranging from early larvae until fully transformed specimens by using characters easily to see in preserved and anesthetized living specimens by means of a dissection microscope at various magnifications and reflected light. Distinctive characters for classification were growth of the maxillae, the anterolateral expansions and posterior outgrowths (vomerine bar) of the vomeres, complete degradation of both, the bony "bridge" connecting the pterygoid, and the palatine and the palatine itself. Larger specimens (length of ca. ≥ 3 cm) can be inspected non-invasively by fixing them in a simple holder. As we used elements that are always present in metamorphic salamanders either exclusively in their larvae (palatine) or in larvae as well as in transformed specimens (premaxillae, maxillae and vomeres), the classification proposed herein appears to be applicable not only to the type form of S. salamandra, but also to other Salamandridae and, appropriately modified, very probably even to urodele taxa that may considerably differ for instance in the shape of the vomeres and dentition. Obviously metamorphic Urodela appear to be constrained by a largely similar developmental sequence with regard of growth and remodelling of the palate, which may be categorized in a standard manner

Organisation of the palate in a spontaneously transforming Mexican axolotl (Ambystoma mexicanum): a case report

We describe the organisation of the palate in a specimen of the Mexican axolotl (Ambystoma mexicanum), which spontaneously started metamorphosis in an advanced age after several breeding cycles, but had not yet completed transformation at an age of > 7 years when it was euthanized. The palate shows a mosaic of paedomorphic (absence of the edentate vomerine plate, monocuspid pedicellate teeth) and transformed traits (separation of palatine and pterygoid, fusion of the vomer and anterior parts of the palatine (vomeropalatinum), bicuspid teeth at least on the upper jaw). The vomeropalatinum has a broad pars palatina along its inner side (typical for paedomorpic specimens). We think that the transformation process has started after the specimen has reached the full paedomorphic status

Formation of the secondary tongue in Hynobius leechi and Ambystoma mexicanum (Amphibia: Urodela)

Tongue development in several developmental stages of the metamorphosing newt Hynobius leechi (Hynobiidae) and the paedomorphic Ambystoma mexicanum (Ambystomatidae) before and after artificially induced metamorphosis was studied by light microscopy (LM) and scanning electron microscopy (SEM). In H. leechi the anlage of the glandular field (lingual glands) of the secondary tongue appears under the free tip of the primary tongue and is clearly seen in late larvae (developmental stage approx. 65). The epithelium of the primary tongue is stratified and composed of epithelial cells, AB-positive goblet cells, some superficial ciliated cells, very few Leydig cells and typical taste buds. Later more or less radially arranged tubular glands (lingual glands) develop in the anterior portion of the prospective secondary tongue, which open in furrows lined in their upper region by the surface epithelium ("neck portion"). Fully developed glands are variously long, moderately branched and contain columnar secretory cells that are preferably AB-positive in their upper region, but AB- and PAS-negative in their terminal portions. Posteriorly the tubular glands become shorter in favour of the neck portion and then are abruptly replaced by a heavily ciliated area containing indentations of the epithelium interspersed with numerous goblet cells (crypts). This zone is considered as a modified remnant of the former primary tongue. Formation of the secondary tongue, often described as "fusion" of the glandular field with the primary tongue, is considered as a process levelling the free, probably regressive end of the primary tongue and the posterior part of the growing glandular field. The development of the secondary tongue of metamorphosing A. mexicanum follows the same pattern. However, the putative anlage of lingual glands in semiadult paedomorphic specimens may be considered as a further character indicating partial metamorphosis in this species. In the transformed axolotl we demonstrate the secondary tongue with lingual glands, epithelial folds with noticeable numbers of AB-PAS-positive goblet cells at the lower surface of the free tip of the secondary tongue, and, contrary to H. leechi, tubular glands immediately behind the dentary

Notes on the cranium of the paedomorphic Eurycea rathbuni (STEJNEGER, 1896) (Urodela: Plethodontidae) with special regard to the dentition

We illustrate the skull, the lower jaw, and especially the dentition of the highly specialized troglobitic salamander Eurycea rathbuni using cleared and stained specimens, scanning electron microscopy, and a few histological sections. Arrangement of the skeletal elements corresponds to earlier descriptions. Dentigerous bones are the fused premaxillae, the vomeres, the palatinal portions of the palatopterygoid in the upper jaw and palate, and the dentaries and coronoids in the lower jaw. Each bone bears a single row of teeth, which are especially numerous and small on the premaxillae and dentaries. Generally, teeth are monocuspid and most are recurved. The dividing zones are unrecognisable to slightly distinct, but do not reach the fully transformed state. Absence or only traces of pedicellation, monocuspidity and an enlarged base are typical for urodele teeth in a relative early stage of development

The Source of Melanocytes in Ortho- and Heterotopic Tail Regenerates of Axolotls and the Dependence of the Regenerative Response on the Presence of Neural Tissue

We studied the regeneration of orthotopic and heterotopic tails in larval axolotls. First, we analyzed tail regeneration following reciprocal exchange of cuffs of tail integument between dark-colored (wild-type) and yellow-colored (hybrid) larval animals. Second, we studied tail regeneration in larval axolotls following transplantation of cuffs of tail integument from metamorphosed dark-colored conspecifics and from an adult fire salamander. In all cases, the amputation planes involved the transplanted integumental cuffs. In the first experiment, the regenerated tails showed the color of the host animals, not that of the transplanted cuffs. This suggests that the melanocytes of the regenerated tails were derived from the host hypodermis. Following transplantation of metamorphosed skin from axolotls and a fire salamander onto larval axolotls, the metamorphosed epidermis reverted to a larval condition. This indicates that the state of differentiation of the metamorphosed epidermis was not permanent. Rather, in order to maintain the metamorphosed epidermal structure, a continuous exposure of the animals to sufficient levels of thyroid hormones was required. Transplantation of tail buds from yellow-colored onto dark-colored axolotl embryos caused the formation of yellow-colored tails both in the head and the anterior limb region of the hosts. Incomplete resection of these heterotopic tails was followed by tail regeneration, while no tail regeneration occurred following complete resection of the heterotopic tails. Successful tail regeneration depended on the presence of neural tissue along the resection plane