Günter Clemen

Dentition and dentigerous bones in juveniles and adults of Polypterus senegalus (Cladistia, Actinopterygii).

Tooth types, their arrangement and the mode of tooth replacement were studied in juvenile and adult specimens of Polypterus senegalus by means of scanning electron microscopy of cleared and stained specimens as well as sections. All the dermal bones of the oropharynx are almost completely covered with teeth except for the angulare. The same is true for the branchial apparatus where only the hyoid skeleton is toothless. The teeth are uniformly monocuspid and conical, but can be classified according to shape and size into three types. These types and the mode of tooth replacement are characteristic for each dermal bone. In some of the jaw bones each tooth possesses a lingually situated replacement tooth. This is true for the teeth of the premaxillary, the maxillary, and the dentary which are arranged in a single line, and those of the dermopalatine, the coronoids, and the vomer which are in several lines and graded in size. Replacement teeth of all the other dentigerous elements develop on top of existing pulpal openings, forming an anastomosing common pulpal complex only after resorption of the previous tooth. The tooth plates of the dermal bones of the branchial apparatus are connected by syndesmosis only to the perichondrally ossified and to the cartilaginous or connective tissue material of the elements of the gill-arches. The dentition and its association with the bones of the head in Polypterus senegalus bear resemblances to advanced actinopterygians on the one hand (e.g. differentiation of tooth-types, arrangement), but also some similarities to living Amphibia (anchoring material and mode of replacement) on the other. The accentuation of a single marginal line of large teeth in both, the outer and the inner dental arcade of the jaws is a peculiarity of Polypterus that in a way parallels the derived state of similar monolinear tooth arrangements in Actinopterygii and Tetrapoda.

Light microscopic observations on the ossification process in the early developing pedicle of fallow deer (Dama dama)

The ossification process of the early developing pedicle was studied in five male fallow deer fawns, aged about seven months. The incipient pedicle was covered by a periosteum, the cambial layer of which was significantly thicker at the apex of the outgrowth than in the more peripheral areas of the pedicle. As was demonstrated histologically, in the central part of the pedicle elongation occurred by a process corresponding to endochondral ossification, whereas in the more peripheral areas the pedicle became enlarged by typical intramembranous ossification. Thus, cartilage formation must be regarded as a normal feature in pedicle growth of fallow deer. The assumption that the transition from pedicle to first antler growth in cervids is reflected by a switch from intramembranous ossification to chondrogenesis at the apex of the growing primary cranial appendage, based mainly on observations in roe deer, does, therefore, not hold for fallow deer. Furthermore, histogenesis of the central part of the fallow deer pedicle closely resembles the developmental events leading to formation of subsequent antlers.

The development of the tooth pattern and dentigerous bones in Polypterus senegalus (Cladistia, Actinopterygii)

The formation sequence of the tooth-bearing bones and the tooth pattern in early ontogeny of Polypterus senegalus is investigated using transparent preparation, histological sections, and SEM. During the attachment step of the yolk-sac larva the first dermal bones and teeth are formed. Teeth appear simultaneously in the areas of the maxillary, dentary, dermopalatine, prearticular, and coronoid 1 along with the first separate anlagen of these bones. A monostichous arrangement of primary teeth is established on the maxillary, dentary, and dermopalatine. Polystichous tooth arrangements do not occur before the early pterolarval phase, and then only in connection with bones of the palate and inner dental arcades. Especially pronounced is the influence of tooth formation on the structure of the parasphenoid that becomes much thickened by accretion of denticulate platelets, but we found neither evidence for a distinct vomeral contribution to the parasphenoid, nor a composite origin of the ectopterygoid in ontogeny. First replacement teeth are found in association with the maxillary and dentary as early as the late apterolarval phase. Primary teeth are of a single general type, whereas from the pterolarval phase onward three tooth types can be distinguished that are restricted to certain tooth bearing bones. Relatively late in ontogeny, dermo-metapterygoid and entopterygoid become formed and colonised by teeth, whereas first branchial teeth and tooth plates appear earlier during the first phase of extrinsic larval feeding. Characteristics of development of the dentition are discussed in comparison with character states of other better known fossil and recent taxa among Actinopterygii and Sarcopterygii. Compared to the assumed basic pattern of actinopterygian fishes, Polypteriformes show a derived condition with respect to structure, arrangement, replacement, and differentiation of teeth, which arises in sequence during larval development. This also corresponds to observed changes of feeding behaviour and functional demands during larval life.

A light microscopic study of primary antler development in fallow deer (Dama dama)

Summary The ossification process of primary antlers was studied in five fallow bucks aged about ten months. In the growing primary antler the following tissue zones could be distinguished histologically in a disto-proximal direction: a proliferative zone, a zone of cartilage maturation and hypertrophy, a zone of cartilage mineralization and degeneration, a zone of primary spongiosa and a zone of secondary spongiosa. Although the strict disto-proximal zonation of tissue changes typical for endochondral ossification of somatic cartilage was not seen in primary antler formation, the histological and histochemical findings clearly demonstrated that this process can be best described as a modified form of endochondral ossification. Our study produced no evidence for a direct (metaplastic) conversion af cartilage into bone during primary antler development. Generally, the histogenesis of primary antlers closely resembles the process of secondary antler growth as described by others. Minor differences between the two processes can be ascribed to the fact that during secondary antler formation a much bigger structure has to be built up in an even shorter time span than in the case of primary antler development and, therefore, the processes of cartilage formation, cartilage destruction and bone remodelling interdigitate even more closely during subsequent antler growth, compared to that of primary antlers. Our study clearly revealed that the transition from pedicle to first antler growth in fallow deer cannot be defined as a change in the ossification pattern, as was previously assumed for cervids in general.

Teeth and their sex-dependent dimorphic shape in three species of costa rican plethodontid salamanders (Amphibia: Urodela)

The shape of the teeth and their sex-dependent dimorphic expression in three species of Costa Rican plethodontids (Bolitoglossa subpalmata, Oedipina uniformis and Nototriton abscondens) were studied using light microscopy and scanning electron microscopy. The teeth of the vomerine tooth patches are about one third larger than the teeth of the jaws in B. subpalmata and O. uniformis, whereas all teeth of N. abscondens are of about uniform size. The occurrence of bicuspid tooth germs in the fetus proves that primary teeth are bicuspid in these directly developing plethodontids. Females possess only bicuspid teeth consisting of a pedicel and a crown, as is considered characteristic for urodeles after metamorphosis. Adult males possess conical monocuspid teeth on the premaxillary. These teeth--which are similar to the typical late larval tooth of salamanders presenting a larval stage--are about twice as big as the neighbouring bicuspid maxillary teeth. N. abscondens males possess some monocuspid teeth and teeth of aberrant shapes on the premaxillary and the maxillaries. A tendency to build more monocuspid teeth in the premaxillary region than in the maxillary region can be observed in this species. We suppose that different degrees of sensitivity to androgens in each section of the dental lamina of the upper jaw cause the secondary occurrence of conical monocuspid teeth predominantly on the premaxillary section.

Histochemical and ultrastructural studies of cartilage resorption and acid phosphatase activity during antler growth in fallow deer (Dama dama)

Cartilage resorption in forming primary fallow deer antlers was studied by histochemistry and electron microscopy. A high activity of tartrate‐resistant acid phosphatase (TRAP), a histochemical marker of skeletal resorbing cells, was first detected in cells located in the mesenchymal tissue separating the columns of hypertrophic cartilage. No cartilage resorption was observed in this region. Intense TRAP staining occurred in large multinucleated cells (identified as inactive osteoclasts) as well as in smaller cells (regarded as mononuclear osteoclast progenitors). On the basis of these findings it was concluded that this was the region where osteoclasts differentiated from progenitor cells. Further proximally, the mineralized cartilage was eroded by active osteoclasts that were located in Howships lacunae and exhibited an intense TRAP staining. Electron microscopy showed that the cells identified as inactive osteoclasts lacked a polarized organization. In contrast, the active osteoclasts in the zone of cartilage resorption exhibited a typical polarized organization: the nuclei congregated near the basolateral cell surface, and there was a zone of deep membrane infoldings (ruffled border) surrounded by a clear zone at the apical cell pole adjacent to the resorption surface of the mineralized cartilage. The multinucleated cartilage‐resorbing cells of the forming antler thus exhibited the typical histochemical and morphological features of active mammalian osteoclasts. Low levels of TRAP activity were also observed in hypertrophic chondrocytes; however, the specificity and potential significance of this staining remain to be elucidated. Anat Rec 268:66–72, 2002.

Ultrastructural aspects of cartilage formation, mineralization, and degeneration during primary antler growth in fallow deer (Dama dama)

Due to their rapid growth, regular replacement and easy accessibility, deer antlers are considered a useful model for the study of cartilage and bone differentiation and mineralization in mammals. The present study describes, for the first time, the cellular and extracellular matrix changes associated with cartilage formation, mineralization and degeneration in primary antlers on the ultrastructural level. Growing primary antlers of 3 to 4 cm length were obtained from six fallow bucks, aged about 10 months. It was shown that the chondroblasts were derived from progenitor cells of the antler perichondrium and differentiated into mature chondrocytes that subsequently underwent hypertrophic changes. Concomitant with cell hypertrophy, formation of a lacunar and a perilacunar extracellular matrix was observed, the latter containing numerous collagenous fibers. Mineralization of the extracellular matrix occurred via matrix vesicles and the formation of apatite crystals at distinct sites of the collagenous fibers. The hypertrophic chondrocytes of the mineralized cartilage then degenerated, a process that was also occasionally observed in more distally located cells surrounded by still unmineralized matrix. No morphological indications of a transdifferentiation of hypertrophic chondrocytes into bone forming cells, i.e., co-occurrence of a degenerating chondrocyte and a viable osteogenic cell in intact lacunae, were found. The cellular and extracellular matrix changes seen in primary antlers resemble those described for secondary antlers. Our results further indicate that the hypertrophic chondrocytes of primary antlers eventually undergo apoptosis, thereby providing further evidence that metaplastic conversion of cartilage into bone does not play a role in antler growth.

Development of the pattern of dentition and dental laminae of costa rican plethodontid salamanders (Amphibia: Urodela)

The pattern of development of teeth and dental laminae of three Costa Rican plethodontids (Amphibia, Urodela, Plethodontidae) was investigated using transparent preparations, light microscopy and scanning electron microscopy. The teeth of the jaws are monostichously positioned, those of the posterior vomeral parts are polystichously arranged. The anterior vomeral parts carry monostichously positioned teeth at the caudal margin; yet, the adult Bolitoglossa subpalmata possesses two lines. As a sex dimorphism adult males display long monocuspid premaxillary teeth which protrude to the outside of the mouth cavity. All species studied possess paired dental laminae in the lower jaw. Nototriton abscondens possesses an unpaired dental lamina in the upper jaw, which is constricted between the unpaired premaxillary and the maxillaries. In contrast, the dental laminae in the upper jaw of B. subpalmata and Oedipina uniformis are segmented into a premaxillary and two maxillary laminae. All species possess a pair of anterior vomeral and a pair of posterior vomeral dental laminae in the adults, whereas the vomeral dental laminae of the subadults are unsegmented. The pattern of dentition is compared with that of Gyrinophilus and Eurycea.

The development of the dentigerous bones and teeth in the hemiramphid fish Dermogenys pusillus ( Atheriniformes, Teleostei)

The development of the dentition and dentigerous bones was studied in the hemiraphid Dermogenys pusillus using histological sections, scanning electron microscopy, and cleared and stained specimens. Five days after birth, the toothless tip of the lower jaw begins to grow longer than the tip of the upper jaw. The growth originates from small cartilaginous triangles connected with Meckels cartilage. The peri- and enchondrial ossification of the growing cartilaginous bars advances rostrad. The pharyngeal tooth plates are formed by fusion of the slat-like dentigerous dermal bones with the bony fractions of the gill branches. Hence, the tooth plates are composite bones. The ventral tooth plate ist formed by the two ceratobranchials V and the basibranchial IV together with the respective dermal bones. The paired pharyngobranchials III and IV are fused with the dorsal tooth plate, and the pharyngobranchials II is fused with the two respective lateral tooth plates. Mineralization starts after birth in elements of the pharyngeal tooth plates and their teeth. There are no indications that the pedicel on which the tooth is established is formed by the enamel organ, which is covered by pulpal cells. The enamel organ originates from the stratum basale of the oropharyngeal epithelium and moves within the jaw from labial toward lingual, the site of the establishment of the tooth. The anlage of the tooth on the tooth plates of the pharynx lies at the level of the tooth base.

Targeted expression of androgen receptors in tooth-forming tissues of a neotropical salamander (Bolitoglossa schizodactyla) enables region-specific formation of dimorphic types of teeth in the male.

Bicuspid teeth are a morphological characteristic in adult metamorphosed urodeles. However, during the mating season the males of many species of plethodontid salamanders (Urodela, Plethodontidae) from northern America bear large monocuspid teeth on their jaws. Since androgen levels are elevated during the mating season, it is likely that the formation of monocuspid teeth is controlled by androgens. However, many plethodontid salamanders from southern Central America live in habitats which lack significant seasonal changes and therefore are no seasonal breeders. These males constantly bear monocuspid teeth, however their presence is restricted to the premaxillary bone, which forms the most frontal element of the upper jaw. Teeth of all other dentigerous bones are bicuspid. This study is based on an immunohistochemical localisation of androgen receptors and reveals a differential expression of androgen receptors in the three separate dental laminae of the upper jaw of Bolitoglossa schizodactyla (Urodela, Plethodontidae) from western Panama. Our findings suggest that the restriction of androgen receptor expression to the premaxillary dental lamina of the male and its absence in all other tooth-forming tissues leads to the differences in tooth morphogenesis. In conclusion, a differential and cell-specific expression of androgen receptors in urodeles-similar to the situation in mammalian species-allows a selective response of differentiating tissues to the hormonal status of the animal.