True enamel matrix of the newt, Triturus pyrrhogaster, contains no sulfated glycoconjugates

Immunohistochemical and Western blot analyses of collar enamel in the jaw teeth of gars, Lepisosteus oculatus, an actinopterygian fish

Although most fish have no enamel layer in their teeth, those belonging to Lepisosteus (gars), an extant actinopterygian fish genus, do and so can be used to study amelogenesis. In order to examine the collar enamel matrix in gar teeth, we subjected gar teeth to light and electron microscopic immunohistochemical examinations using an antibody against bovine amelogenin (27 kDa) and antiserum against porcine amelogenin (25 kDa), as well as region-specific antibodies and antiserum against the C-terminus and middle region, and N-terminus of porcine amelogenin, respectively. The enamel matrix exhibited intense immunoreactivity to the anti-bovine amelogenin antibody and the anti-porcine amelogenin antiserum in addition to the C-terminal and middle region-specific antibodies, but not to the N-terminal-specific antiserum. These results suggest that the collar enamel matrix of gar teeth contains amelogenin-like proteins and that these proteins possess domains that closely resemble the C-terminal and middle regions of porcine amelogenin. Western blot analyses of the tooth germs of Lepisosteus were also performed. As a result, protein bands with molecular weights of 78 kDa and 65 kDa were clearly stained by the anti-bovine amelogenin antibody as well as the antiserum against porcine amelogenin and the middle-region-specific antibody. It is likely that the amelogenin-like proteins present in Lepisosteus do not correspond to the amelogenins found in mammals, although they do possess domains that are shared with mammalian amelogenins.

High resolution transmission electron microscopy of developing enamel in the Australian lungfish, Neoceratodus forsteri (Osteichthyes: Dipnoi)

The permanent tooth plates of the Australian lungfish, Neoceratodus forsteri, are covered by enamel that develops initially in a similar manner to that of other vertebrates. As the enamel layer matures, it acquires several unusual characteristics. It has radially oriented protoprismatic structures with the long axes of the protoprisms perpendicular to the enamel surface. Protoprisms can be defined as aggregations of hydroxyapatite crystals that lack the highly ordered arrangement of the rods of mammalian enamel but are not without a specific structure of their own. The protoprisms are arranged in layers of variable thickness that are deposited sequentially as the tooth plate grows. They may be confined to the separate layers, or may cross the boundary between each layer. Crystals within the protoprisms are long and thin with hydroxyapatite c-axis dimensions of between 30 and 350 nm, and with typical a-b axis dimensions of 5-10 nm. The hydroxyapatite crystals of lungfish enamel have no centre dark lines of octacalcium phosphate, an unusual character among vertebrates. As each crystal develops, arrays of atoms may change direction, and regions exist where dislocations and extra lattice planes are inserted into the long crystal. The resulting hydroxyapatite crystal is not straight, and has a rough surface. The crystals are arranged in tangled structures with their crystallographic c-axes closely aligned with the long axis of the protoprism. Lungfish enamel differs from the enamel of higher vertebrates in that the hydroxyapatite crystals are of different shape, and, in mature enamel, the protoprisms remain as protoprisms and do not develop into the conventional prismatic structures characteristic of mammalian enamel.

Amphibian teeth: current knowledge, unanswered questions, and some directions for future research

Elucidation of the mechanisms controlling early development and organogenesis is currently progressing in several model species and a new field of research, evolutionary developmental biology, which integrates developmental and comparative approaches, has emerged. Although the expression pattern of many genes during tooth development in mammals is known, data on other lineages are virtually non-existent. Comparison of tooth development, and particularly of gene expression (and function) during tooth morphogenesis and differentiation, in representative species of various vertebrate lineages is a prerequisite to understand what makes one tooth different from another. Amphibians appear to be good candidates for such research for several reasons: tooth structure is similar to that in mammals, teeth are renewed continuously during life (=polyphyodonty), some species are easy to breed in the laboratory, and a large amount of morphological data are already available on diverse aspects of tooth biology in various species. The aim of this review is to evaluate current knowledge on amphibian teeth, principally concerning tooth development and replacement (including resorption), and changes in morphology and structure during ontogeny and metamorphosis. Throughout this review we highlight important questions which remain to be answered and that could be addressed using comparative morphological studies and molecular techniques. We illustrate several aspects of amphibian tooth biology using data obtained for the caudate Pleurodeles waltl. This salamander has been used extensively in experimental embryology research during the past century and appears to be one of the most favourable amphibian species to use as a model in studies of tooth development.

Enameloid/enamel transition through successive tooth replacements in Pleurodeles waltl (Lissamphibia, Caudata)

Study of the evolutionary enameloid/enamel transition suffers from discontinuous data in the fossil record, although a developmental enameloid/enamel transition exists in living caudates, salamanders and newts. The timing and manner in which the enameloid/enamel transition is achieved during caudate ontogeny is of great interest, because the caudate situation could reflect events that have occurred during evolution. Using light and transmission electron microscopy, we have monitored the formation of the upper tooth region in six successive teeth of a tooth family (position I) in Pleurodeles waltl from late embryos to young adult. Enameloid has only been identified in embryonic tooth I(1) and in larval teeth I(2) and I(3). A thin layer of enamel is deposited later by ameloblasts on the enameloid surface of these teeth. From post-metamorphic juvenile onwards, teeth are covered with enamel only. The collagen-rich enameloid matrix is deposited by odontoblasts, which subsequently form dentin. Enameloid, like enamel, mineralizes and then matures but ameloblast participation in enameloid matrix deposition has not been established. From tooth I(1) to tooth I(3), the enameloid matrix becomes ever more dense and increasingly comes to resemble the dentin matrix, although it is still subjected to maturation. Our data suggest the absence of an enameloid/enamel transition and, instead, the occurrence of an enameloid/dentin transition, which seems to result from a progressive slowing down of odontoblast activity. As a consequence, the ameloblasts in post-metamorphic teeth appear to synthesize the enamel matrix earlier than in larval teeth.

Tooth development in Ambystoma mexicanum: phosphatase activities, calcium accumulation and cell proliferation in the tooth-forming tissues

Prerequisites of tooth formation, cell proliferation in the tooth-forming tissues, calcium accumulation and the enzymatic activities of alkaline (ALP) and acid phosphatases (ACP) were investigated by immunohistochemical and histochemical methods in various developmental stages of the Mexican Axolotl, Ambystoma mexicanum. During the growth of replacement teeth, the tooth-forming tissues continually recruit cells from the surrounding regions. The basal layer of the oral epithelium, the dental lamina and sometimes even the outer enamel epithelium provide cells for the differentiated inner enamel epithelium, in which the active ameloblasts are localized. The differentiating odontoblasts are derived from proliferating cells situated basally to the replacement teeth in the mesenchymal tissue. When differentiation has started and the cells have become functional, proliferative activity can no longer be observed. Calcium is accumulated close to the site of mineralization in the inner enamel epithelium and in the odontoblasts as it is in mammals, elasmobranchii and teleostei. The activities of ACP and ALP related to the mineralization of the replacement teeth are separated spatially and not sequentially as they are in mammals. However, the results indicate a similar function of these enzymatic components in relation to tooth formation and maturation of mineral deposition. Most of the substantial processes related to tooth formation reported from other vertebrates occur in a manner similar to that in Ambystoma mexicanum, but there also seem to be basic mechanisms present that are realised in a unique way in this urodele.

Development of larval and transformed teeth in Ambystoma mexicanum (Urodela, Amphibia): an ultrastructural study

Odontogenesis of early larval non-pedicellate teeth, late larval teeth with a more or less distinct dividing zone and fully transformed pedicellate teeth in Ambystoma mexicanum (Urodela) was studied to obtain insights into the development of differently structured teeth in lower vertebrates. Using transmission electron microscopy we investigated five developmental stages: (1) papilla; (2) bell stage (secretion of the matrix begins); (3) primordium (mineralization and activity of ameloblasts starts); (4) replacement tooth (young, old); and (5) established, functional tooth. Development of the differently structured teeth is largely identical in the first three stages. Mineralization takes place in apico-basal direction up to the (prospective) pedicel (early and some late larvae) or up to the zone that divides the late larval and transformed tooth in pedicel and dentine shaft (pedicellate condition). Mineralization starts directly at the collagen and by means of matrix vesicles. First odontoblasts develop small processes that extend to the basal lamina of the inner epithelial layer of the enamel organ. The processes are small and lack organelles in early larval teeth, but become larger, arborescent, and contain some organelles in late larval and transformed teeth. The processes are surrounded by unmineralized matrix (predentine). Odontoblasts at the basis of the teeth, at the pedicel, and in the zone of division do not develop significant cytoplasmic processes that extend into the matrix. Cells of the inner enamel epithelium differentiate to ameloblasts that secrete the enamel. In the early larval tooth they show an extensive basal labyrinth that becomes regressive when the enamel layer is completed. In late larval and transformed teeth, however, a large cavity arises between the basal ruffled border of ameloblasts and their basal lamina. This cavity appears to mediate amelogenesis. A small apical zone in early, but not in late larval teeth directly below the thin enamel layer consists of enameloid and is free of dentine channels.

Abortive secretion of an enamel matrix in the inner enamel epithelial cells during an enameloid formation in the gar-pike, Lepisosteus oculatus (Holostei, Actinopterygii)

The tooth in the gar-pike, Lepisosteus oculatus, an actinopterygian fish, is characterized by the occurrence of both enamel and enameloid, the former covering the tooth shaft and the latter, the tooth cap. Our previous research demonstrated that the enamel in this species was, as in the lungfish, immunoreactive for amelogenin, indicating its homologous nature with the mammalian tooth enamel, whereas the enameloid was completely immunonegative. The present study demonstrates that, during the early maturation stage of the enameloid formation, the inner enamel epithelial cells (IEECs) synthesize through a well-developed Golgi apparatus a fine-granular substance which is intensely immunoreactive for amelogenin. This substance was accumulated in a large saccule extended in a suprabasal zone of the cell; we were unable to find any morphological sign indicating a connection of the substance with the enameloid matrix. The abortive secretion of the enamel matrix-like substance in the IEEC during an enameloid formation was considered to be an instance of rudimental enamel formation. In the gar-pike, the synthesis of amelogenin in the IEEC has been demonstrated to occur independently from that of the enameloid matrix. The present findings demonstrate a prominent difference between the tooth enamel and enameloid.

An immunocytochemical study of amelogenin proteins in the developing tooth enamel of the gar-pike, Lepisosteus oculatus (Holostei, Actinopterygii)

Previous studies have demonstrated the morphological similarity of the enamel-like layer found in the teeth of the coelacanth, lungfish and gar-pike to the enamel of tetrapods. In order to clarify the phylogenetic continuity between both structures, tooth germs of the gar-pike were immunocytochemically studied using an anti-bovine amelogenin polyclonal antibody. Intense immunoreaction was shown over the enamel-like matrix layer. Certain cell organelles associated with the secretory pathway of the ameloblasts were recognized as immunoreactive. These results indicate that the enamel-like layer of the gar-pike is a tissue homologous with the mammalian enamel because both possess a common, amelogenin-like substance.

Proteoglycans (PGs) in the larval amphibian tooth as visualized by cuprolinic blue

Proteoglycans (PGs) were localized in the predentine and dentine of young larvae from the urodelan species Salamandra salamandra. After cuprolinic blue (CB) staining at the critical electrolyte concentration of 0.1 M MgCl2, CB-positive, electron dense filaments with considerable variations in length and width were found in the collagen-free zone adjacent to the odontoblast processes (length up to 1.3 microns, width 21 nm), in predentine (660 nm/3.2 nm), in dentine around (20 nm/9 nm) and in the dentine tubules (35-150 nm/8 nm). Size classes very likely represent different PGs.

Hard tissue of teeth and their calcium and phosphate content in Ambystoma mexicanum (Urodela: Ambystomatidae)

The wall of the pulp cavity, fracture faces and the demineralized surfaces of teeth from larvae and adults of Ambystoma mexicanum were investigated by scanning electron microscopy (SEM). Calcium and phosphate contents were determined by microanalysis. The apical part of the tooth (crown, tooth apex) contains dentin canals. In the larva, these do not reach the enamel-dentin border but end below this border in front of a denser hard substance, possibly enameloid. The pedicel in the adult and the basal portion of the tooth in the larva (base) are without dentin canals. These parts of the teeth are characterized by longitudinally arranged collagen fibres as visualized on the demineralized surfaces. These observations indicate a congruency in early-larval and adult teeth between base and pedicel as well as apex and crown. This partition is also confirmed by the calcium and phosphate values which were identical in larvae and adults. Highest values are found in enamel and lowest values in the tooth-bearing bone. Calcium and phosphate content show a clear difference between dentin and the basal part of the tooth (pedicel and base). The ring-like dividing zone in the adult tooth is less well mineralized.

Ganoine formation in the scales of primitive actinopterygian fishes, lepisosteids and polypterids

The scales of primitive living actinopterygian fishes, lepisosteids and polypterids, have retained ganoine, a hypermineralized layer which covered the scales of the osteichthyan ancestors. To know finally its tissue origin in the actinopterygian lineage, ganoine formation was described in Lepisosteus oculatus, with scales devoid of dentin, and was compared to ganoine formation in two polypterids, Calamoichthys calabaricus and Polypterus senegalus, with scales possessing a dentin layer. The events taking place before, during and after ganoine deposition were studied in experimentally regenerated scales using light and transmission electron microscopy. In spite of differences in tissue composition and in organization of the epidermal cells on the scale surface, ganoine formation is similar in both types of scales. Preganoine is deposited by epidermal cells and constitutes a thick layer which mineralizes progressively to become ganoine, a true enamel. The cellular processes involved in ganoine formation were compared to those described for enamel in mammalian teeth.

Light and TEM study of nonregenerated and experimentally regenerated scales of Lepisosteus oculatus (Holostei) with particular attention to ganoine formation

BACKGROUND

The structure of nonregenerated and experimentally regenerated scales of the holostean fish Lepisosteus oculatus and the events taking place before and during ganoine deposition on the scale surface were studied. The aim of this study was to answer the question of the origin of the ganoine in lepisosteids, the scales of which are devoid of dentine, and to compare them to ganoine formation in polypterid scales and to enamel formation in teeth.

METHODS

Two adult specimens were used and the scale structure was studied using light and transmission electron microscopy. Regeneration was used as an alternative to the lack of developmental stages and to induce ganoine deposition on the scale surface.

RESULTS

Nonregenerated scales are composed of a thick, avascular bony plate capped by ganoine that is covered either by the epidermis or by dermal elements. The ganoine surface is separated from the covering soft tissues by an unmineralized layer, the ganoine membrane. During the first 2 months of regeneration, the bony plate forms. It differs from the bony plate of nonregenerated scales only by its large, woven-fibered central region and by the presence of numerous vascular canals. Shortly before ganoine deposition, the osteoblasts cease their activity and an epithelial sheet comes to contact them and spreads on the bony surface. This epithelial sheet is connected to the epidermis by a short epithelial bridge only and is composed of two layers: the inner ganoine epithelium (IGE), in contact with the bone surface and composed of juxtaposed columnar cells that synthesize the ganoine matrix, preganoine; the outer ganoine epithelium (OGE), composed of elongated cells, the surface of which is separated from the overlying dermal space by a basal lamina. Isolated patches of preganoine are deposited by the IGE cells in the upper part of the osteoid matrix of the scale. The interpenetrated preganoine and osteoid matrices constitute an anchorage zone between ganoine and bone. Preganoine patches fuse and a continuous layer of preganoine is progressively synthesized by the IGE cells. Preganoine progressively mineralizes to become ganoine.

CONCLUSIONS

The processes of ganoine formation are similar to those known for the ganoine in the polypterid scales and to those described for enamel deposition in teeth. Ganoine is enamel.