Presence of plicidentine in the oral teeth of the coelacanth Latimeria chalumnae Smith 1939 (Sarcopterygii; Actinistia).

The extant coelacanth Latimeria is a sarcopterygian predatory fish with caniniform teeth on its upper and lower jaws. The teeth are constituted of a cone of dentine with an apical cap of enamel, and they are fixed to the osseous component of the jaws by an attachment bone. Internal walls of the tooth base show folds that have been firstly interpreted in the past as radial vascular canals. Three-dimensional visualisation of these foldings using X-ray tomographic techniques and new histological interpretation lead to reconsider these structures as true plicidentine. The folds of the dentine do not invade the whole pulp cavity of the tooth contrary to the plicated condition of most fossil sarcopterygian fishes (e.g., Eusthenopteron, Porolepis, Megalichthys) certain fossil marine reptiles (ichthyosaurs) and extant varanids; in Latimeria they are limited to the lower third to the half of the pulp cavity. The presence of plicidentine in Latimeria's teeth is proposed to be a plesiomorphic character for sarcopterygians.

Demineralization of the vertebral skeleton in Atlantic salmon Salmo salar L. during spawning migration.

In Atlantic salmon, Salmo salar, the mineral rate of vertebrae in a given fish varies according to the position of the vertebra along the rachidian axis. Indeed, the mean rate goes from 49% in the anterior vertebrae and raises to 51% in post-truncal vertebrae. Although no significant difference in the mineral rate was noticed between males and females either in the lower river basin or after spawning, the mineral rate of vertebral bone decreased significantly (1-2%) during spawning migration. Vertebrae, like scales, are an important reservoir of calcium from which fasting salmon draws the minerals and organic materials necessary for the substantial remodeling of cranial bones in males and for sexual maturation. We hypothesize that mineral decrease in vertebrae may be the result of a halastasic demineralization of the vertebral tissues.

Ultrastructural data on the melanophores associated with the cellular elasmoid scales in Leporinus friderici (Teleostei: Ostariophysi, Anastomidae): Their putative participation in scale matrix formation.

Transmission electron microscopic (TEM) examination of cellular scales of Leporinus friderici reveals the presence of melanophores associated with the hyposquama, a continuous cellular layer lining the inner surface of the scale. Hyposquamal scleroblasts synthesize the collagen fibrils forming the scale matrix. Some scleroblasts lining the deep surface of the scale margin become trapped within the collagenous matrix. Neighboring melanophores become inserted within the hyposquama. They contact the scale matrix and show morphological features resembling those of the adjacent hyposquamal scleroblasts with which they are connected and, like them, they appear to be involved in the production of the collagenous scale matrix. The present ultrastructural study favors the hypothesis that melanophores in vivo are like tumorous cell lines in vitro in that they maintain a degree of plasticity allowing changes in their phenotype according to environmental conditions. The close morpho-functional links between scale scleroblasts and melanophores suggest that they could be closely related lineages derived from the same basic stem cells and support the hypothesis that scale scleroblasts as well as melanophores are neural crest derivatives. © 1996 Wiley-Liss, Inc.

Comparative study of lower pharyngeal jaw structure in two phenotypes of Astatoreochromis alluaudi (teleostei: Cichlidae).

The potentially molluscivorous East-African cichlid Astatoreochromis alluaudi is known to exhibit phenotypic plasticity in its pharyngeal jaw apparatus. We examined wild-caught (snail-eating) fish and specimens experimentally reared on soft food for differences in bone structure in their lower pharyngeal jaw (LPJ). The LPJ is built up of two halves, each of which consists of four structural units: a bony dentigerous, sutural and cortical plate, surrounding a medullary cavity containing sparse bone. Histomorphometric data and associated statistical analysis on serial microradiographs through the posterior third region of the LPJ, where crushing forces are assumed to be the highest, reveal differing growth trajectories: (1) compensating for fish size (standard length) the LPJ grows to a significantly larger size and volume in snail-eating specimens, (2) all structural units distinguished contribute to the volume increase of the LPJ in the hard versus the soft phenotype, and (3) the bone volume fraction in each of the units keeps pace with the growth of the unit proper, indicating that porosity does not change on one growth trajectory or from one phenotype to another. In addition, morphological observations show in hard food specimens: (1) the development of a structurally different bony layer along the inner side of the cortical plate, and (2) a reinforcement of the medullary cavity in the form of oriented trabeculae. Both are interpreted as a consolidation of the medullary cavity to resist the compressive forces exerted when hard food particles (mollusc shells) are crushed. © 1994 Wiley-Liss, Inc.

On the origin of ganoine: histological and ultrastructural data on the experimental regeneration of the scales of Calamoichthys calabaricus (Osteichthyes, Brachyopterygii, Polypteridae).

In order to understand the process of ganoine formation on the ganoid scales, scale regeneration has been studied to overcome the lack of a growth series of scale ontogeny. Seven stages of ganoid scale regeneration have been defined over a period of five months in the polypterid fish Calamoichthys calabaricus. The study has been carried out using transmission electron microscopic techniques. After wound healing and differentiation of the osseous basal plate, a layer of vascular dentin is deposited at the upper surface of the basal plate owing to the presence there of odontoblasts closely applied to the dentin. When these cells move away, a close contact is then established between the stratified epidermis and the regenerating scale. Numerous alterations of the epidermal-dermal boundary occur until its disappearance and a thick layer of pre-ganoine is formed. This layer is progressively mineralized; and finally an organic intermediate layer differentiates between the ganoine, which is a hyper-mineralized tissue, and the overlying epidermis. This ultrastructural study demonstrates rather unequivocally the involvement of the inner epidermal layer (IEL) in the appearance and growth of the ganoine. It is suggested that these epidermal cells can be compared functionally to the inner dental epithelium (IDE) described during mammal tooth morphogenesis. Consequently, our results allow us to propose that ganoine can be identified as true enamel, although additional data are necessary to analyze the proteinaceous component or its organic matrix.

[Histological data on the pharyngeal jaws of Trachinotus teraia (Cuvier 1832), Carangidae (Osteichthyes, Perciformes) of tropical Africa]. / Données histologiques sur les mâchoires pharyngiennes de Trachinotus teraia (Cuvier 1832), Carangidae (Osteichthyen, Perciforme) d'Afrique tropicale.

The thick pharyngeal jaws of Trachinotus teraia are made up of small teeth embedded in an abundant acellular vascularized bone tissue. The diameter of the vascular canals is progressively reduced from the center of the jaw to the abrasive surface where they are wholly obliterated by bone tissue. Each tooth consists of a small cone of orthodentine without an apical hypermineralized cape. The teeth have a weak masticatory function compared to the surrounding bone tissue. The genesis of these teeth probably takes place in the inner part of the pharyngeal jaws; they then move progressively towards the abrasive surface. The inner localization of the first stages of dentinogenesis raises the question of an effective epidermal participation in the ontogenesis of the pharyngeal teeth in T. teraia.

Proline hydroxylation of collagens synthesized at different temperatures in vivo by two poikilothermic species.

Extent of prolyl hydroxylation in newly synthesized viper collagen is decreased at 10 degrees C to approximately 23% of normal on skin and to approximately 57% of normal in bone collagen. At 20 degrees C, prolyl hydroxylation is approximately 50% of normal in skin and normal in bone. At 10 degrees C and 20 degrees C, prolyl hydroxylation is decreased approximately 32% in the skin collagen of carp. In contrast, prolyl hydroxylation is unchanged at 10 and 20 degrees C in bone, scale and lepidotrichia. Prolyl hydroxylation of cartilaginous endoskeleton showed an approximately 25% decrease at 20 degrees C.

Structure and comparative morphology of camptotrichia of lungfish fins.

The present work is devoted to the organization and ultrastructure of the fin rays or camptotrichia of two living Dipnoi (lungfishes) Protopterus and Neoceratodus. In both species, these rods have a dual structure: only the superficial region facing the stratified epidermis is mineralized while the deep one is made of a dense unmineralized network of collagen fibrils forming a permanent pre-osseous tissue. Only the camptotrichia of Neoceratodus is made of cellular bone. This study confirms the structural peculiarities of these camptotrichia when compared to the dermal skeleton of the Actinopterygii constituted by the bony lepidotrichia and the actinotrichia. These results are discussed and compared to fossil dipnoan fin rays.

Comparative fine structure of the Osteichthyan dermotrichia.

This study shows that camptotrichia constitute the only skeleton of the Dipnoan fins. Their fine structure is indeed quite different from the two other Osteichthyan dermotrichia, namely the actinotrichia and the lepidotrichia found simultaneously in the fin margin of both Actinopterygians and Crossopterygians fins. Although different from lepidotrichial bone, the fine structure of camptotrichia is more reminiscent of it than of the elastoidin-formed actinotrichia.

'Twisted plywood' structure and mineralization in the scales of a primitive living fish Amia calva.

The basal plate of the scales of Amia calva is composed of regular double twisted plywood, as in Latimeria and Dipnoan scales. However, the progressive rotation of the fibrils direction is left-handed in Amia and right-handed in the 'Sarcopterygians'. So, the similarity between these peculiar plywoods is probably the result of convergence. The basal plate of Amia scales is incompletely mineralized. There are numerous calcified ovoid corpuscles which look very like the Mandl's corpuscles of Teleost scales. The mineralization probably progresses essentially by the fusion of these corpuscles, as in Teleost scales, and would be inotropic rather than spheritic.

The fine structure of calcified Mandl's corpuscles in teleost fish scales.

Calcified Mandl's corpuscles present in the internal layer (or fibrillary plate) of the teleost fish scale were studied by transmission and scanning electron microscopy for a better understanding of this special type of mineralization process. The corpuscles show a great variability in their structure, form and surface features depending on the arrangement of the collagen fibrils in the internal layer of the different fish species studied, on the localization of the corpuscles in the scale and on the technical treatment to which the scale is subjected.

Elastoidin actinotrichia in Coelacanth fins: a comparison with teleosts.

In this work, we present the first ultrastructural evidences of actinotrichia in the Coelacanth Latimeria. We describe its actinotrichia with the electron microscope (SEM and TEM) and compare their structure to Teleost actinotrichia. Both elements present similar fine structure, i.e. a periodic cross-striation of 60-65 nm; the plesiomorphic character of actinotrichia is discussed in Osteichthyes.

The fibrous structure of coelacanth scales: a twisted 'plywood'.

Isopedin is a network of collagen bundles present in the scales of most fishes. The scales of coelacanths show a remarkable three-dimensional arrangement of this network which is similar to a regularly twisted plywood. The successive fibrous layers cross at an angle which differs slightly from a right angle. It results that the whole system is twisted. The progressive rotation of the fibril direction is right-handed. Certain preferential orientations of fibrils have been observed, namely parallel to the growth rings. Such arrangments also exist in the embryonic cornea of birds and in the cuticle of certain insects, but do not present such an extensive and regular development.