Biomedical subjects

J Y Sire

Publications and source records attributed to J Y Sire.

At least 19 recordsLinked to original sources

PubMed2002

Developmental differences between cranial bones in the zebrafish (Danio rerio): some preliminary light and TEM observations.

To test whether cartilage bones and dermal bones, which belong to two different units of the vertebrate skeleton, have distinct developmental programs possibly reflected in a different molecular control of their ossification process, we currently investigate the development of some selected cranial bones in the zebrafish, Danio rerio. Here we present some light microscopical and ultrastructural findings with respect to the maxillary bone (a dermal bone that is edentulous in the zebrafish) and the basioccipital bone (a cartilage bone, i.e., with a perichondral phase followed by endochondral invasion). The two bones differ in (a) the area where matrix is first deposited--an unstructured extracellular domain in the former versus intermingling of bone matrix elements with cartilage matrix in the latter--and (b) the progression of ossification--continuously from an initium in the former versus through fusion of separate anlagen in the latter. These findings seem to support the hypothesis that the two types of bone have at least some distinctive features in their developmental programs.

Animals↗

PubMed2001

A fourth teleost lineage possessing extra-oral teeth: the genus atherion (teleostei; atheriniformes).

In the course of an evolutionary and developmental study on the dermal skeleton, our attention was drawn to the existence of denticles located outside the oral cavity in the atheriniform species Atherion elymus. These denticles, attached to the surface of most dermal bones of the head, are especially numerous on the snout, chin and the undersides of the lower region of the head, where they are aligned forming a crenulated keel. Using light, scanning and transmission electron microscopy, we clearly demonstrate the dental (vs bony) nature of these denticles. They are small, conical elements mostly oriented backwards and are not ankylosed to the bone support. Ligaments originating from the internal and external surface of the base of the dentine cone link the denticles to the attachment bone, which itself merges with the bone support below. The denticles have the same form and structure as teeth, from which they differ only in having a larger base and a pulp cavity that is nearly completely filled with secondary dentine by centripetal deposition. This suggests that the denticles have a longer functional history than teeth. Atherion is now the fourth teleost lineage found to develop such denticles on the head.

Animals↗

PubMed2001

Molecular evidence for precambrian origin of amelogenin, the major protein of vertebrate enamel.

Although molecular dating of cladogenetic events is possible, no molecular method has been described to date the acquisition of various tissues. Taking into account the specificity of the major protein in enamel in formation (amelogenin), we were able to develop such a method for enamel. Indeed, because the amelogenin protein is exclusively involved in enamel formation and mineralization and because it lacks pleiotropic effects, this protein is a good candidate to estimate the date of acquisition of this highly mineralized tissue. We searched DNA banks for similarities between the amelogenin sequence and other sequences. Similarities were found only to exon 2 of SPARC (osteonectin) in two protostomians and in eight deuterostomians, and to exon 2 of three SPARC-related deuterostomian genes (SC1, hevin, and QR1). The other amelogenin exons did not reveal significant similarities to other sequences. In these proteins, exon 2 mainly encodes the peptide signal that plays the essential role in enabling the protein to be ultimately localized in the extracellular matrix. We tested the significance of the exon 2 similarities. The observed values were always significantly higher than the expected randomly generated similarities. This demonstrates a common evolutionary origin of this exon. The phylogenetic analyses of exon 2 sequences indicated that exon 2 was duplicated to amelogenin from an ancestral SPARC sequence in the deuterostomian lineage before the duplication of deuterostomian SPARC and SC1/hevin/QR1. We were able to date the origin of the latter duplication at approximately 630 MYA. Therefore, amelogenin exon 2 was acquired before this date, in the Proterozoic, long before the so-called "Cambrian explosion," the sudden appearance of several bilateralian phyla in the fossil record at the Proterozoic-Phanerozoic transition. This sudden appearance has been often suggested to reflect intensive cladogenesis during this period. However, molecular dating of protostomian-deuterostomian divergence and of the cladogenesis among several major clades of Bilateralia lead to a different conclusion: many bilateralian clades were already present during the late Proterozoic. It has previously been proposed that these bilateralians were not mineralized and that they had low fossilization potential. Our results strongly suggest that late Proterozoic fossils possessing a mineralized tissue homologous to enamel might be found in the future.

Amelogenin↗

PubMed2000

Marking zebrafish, Danio rerio (cyprinidae), using scale regeneration.

Tagging or marking small laboratory-bred fish species is not an easy task. This also holds for the zebrafish, Danio rerio, which is widely used throughout the world as a model organism for genetics, developmental biology, etc. We present a simple marking technique based on scale regeneration. A comparative morphological study of various types of zebrafish scales indeed shows that regenerated scales are easily distinguishable from nonregenerated ones. We propose to take advantage of this typical morphology to mark a single or several individuals. This technique, based on a natural biological process, is easy to perform and does not enhance fish mortality in laboratory breeding conditions. It permits assembly of several specimens in a single tank with the possibility of identifying each of them by regenerated-scale coding. Nevertheless, a prerequisite is that the species does not lose and regenerate scales in large numbers in laboratory breeding conditions. To check this, 5,200 scales were removed from a large region of the left flank in 100 zebrafish and the number and position of regenerated scales were statistically analysed. Our results indicate that (1) laboratory-bred zebrafish have only a few regenerated scales (7.48%), (2) the probability of finding a regenerated scale is similar whatever its position in a row (antero-posterior axis), but (3) it differs from one row to another (scales from the back are more frequently lost than those from the pectoral region). This paper presents a procedure to mark small breeding colonies of zebrafish using scale regeneration with the number and position of the scales to be removed with high probability of marking success. J. Exp. Zool. 286:297-304, 2000.

Animal Identification Systems↗

PubMed2000

Development and fine structure of pharyngeal replacement teeth in juvenile zebrafish (Danio rerio) (Teleostei, Cyprinidae).

Teeth are commonly used model systems for the study of epithelial-mesenchymal interactions during organogenesis. We describe here the ultrastructural characteristics of developing pharyngeal replacement teeth in juvenile zebrafish, an increasingly important model organism for vertebrate development. Replacement teeth develop in close association with the dental organ of a functional tooth. Morphogenesis is well advanced prior to the start of cytodifferentiation. Fibrillar enameloid matrix is formed first, followed by the deposition of predentine. Initial mineralization of the enameloid proceeds quickly; maturation involves the presence of ruffled-bordered ameloblasts. Dentine mineralization is inotropic and is mediated by matrix vesicles. Woven-fibred attachment bone matrix is deposited before completion of dentine mineralization. Eruption of fully ankylosed teeth is a fast process and may involve degenerative changes in the pharyngeal epithelium. Mononucleated osteoclasts and clastic cells located in the pulp cavity intervene in tooth resorption prior to shedding. Structural differences with larval, first-generation zebrafish teeth include the presence of dentinal tubules and the absence of an electron-dense covering membrane. Part of these differences may relate to size differences of the teeth. Others, like the site of the replacement tooth bud, suggest that initiation may take place in already committed epithelium from the first initiation event in the larval stage.

Amelogenesis↗

PubMed1999

Spreading, proliferation, and differentiation of the epidermis after wounding a cichlid fish, Hemichromis bimaculatus.

A large superficial wound has been experimentally provoked in the cichlid fish Hemichromis bimaculatus to study the interactions between the epidermal cells and the substrate on which they spread, on the one hand, and the restoration of the subepidermal tissues and the epithelial-mesenchymal interactions preceding scale regeneration, on the other hand. The re-epithelialization process, e.g., migration, spreading, differentiation, and proliferation of the epidermal cells, has been followed step by step, using light, scanning and transmission electron microscopy, and tritiated thymidine incorporation, until complete reorganization of the healing epidermis. Wound healing is fast (500 microm/hr) and proceeds centripetally from the wound margins. The epidermal cells spread on a wound surface which is composed of two different matrices: the remains of basement membrane materials covering the scale-pockets, and collagen fibrils of cut dermal strips. Even though both matrices favour cell spreading and attachment, migrating cells show a different behaviour. The re-epithelialization of the wound follows an orderly sequence similar to amphibian and mammalian wound healing, i.e., a "leap frog" mechanism of cell locomotion involving three epidermal layers. The basal layer cells, which spread on the substrate, and the superficial layer cells which protect the epidermis, differentiate first. Whatever the type of substrate over which the epithelium spreads (basement membrane material or collagen fibrils), the epidermal basal layer cells differentiate as soon as they become attached. The incorporation of tritiated thymidine has revealed that there is no proliferation in the healing epidermis until after complete closure of the wound, but that the rapid re-epithelialization of the large surface requires the recruitment of epidermal cells at the wound margins. The present study offers new data on the dynamics of re-epithelialisation and on the resistance of cichlid skin to such wounds. It is also clearly shown that the epidermal basal layer cells differentiate rapidly, a step which is interpreted as the first stage of epithelial-mesenchymal interactions that will lead to scale regeneration.

Animals↗

PubMed1998

Early development of the zebrafish (Danio rerio) pharyngeal dentition (Teleostei, Cyprinidae).

In order to build a reference system to assess ongoing in vitro and in situ hybridisation experiments on epithelial-mesenchymal interactions governing odontogenesis in the zebrafish, we describe here the generation of the pharyngeal dentition, and the histological development of teeth up to fourteen days post-fertilization, using serial semithin sections, handmade and computer-assisted reconstructions and transmission electron microscopy. The tooth pattern in larval zebrafish is generated in a predictable, and bilaterally symmetrical manner from shortly before hatching onwards. Characteristics related to tooth development and structure differ considerably from those seen in juvenile specimens and those described for other bony fishes. Particular features related to the cyprinid condition include the complex epithelial connectivity and the mode of attachment of the teeth.

Aging↗

PubMed1998

Evolution of patterns and processes in teeth and tooth-related tissues in non-mammalian vertebrates.

The evolutionary links that exist between odontodes and organs that are phylogenetically related to them (teeth and scales) suggest the use of comparative approaches to study these structures. Part one of this review briefly introduces current ideas on how the pattern of odontodes and odontode-derived tissues has been established during evolution to yield the diversity of odontode-related organs currently observed in nature in the cranial and postcranial skeleton. This introductory survey is used to highlight aspects of the developmental processes underlying the formation of some of these organs and the resemblance their development bears to odontogenesis. Part two provides a concise survey of the diversity of tooth structure in the different classes of extant vertebrates, in particular with reference to enamel/enameloid and dentine structure, and tooth attachment. Against this background, the current state of knowledge is reviewed with regard to developmental mechanisms involved in non-mammalian odontogenesis. Common structure and similarities in development demonstrate that teeth and odontode derivatives should not be considered subjects of separate lines of research. On the contrary, results acquired in one of these fields are relevant to the other and may disclose model species that are relevant to studies on mammalian odontogenesis.

Animals↗

PubMed1998

Evolution of the amelogenin gene in toothed and toothless vertebrates.

Amelogenin proteins constitute the major organic contents in forming enamel, but some previously published data indicate that the amelogenin gene could be present in several species lacking teeth or functional enamel. Therefore, amelogenin could have another, still unknown, function other than contributing to enamel formation. In order to test this hypothesis, the presence of this gene has been searched for in various vertebrates. We first compared the mammalian amelogenin sequences available in the literature to obtain the best chance of a successful detection of this gene in phylogenetically distant species. Using this analysis, we have shown that the occurrence of the amelogenin gene in the Y chromosome of primates and of an artiodactyl species is probably due to two independent duplications from genes on the X chromosome. Primers for PCR have therefore been synthesized and tested in eight species, five possessing teeth (human, two phylogenetically distant lizards, and two phylogenetically distant actinopterygians) and three lacking teeth (chicken and two phylogenetically distant turtles). The amelogenin gene has been detected in all species except those lacking teeth. This result indicates that the unique role of amelogenin in amniotes is to contribute to enamel formation.

Amelogenin↗

PubMed1997

Immunodetection of amelogenin-like proteins in the ganoine of experimentally regenerating scales of Calamoichthys calabaricus, a primitive actinopterygian fish.

BACKGROUND: The account of the present study is to test our previous hypothesis that ganoine, a highly mineralized layer found at the scale surface of primitive actinopterygian fish, could be homologous with the enamel covering the crown of vertebrate teeth. METHODS: Immunocytochemical techniques have been carried out on regenerating scales of a primitive polypterid, Calamoichthys calabaricus, with three antibodies to mammalian amelogenins. RESULTS: The present study provides the first evidence that ganoine contains molecules which cross-react with mammalian amelogenin proteins. CONCLUSIONS: This result is consistent with our previous findings that ganoine and enamel can be considered as homologous tissues. Moreover, the presence in ganoine of a primitive actinopterygian of amelogenin-like proteins, which share epitopes with amelogenins of mammalian enamel, indicates that the gene(s) coding for these proteins appeared earlier than previously suggested and supports the hypothesis that amelogenins show a highly conserved structure through vertebrate evolution.

Amelogenin↗

PubMed1997

Scale development in zebrafish (Danio rerio).

In the course of an extensive comparative, structural and developmental study of the cranial and postcranial dermal skeleton (teeth and scales) in osteichthyan fishes, we have undertaken investigations on scale development in zebrafish (Danio (Brachydanio) rerio) using alizarin red staining, and light and transmission electron microscopy. The main goal was to know whether zebrafish scales can be used as a model for further research on the processes controlling the development of the dermal skeleton in general, especially epithelial-mesenchymal interactions. Growth series of laboratory bred specimens were used to study in detail: (1) the relationship of scale appearance with size and age; (2) the squamation pattern; and (3) the events taking place in the epidermis and in the dermis, before and during scale initiation and formation, with the aim of searching for morphological indications of epithelial-mesenchymal interactions. Scales form late in ontogeny, generally when zebrafish are more than 8.0 mm in standard length. Within a population of zebrafish of the same age scale appearance is related to standard length, but when comparing populations of different age the size of the fish at scale appearance is also related to age. Scales always appear first in the posterior region of the body and the squamation then extends anteriorly. Scales develop in the dermis but closely apposed to the epidermal-dermal boundary. Cellular modifications occurring in the basal layer of the epidermis and in the dermis before scale formation clearly indicate that the basal epidermal cells differentiate first, before any evidence of differentiation of the progenitors of the scale-forming cells in the dermis. This strongly suggests that scale differentiation could be initiated by the epidermal basal layer cells which probably produce a molecular signal towards the dermis below. Subsequently dermal cells accumulate close to the epidermis, and differentiate to form scale papillae. The late formation of the scales during ontogeny is due to a late colonisation of the dermis by the progenitors of the scale-forming cells. Because of their late formation during ontogeny and of their regular pattern of development, scales in zebrafish represent a good model for further investigations on the general mechanisms of epithelial-mesenchymal interactions during dermal skeleton development, and in particular for the study of the gene expression patterns.

Animals↗

PubMed1995

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.

Alkaloids↗

PubMed1994

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.

Animals↗

PubMed1992

Development of cartilage and bone tissues of the anterior part of the mandible in cichlid fish: a light and TEM study.

The present paper presents ultrastructural details of chondrogenesis of Meckel's cartilage and of ossification of its associated peri- and parachondral bones in a teleost fish, the cichlid Hemichromis bimaculatus. We have distinguished four stages during chondrogenesis, each of which is characterized by specific cellular and matrix features: blastema, primordium, differentiated cartilage and cartilage surrounded by perichondral bone. The blastema is characterized by prechondroblasts and the lack of cartilage matrix; the primordium by chondroblasts and the onset of secretion of matrix of fibrillar and granular nature; differentiated cartilage is characterized by chondrocytes and larger amounts of typical hyaline cartilage matrix. Once perichondral bone is laid down, the chondrocytes show degenerative features but not true hypertrophy. Differentiation of the cartilage cells is attended with cytoplasmic changes indicative of an increasing secretory activity. There is a regional calcification of the cartilage matrix by fusion of calcospherites. Chondrogenesis of the symphyseal area is continuous with that of the rami but starts slightly later. Formation of perichondral bone at the cartilage surface is attended with the deposition of a transitional zone apparently containing a mixture of the two matrices. The role of the perichondral cells is discussed and it is proposed that they may contribute to the formation of the two matrices. The transitional zone may then result either from a diffusion process or from the simultaneous deposition of elements of the two matrices. Growth of the cartilage is argued to be largely the result of matrix secretion, except in the symphyseal area where appositional growth probably occurs until the region is completely covered by perichondral bone. This paper provides a basis for further studies on the developmental interactions between cartilage, bone and teeth during mandibular development in cichlids.

Animals↗

PubMed1992

Bone and cartilage resorption in relation to tooth development in the anterior part of the mandible in cichlid fish: a light and TEM study.

This paper presents ultrastructural features of the contact region between particular tooth germs and Meckel's cartilage prior to, during, and after initial resorption of the perichondral bone and of the cartilage in the cichlids Hemichromis bimaculatus and Astatotilapia burtoni. Imminent resorption opposite such teeth is announced by the presence, in this region, of a particular cell type, considered to be a stage in the cytodifferentiation of osteoclasts. Slightly later, an osteoclast with typical ruffled border is seen to open a fenestra in the perichondral bone which surrounds Meckel's cartilage. Although the action of the osteoclast is directed primarily towards the bone, it may also affect, to a much lesser extent, the underlying uncalcified cartilage. Typically, fibroblast-like cells invade the resorption cavity along with the osteoclast; the tooth germ soon follows. Capillaries are seen to invade the cartilage only at a later stage when a large cavity has been established. It is proposed that the fibroblast-like cells may have a dual function: degradation of cartilage and deposition of new bone. Although these processes are normally limited to the area surrounding tooth germs at specific loci, tooth germs in other positions may sometimes be seen invade the cartilage. They do so either passively, because of the existence of such a cavity, or as a result of their own resorption-inducing activity. Whatever the mechanism, attachment bone is being deposited within the erosion cavity and on the surface of the exposed perichondral bone. The stimuli possibly eliciting resorption of Meckel's cartilage are discussed. It is hypothesized that pressure exerted by the growing tooth germ may stimulate the osteoblasts covering the bone surface and, in this way, provoke osteoclastic bone resorption.

Animals↗

PubMed1989

Scales in young Polypterus senegalus are elasmoid: new phylogenetic implications.

A component of the basal plate which has a plywood-like organization similar to that of the elasmoid scales of teleosts is described in the scales of Polypterus senegalus for the first time. The origin and development of this structure is studied in young (50-117 mm, standard length) and adult (225 and 240 mm) specimens using light and electron microscopy. In 50 mm fish, the scales are imbricated and composed mainly of a succession of orthogonal collagen layers forming a plywood-like structure, the isopedin. The outer surface of the scale is ornamented locally by irregular patches of collagenous material. The layers are not mineralized, whereas the superficial patches are well calcified. The isopedin thickens until it has 12-15 layers and then stops growing (88 mm fish). It mineralizes irregularly from its upper part, and two vascular regions, surrounded by woven-fibered osseous material, form on the outer and deeper surfaces of the isopedin. These regions thicken while the vascular canals close by centripetal deposition of parallel-fibered osseous tissue. The outer region is the superficial part of the mature scale (called here osteodentin), which is covered by the ganoine deposited by the epidermal cells. The deeper part constitutes the definitive basal plate, composed of parallel-fibered osseous tissue. The results show 1) that the young ganoid scales of Polypterus senegalus have a structure similar to that of typical elasmoid scales; and 2) that the isopedin structure does not change during ontogeny and so represents a permanent record of the first ontogenetic stages. The phylogenetic implication of these results is that the elasmoid scales of teleosts arose by a process of paedomorphosis.

Animals↗

PubMed1988

Cytoskeletal organization and collagen orientation in the fish scales.

Immunofluorescence and electron microscopy were used to analyze the relationships between the organization of collagen fibrils in elasmoid scales, and the orientation of microtubules and actin microfilaments in the scleroblasts producing this collagenous stroma. Attention was focused on the basal plate of the scales because of the highly ordered three-dimensional arrangement of the collagen fibrils in superimposed plies forming an acellular plywood-like structure. The collagen fibrils are synthesized by the scleroblasts forming a monolayered pseudo-epithelium, the hyposquama, at the lowest surface of the scale. Fully developed scales with a low collagen deposition rate were compared with regenerating scales active in fibrillogenesis. When an ordered array of the collagen fibrils is found, the innermost collagen fibrils are coaligned with microtubules and actin microfilaments. Thus, because of this coalignment, microtubules and actin microfilaments of the hyposquamal scleroblasts are subjected to consecutive alterations during the formation of the plies of the basal plate. The sequence of events when the collagen fibrils change their direction from one ply to the other in the basal plate is deduced from immunofluorescence and phase-contrast-microscopic observations. During the formation of the orthogonal plywood-like structure in the regenerating scales, first microtubules may change their curse with a rotating angle of about 90 degrees; then, actin microfilaments are disorganized and reorganized by interacting mechanically with the microtubules with which they are coaligned. Collagen fibrils are synthesized in a direction that is roughly perpendicular to that of the preceding ply. The unknown signals inducing the change in direction of the cytoskeleton may be transmitted throughout the hyposquama via gap junctions.

Animals↗

PubMed1988

Evidence that mineralized spherules are involved in the formation of the superficial layer of the elasmoid scale in cichlids Cichlasoma octofasciatum and Hemichromis bimaculatus (Pisces, Teleostei): an epidermal active participation?

The region between the epidermis and the surface of the overlapping part of scales has been studied in two cichlid teleosts using transmission electron microscopy. In a few specimens only, numerous mineralized spherules (approximately 1 micron in diameter) are observed in the loose dermis and at the scale surface, and form a large part of the superficial outer limiting layer of the scale. In the loose dermis (stratum laxum) and close to the scale surface spherules are either free or included in dermal cells. When free, they are dispersed in the extracellular matrix of the dermis, among the fibrils of anchoring bundles, and fused with the scale surface. When included in cell vacuoles, they lie close to the lamina densa and to the scale surface. Steps in the formation of the mineralized spherules are only seen in the lamina densa of the basement membrane. The spherules contain needle-like mineral crystals radially orientated and an organic matrix of stippled material and dense granules, some of which form concentric lines around the centre of the spherules. The results suggest that mineralized spherules form in the lamina densa and pass through the dermis to the scale surface in which they are incorporated.

Animals↗