The structure of the osteoderms in the Gekko: Tarentola mauritanica

Histological and cytological analysis reveals that the osteoderms of Tarentola mauritanica are composed of an outer part superimposed on a basal region. The structure of both parts can be related to that of the surrounding dermis. The basal part of the osteoderms, inserted in the dense dermis, is made up of abundant closely packed collagen fibrils that orient the mineral deposit. The outer part, located in the superficial loose dermis, is crossed by few bundles of mineralized collagen fibrils arising from the basal part. These bundles connect the osteoderm to the overlying loose dermis. The outer superficial part is characterized by the presence of mineralized globules surrounding the mineralized collagen bundles. In these globules, the crystals are deposited on a microfibrillar matrix rich in acidic mucosubstances and composed of radially oriented, tangled microfilaments that lie among the collagen bundles. The two different mineralizing systems in the osteoderms of Tarentola mauritanica may reflect two different organic matrices. The mineral is deposited in a preexisting dermal tissue, as a "metaplastic ossification," and is another expression of the potential retained by the reptilian dermis to form mineralized structures.     

Collagen fibril assembly and deposition in the developing dermis: segmental deposition in extracellular compartments

The hierarchy of extracytoplasmic compartmentalization and fibrillar organization as well as the assembly and deposition of collagen fibrils was characterized in the 15-day chick embryo dermis using transmission electron microscopy. At least two levels of extracellular compartmentalization are recognizable at this stage of dermal development. The first compartment consists of a series of narrow channels containing single or small groups (less than 5) of collagen fibrils. These channels course deep within the cell and are open to the extracellular space. The second extracellular compartment consists of fibrils grouped as small bundles in close association with the cell surface and is most often defined by a single fibroblast. A third level of fibril organization and compartmentalization is sometimes apparent at this stage of dermal development consisting of laterally associated bundles, more characteristic of the mature dermis. This compartment is associated with the fibroblast surface, but is less well defined than the fibril channels or bundle-forming compartments. Dermal collagen fibrils within bundles are discontinuous. Numerous fibrils ends are identified from serial sections and the ends gradually taper. These data indicate that the dermal fibroblast compartmentalizes the extracellular space and deposits collagen fibril segments during dermal morphogenesis. A model for the genesis of the extracellular compartments and their role in collagen fibrillogenesis and development of regularly arranged connective tissues, tendon, and cornea has been proposed. Dermal development conforms to this model and we suggest that extracytoplasmic compartmentalization of the steps in matrix assembly and segmental deposition of collagen fibrils are important mechanisms in the development of a wide variety of connective tissues.     

Structure and Development of the Odontodes in an Armoured Catfish,Corydoras aeneus (Siluriformes,Callichthyidae)

Abstract In some living osteichthyans (e.g. the armoured catfishes) the postcranial dermal skeleton exhibits tooth-like structures (odontodes) similar to those present in the dermal skeleton of the ancient craniates. We have undertaken this work to compare odontode with tooth development, structure, attachment to a bony support and replacement. We studied the odontodes fixed on the scutes (i.e. postcranial dermal plates) in a growth series of Corydoras aeneus using light, scanning and transmission electron microscopy. Odontodes are constituted of a pulp cavity surrounded by a cone of dentine itself capped with hypermineralized substance. The pulp cavity is devoid of nerves and blood vessels and there are no odontoblastic processes in the dentine. The dentine cone is firmly attached to a circular bony protuberance of the scute surface, the pedicel or attachment bone, by means of a ligament. An odontode anlage develops as a small invagination of a dermal papilla projecting into the epidermis, the basal cell layer of which constitutes a dental epithelium. First, dentine is deposited, next the hypermineralized substance, then the ligament and attachment bone. Odontodes develop in two positions with regard to the scute surface: a primary position when new odontodes form at the posterior border of the enlarging scute; a secondary position when new odontodes replace old odontodes that have been shed during thickening of the scute. In this case, the ligament and part of the base of the dentine cone are resorbed but not the pedicel of attachment bone, which is covered by deposition of scute matrix after the odontode has been shed. Within the scute matrix, the embedded pedicels of successive generations of odontodes are preserved, forming piles in the scutes of adult specimens.     

Striae albae: a morphological study on the human skin

Specimens of abdomen skin, comprising alternate areas of striae albae and healthy skin, were removed during surgical lipectomy from multiparous and obese women between the ages of 24 and 53 years. A flattening and thinning of the striae albae surface and the almost complete disappearance of dermal papillae was observed in paraffin and thin sections. The papillary dermis was found to be almost completely replaced by straight bundles of collagen fibres running parallel to the skin surface. Immunofluorescence data revealed in these bundles high positivity for type I collagen. The underlying reticular dermis was also found to contain large densely packed bundles of collagen fibres running parallel to the skin surface. Both papillary and reticular dermis collagen fibres were mainly arranged orthogonally to the main axis of the stria. Furthermore, the density of the collagen fibre bundles and the diameter of the collagen fibrils was found to be greater than that of the clinically healthy skin. A larger number of elastic fibres, which presented an abnormal ultrastructural appearance, were visible in pathological papillary and reticular dermis.     

Collagen degradation in the rabbit skin during short-term tissue culture

Full thickness rabbit skin explants were cultured on plastic dish for 1 week and the sequential morphological changes were examined daily by light and electron microscopy. During the cultured period, bundles of dermal collagen fibres gradually loosened and were removed from the upper dermis and from the cut margin of the explant, which was covered by a sheet of migrating epidermal cells. In these areas, cells containing phagocytosed collagen fibrils were observed from the 3rd day to the end of the culture period. These cells containing phagocytosed collagen fibrils included dermal fibroblasts and macrophages, epidermal keratinocytes and endothelial cells lining blood vessels. The presence of acid phosphatase activity in vacuoles containing the collagen fibrils suggested that intracellular degradation of collagen was occurring. In addition, extracellular collagen degradation was recognized around fibroblasts and beneath the migrating epidermis by the high collagenolytic activity at these sites. These findings suggest that both intra- and extracellular collagen degradation may participate in collagen removal from dermal connective tissue in cultured skin explants.     

Formation and structure of scales in the Australian lungfish,neoceratodus forsteri (Osteichthyes: Dipnoi)

The large elasmoid scales of the Australian lungfish, Neoceratodusforsteri, are formed within the dermis by unpigmented scleroblasts, growing within a collagenous dermal pocket below a thick glandular epidermis. The first row of scales, on the trunk of the juvenile lungfish, appears below the lateral line of the trunk, single in this species, at around stage 53. The scales, initially circular in outline, develop anteriorly and posteriorly from the point of initiation in the mid‐trunk region, and rows are added alternately below the line, and above the line, until they reach the dorsal or ventral midline, or the margins of the fins. Scales develop later on the ventral surface of the head, from a separate centre of initiation. Scales consist of three layers, all produced by scleroblasts of dermal origin. The outermost layer of interlocking plates, or squamulae, consists of a mineralised matrix of fine collagen fibrils, covered by unmineralised collagen and a single layer of cells. Squamulae of the anterior and lateral surfaces are ornamented with short spines, and the mineralised tissue of the posterior surface is linked to the pouch by collagen fibrils. The innermost layer, known as elasmodin, consists of bundles of thick collagen fibrils and cells arranged in layers. An intermediate layer, made up of collagen fibrils, links the outer and inner layers. The elasmoid scales of N. forsteri can be compared with scale types among other osteichthyan groups, although the cellsand canaliculi in the mineralised squamulae bear littleresemblance to typical bone. J.Morphol., 2012. © 2011 Wiley Periodicals, Inc.     

Collagen types I, III, and V in human embryonic and fetal skin

The dermis of human skin develops embryonically from lateral plate mesoderm and is established in an adult-like pattern by the end of the first trimester of gestation. In this study the structure, biochemistry, and immunocytochemistry of collagenous matrix in embryonic and fetal dermis during the period of 5 to 26 weeks of gestation was investigated. The dermis at five weeks contains fine, individual collagen fibrils draped over the surfaces of mesenchymal cells. With increasing age, collagen matrix increases in abundance in the extracellular space. The size of fibril diameters increases, and greater numbers of fibrils associate into fiber bundles. By 15 weeks, papillary and reticular regions are recognized. Larger-diameter fibrils, larger fibers, denser accumulations of collagen, and fewer cells distinguish the deeper reticular region from the finer, more cellular papillary region located beneath the epidermis. The distribution of collagen types I, III, and V were studied at the light microscope level by immunoperoxidase staining and at the ultrastructural level by transmission (TEM) and scanning electron microscopy (SEM) with immunogold labeling. By immunoperoxidase, types I and III were found to be evenly distributed, regardless of fetal age, throughout the dermal and subdermal connective tissue with an intensification of staining at the dermal-epidermal junction (DEJ). Staining for types III and V collagen was concentrated around blood vessels. Type V collagen was also localized in basal and periderm cells of the epidermis. By immuno-SEM, types I and III were found associated with collagen fibrils, and type V was localized to dermal cell surfaces and to a more limited extent with fibrils. The results of biochemical analyses for relative amounts of types I, III, and V collagen in fetal skin extracts were consistent with immunoperoxidase data. Type I collagen was 70-75%, type III collagen was 18-21%, and type V was 6-8% of the total of these collagens at all gestational ages tested, compared to 85-90% type I, 8-11% type III, and 2-4% type V in adult skin. The enrichment of both types III and V collagen in fetal skin may reflect in part the proportion of vessel- and nerve-associated collagen versus dermal fibrillar collagen. The accumulation of dermal fibrillar collagen with increasing age would enhance the estimated proportion of type I collagen, even though the ratios of type III to I in dermal collagen fibrils may be similar at all ages.     

The twisted collagen network of the box-fish scutes

The present article describes the three-dimensional arrangement of collagen fibrils in dermal plates of different species of Ostraciidae. These dermal plates or 'scutes' are transformed scales, which have a polygonal shape and form a rigid tiling. They are natural composites, associating a fibrous network with a mineral deposit lying at two different levels of the scute, the 'ceiling' and the 'floor', plus a set of similarly mineralized walls joining the two levels. The three-dimensional structure of the collagen network can be compared to that of 'plywood': fibrils align parallel within superposed layers of uniform thickness, and their direction changes from layer to layer. In the dermal plate, two types of plywood have been evidenced: (1) one lying between the two mineralized plates, where the orientation of fibrils rotates continuously, and (2) one under the lower plate, with thick layers of fibrils, each showing a constant orientation, but abrupt angular changes are observed at the transition from one layer to the following one. In oblique sections, both types of plywood reveal large series of arced patterns, testifying to a twisted arrangement of collagen fibrils, analogous to the arrangement of molecules or polymers in cholesteric liquid crystals. The network is reinforced by some collagen fibrils running unidirectionally and almost normally to the lamellate structure. Moreover in the overall organization of the scute, these plywood systems form a set of nested boxes. This original architecture is compared to the arrangement of the collagenous network previously described in most fish scales and in other extracellular matrices.     

Dermal tracts in frog skin: fibronectin pathways for cell migration

The dermis of the frog skin (Rana esculenta) displayed a remarkable organization of vertical and horizontal tracts. Vertical thick tracts connected the dermal Stratum spongiosum with the subcutaneous tissue. Horizontal thin tracts were found alongside and contiguous to them. The thick tracts were sheathed by collagen fibrils of the Stratum compactum which were vertically oriented (i.e. parallel to the axes of the tracts) according to the horizontal and orthogonal arrangement of the collagen bundles of the Stratum compactum. The thin tracts devoid of collagenous sheath were formed by clear spaces between superimposed collagen bundles of the dermal Stratum compactum. On vertical sections, the thick tracts were seen to contain fibronectin (FN), detected by indirect immunoperoxidase. Continuous vertical FN lines were centred in these tracts. On horizontal sections, a clear zone around these FN-centred lines was also sheathed by FN. The thick tracts contained flattened pigmentary cells and fibroblasts; these cells were FN-outlined. The thin tracts contained patches of FN and FN-outlined fibroblasts. In culture, in vertical thick tracts, both pigmentary cells and fibroblasts disappeared when antiserum to FN was added to the culture medium. This suggested that thick tracts were pathways allowing pigmentary cells to move upward or downward between their usual upper dermal and lower subcutaneous localizations. Fewer fibroblasts were found in the thin tracts in the presence of antiserum to FN.     

Reconstitution of human epidermis in vitro is accompanied by transient activation of basal keratinocyte spreading

Frozen human cadaver skin obtained from the skin bank was thawed and incubated in serum-free medium for 1–2 days, after which the original epidermis could be removed mechanically. Transmission electron microscopic observations showed that the dermal matrix remaining behind contained intact bundles of collagen fibrils but no live cells and that a continuous lamina densa persisted in the basement membrane region. Indirect immunofluorescence analyses demonstrated linear staining of the basement membrane region by antibodies against laminin and type IV collagen and discontinuous staining with antibodies against fibronectin. Scanning electron microscopic observations revealed a normal topographical arrangement of dermal matrix papilla and interspersed crypts on the surface of the matrix. Epidermal cells placed on the dermal matrix attached in 1–2 h and spread by 24 h. After 1 week of culture the epidermis was reconstituted, at which time approximately 30% of the epidermal cells were basal keratinocytes and the remainder were more differentiated keratinocytes. A high degree of differentiation of the reconstituted epidermis was shown by the formation of hemidesmosomes along the basement membrane, the formation of desmosomes characterized by intercellular dense lines, and the presence of a cell layer containing keratohyalin granules. At various times during epidermal reconstitution, cells were harvested and tested in short-term assays for adhesion to fibronectin substrata. During the first several days there was a transient activation of basal keratinocyte spreading analogous to the modulation of keratinocyte spreading that we have observed during epidermal reconstitution in vivo.     

Differential ultrastructural aberrations of collagen fibrils in Ehlers-Danlos syndrome types I–IV as a means of diagnostics and classification

Among the different subtypes of Ehlers-Danlos syndrome (EDS), the dominant types I–III have, so far, been uninformative biochemically and molecular genetically, and diagnostic problems with subgroup boundaries often arise. We have investigated the ultrastructural pattern of connective tissue macromolecules in skin biopsy specimens of some 85 patients aged 4 months-54 years who exhibit clinical symptoms or the suspicion of EDS I–IV. Based on the differential features of collagen fibrils and ground substance material, four distinct groups could be established. Group I (clinically EDS type I) showed disorganized collagen bundles and dense aggregations of collagen fibrils with bizarre shapes. Group II (clinically varying from EDS types I–III) revealed collagen bundles that regularly contained numerous “composite collagen fibrils” with enlarged “flower-like” cross-sections and rope-like longitudinal sections, often associated with increased amounts of matrix substances in the form of electron-dense irregular strands and filaments in a branched network. Group III (clinically EDS types II–III) presented smaller isolated collagen flowers and ropes associated with excessive filamentous ground substance material and flocculent material. Group IV (with clinical symptoms of EDS type IV) had a dermis thinned to one third of the normal and a reduced number of collagen bundles with small diameter fibrils. In 13 patients, the abnormal ultrastructural dermal architecture did not coincide with any of these four groups or with the pattern of any other inherited connective tissue disorder. In 16 additional patients with mostly mild clinical symptoms, such as muscle weakness and small joint hyperlaxity, no ultrastructural aberrations could be found. Even though the primary defects underlying the respective aberration of the collagen fibrils are still unknown, the differential ultrastructural changes of the collagen fibrils together with clinical symptoms should, as in other heterogeneous genetic disorders, facilitate the (provisional?) classification of EDS and permit the diagnosis of individual cases.     

Immunofluorescent localization of collagen types I,III and IV,fibronectin, laminin,and basement membrane proteoglycan in developing mouse skin

Summary The distribution of various extracellular matrix components was studied in frozen sections of embryonic (14–18 days) and early postnatal (birth and 4 days post parturn) dorsal mouse skin using monospecific antibodies and indirect immunofluorescence. Basement membrane zone components — type IV collagen, laminin and heparan sulphate proteoglycan — were found to be uniformly and unchangingly distributed along the dermal-epidermal junction. In contrast, the distribution of interstitial matrix components — types I and III collagen, and fibronectin — was heterogeneous and varied with the stages of hair development. Collagens became sparse and were eventually completely removed from the prospective dermal papilla and from a one-cell-thick sheath of dermal cells around hair buds. They remained absent from the dermal papilla throughout hair organogenesis. Fibronectin was always present around dermal papilla cells and was particularly abundant along the dermal-epidermal junction of hair rudiments, as well as underneath hair buds. In contrast, in interfollicular skin, collagens accumulated in increasing density, while fibronectin became progressively sparser. It thus appears that interstitial collagens and fibronectin are distributed in a manner which is related to hair morphogenesis. In morphogenetically active regions, collagen density is low, while that of fibronectin is high. Conversely, in histologically stabilized zones, collagen is abundant and fibronectin is sparse. This microheterogeneous distribution of interstitial collagens and of fibronectin might thus constitute part of the morphogenetic message that the dermis is known to transmit to the epidermis during the development of skin and of cutaneous appendages.     

A biomechanical mathematical model for the collagen bundle distribution-dependent contraction and subsequent retraction of healing dermal wounds

A continuum hypothesis-based, biomechanical model is presented for the simulation of the collagen bundle distribution-dependent contraction and subsequent retraction of healing dermal wounds that cover a large surface area. Since wound contraction mainly takes place in the dermal layer of the skin, solely a portion of this layer is included explicitly into the model. This portion of dermal layer is modeled as a heterogeneous, orthotropic continuous solid with bulk mechanical properties that are locally dependent on both the local concentration and the local geometrical arrangement of the collagen bundles. With respect to the dynamic regulation of the geometrical arrangement of the collagen bundles, it is assumed that a portion of the collagen molecules are deposited and reoriented in the direction of movement of (myo)fibroblasts. The remainder of the newly secreted collagen molecules are deposited by ratio in the direction of the present collagen bundles. Simulation results show that the distribution of the collagen bundles influences the evolution over time of both the shape of the wounded area and the degree of overall contraction of the wounded area. Interestingly, these effects are solely a consequence of alterations in the initial overall distribution of the collagen bundles, and not a consequence of alterations in the evolution over time of the different cell densities and concentrations of the modeled constituents. In accordance with experimental observations, simulation results show furthermore that ultimately the majority of the collagen molecules ends up permanently oriented toward the center of the wound and in the plane that runs parallel to the surface of the skin.

     

Collagen fibrils of an invertebrate (Sepia officinalis) are heterotypic: immunocytochemical demonstration

Collagen fibrils from the dermis of Sepia officinalis were processed for immunoelectron microscopy to reveal reactions to antibodies against mammalian types I, III, and V, teleost type I and cephalopod type I-like collagens, by single and double immunogold localization. The fibrils were observed: (a) in suspensions of prepared fibrils, (b) in ultrathin sections of embedded fibril preparations, and (c) in ultrathin sections of dermal tissue. Some samples were subjected to acetic acid or urea dissociation. It was found that collagen fibrils from Sepia dermis are heterotypic in that they are composed of type I-like and type V collagens. Type I-like collagen epitopes were present mainly at the periphery of the fibrils; type V collagen epitopes were present throughout the fibrils. This is the first demonstration that collagen fibrils from an invertebrate are heterotypic, suggesting that heterotypy may be an intrinsic characteristic of the fibrils of fibrillar collagens, independent of evolutionary or taxonomic status.     

Copolymerization of pNcollagen III and collagen I. pNcollagen III decreases the rate of incorporation of collagen I into fibrils, the amount of collagen I incorporated, and the diameter of the fibrils formed

Previous observations suggested that pNcollagen III, the partially processed form of type III procollagen, coats fibrils of collagen I and thereby helps regulate the diameter of fibrils formed by collagen I. The previous observations, however, did not exclude the possibility that pNcollagen III was deposited on preformed collagen I fibrils after the fibrils were assembled. Here, mixtures of pNcollagen III and collagen I were generated simultaneously by enzymatic cleavage of precursor forms of the proteins. The results demonstrated that pNcollagen III forms true copolymers with collagen I. The presence of pNcollagen III both inhibited the rate at which collagen I assembled into fibrils and decreased the amount of collagen I incorporated into fibrils at steady-state equilibrium. In addition, the results demonstrated that copolymerization of pNcollagen III with collagen I generated fibrils that were thinner than fibrils generated under the same conditions from collagen I alone. Increasing the initial molar ratio of pNcollagen III to collagen I in the solution-phase increased the amount of pNcollagen III copolymerizing with collagen I and progressively decreased the diameter of the fibrils. Therefore, the copolymers were heterogeneous in that the stoichiometry of the two monomers in the fibrils varied. The results are consistent with a model in which pNcollagen III can regulate the diameter of collagen I fibrils by coating the surface of the fibrils and thereby allow tip growth but not lateral growth of the fibrils.     

Evidence for participation of the epidermis in the deposition of superficial layer of scales in zebrafish (Danio rerio): A SEM and TEM study

Comparative studies on scale structure and development in bony fish have led to the hypothesis that elasmoid scales in teleosts could be dental in origin. The present work was undertaken to determine whether the scales in zebrafish (Danio rerio), a species widely used in genetics and developmental biology, would be an appropriate focus for further studies devoted to the immunodetection of dental components or to the detection of the expression of genes coding for various dental proteins in fish scales. The superficial region of mature and experimentally regenerated scales and its relationships to the epidermal cover were studied in adult zebrafish using scanning (SEM) and transmission (TEM) electron microscopy. The elasmoid scales are relatively large, thin, and are located in the upper region of the dermis, close to the epidermis. In adults, the surface of the posterior region appears smooth at the SEM level and is entirely covered by the epidermis. During regeneration, the relationship of the epidermal cover to the scale surface is established within 4 days. This interface is easier to study in regenerating than in mature scales because the former are poorly mineralized. TEM revealed that: (1) the epidermis is in direct contact with the scale surface, from which it is separated only by a basement membrane-like structure, (2) there are no dermal elements at the scale surface except at the level of grooves issuing from the focus and crossing the scale surface radially, (3) the mineral crystals located in this superficial region are perpendicular to the scale surface, whereas those located deeper within the collagenous scale matrix are randomly disposed, and (4) when decalcified, the matrix of the superficial region of the scale appears devoid of collagen fibrils but contains thin electron-dense granules, some of which are arranged into layers. The continuous epidermal covering, the absence of dermal elements, as well as the fine structure of the matrix and its type of mineralization, strongly suggest that epidermal products, possibly enamel-like proteins, are deposited at the scale surface and contribute to the thickening of the upper layer in zebrafish scales. J. Morphol. 231:161–174, 1997. © 1997 Wiley-Liss, Inc.     

Reduced type I collagen utilization: a pathogenic mechanism in COL5A1 haplo-insufficient Ehlers-Danlos syndrome

To examine mechanisms by which reduced type V collagen causes weakened connective tissues in the Ehlers-Danlos syndrome (EDS), we examined matrix deposition and collagen fibril morphology in long-term dermal fibroblast cultures. EDS cells with COL5A1 haplo-insufficiency deposited less than one-half of hydroxyproline as collagen compared to control fibroblasts, though total collagen synthesis rates are near-normal because type V collagen represents a small fraction of collagen synthesized. Cells from patients with osteogenesis imperfecta (OI) and haplo-insufficiency for proalpha1(I) chains of type I collagen also incorporated about one-half the collagen as controls, but this amount was proportional to their reduced rates of total collagen synthesis. Collagen fibril diameter was inversely proportional to type V/type I collagen ratios (EDS > control > OI). However, a reduction of type V collagen, in the EDS derived cells, was associated with the assembly of significantly fewer fibrils compared to control and OI cells. These data indicate that in cell culture, the quantity of collagen fibrils deposited in matrix is highly sensitive to reduction in type V collagen, far out of proportion to type V collagen's contribution to collagen mass.     

Collagen XII and XIV, new partners of cartilage oligomeric matrix protein in the skin extracellular matrix suprastructure

The tensile and scaffolding properties of skin rely on the complex extracellular matrix (ECM) that surrounds cells, vasculature, nerves, and adnexus structures and supports the epidermis. In the skin, collagen I fibrils are the major structural component of the dermal ECM, decorated by proteoglycans and by fibril-associated collagens with interrupted triple helices such as collagens XII and XIV. Here we show that the cartilage oligomeric matrix protein (COMP), an abundant component of cartilage ECM, is expressed in healthy human skin. COMP expression is detected in the dermal compartment of skin and in cultured fibroblasts, whereas epidermis and HaCaT cells are negative. In addition to binding collagen I, COMP binds to collagens XII and XIV via their C-terminal collagenous domains. All three proteins codistribute in a characteristic narrow zone in the superficial papillary dermis of healthy human skin. Ultrastructural analysis by immunogold labeling confirmed colocalization and further revealed the presence of COMP along with collagens XII and XIV in anchoring plaques. On the basis of these observations, we postulate that COMP functions as an adapter protein in human skin, similar to its function in cartilage ECM, by organizing collagen I fibrils into a suprastructure, mainly in the vicinity of anchoring plaques that stabilize the cohesion between the upper dermis and the basement membrane zone.     

Skin development in bony fish with particular emphasis on collagen deposition in the dermis of the zebrafish (Danio rerio)

The first part of this article is a review of the current status of knowledge of the fish skin, with particular attention to its development. In the second part we present original results obtained in zebrafish (Danio rerio), with particular emphasis on the deposition and organisation of the dermal collagenous stroma. Using a series of zebrafish specimens aged between 15 hours postfertilization (hpf) and 4.5 years old, we have combined Transmission Electron Microscopy (TEM) observations and in situ hybridisation using type I collagen a2 chain (Col1a2) probe. Collagen fibrils, with a diameter of 22 nm, appear first in an acellular subepidermal space at 24 hpf, are first all oriented in the same direction, and form the primary dermal stroma. Subsequently, three events occur. (1) From 5-7 days pf (dpf) onwards the collagen fibrils self-organise into several lamellae arranged in a plywood-like structure, starting in the upper layers and progressing throughout the entire thickness of the dermis. (2) At 20-26 dpf, fibroblasts of unknown origin progressively invade the acellular collagenous stroma, some of them accumulating below the epidermis. (3) Concomitant with the invasion of fibroblasts, the collagen fibrils increase progressively in diameter to reach 160 nm towards the end of the fish life. In situ hybridisation experiments reveal that, between 24 and 48 hpf, the collagen matrix is produced by the epidermis only. From 72 hpf to 20-26 dpf, both the basal epidermal cells and the dermal cells bordering the deep region of the dermis are involved in the production of collagen. When the fibroblasts invade the plywood-like structure, the epidermal cells progressively cease to synthesise collagen, which from this point is produced only by the fibroblasts. This suggests that the fibroblasts secrete a still unidentified signalling molecule that downregulates collagen production by the epidermis.     

Scale morphogenesis and ultrastructure of dermis during embryonic development in the alligator (Alligator mississippiensis, Crocodilia, Reptilia)

Formation of scales in different body regions of embryonic alligators is described using light and electron microscopy. Transformation of the skin surface to produce scales takes place between stages 19 and 23, after which the shape of scales is complete over most of the embryonic surface. Scalation is not synchronous; different regions develop scales at different rates. Initially scales are formed on the back and dorsal side of the proximal tail and appear as undulations of the epidermis which form symmetrical (bumps) or asymmetrical (serrated) scale anlagen. No dermal condensations are apparent beneath the epidermis, although in some areas of the skin (belly, limbs) mesenchymal cells are more numerous within the bumps than in other areas. At stage 21, scalation has spread to the neck and belly but is absent or poorly developed over most areas of the flank, gular, jaw, limb and head regions. Grooves form between the outer edges of adjacent scales or interbump regions. A superficial denser dermis and a reticulated deep subdermis are visible in many scales from stage 21. The dermis forms a superficial loose and a deep dense layer from stage 22. Both loose and deep dermis, and sometimes the deep reticulate subdermis, move towards the surface to form the dermal core of scales, although the mechanism of this movement is not known. Bundles of collagen fibrils, with almost no elastic fibrils, are progressively deposited, especially in the denser dermis. At stage 22, the flank, gular and proximal areas of limbs form scales, but the head, jaw, distal limbs and digits still lack scales. The digits become scaled at stage 23 when scalation is well advanced in the other regions. By stage 24 most of the body is scaled and subsequent scale modifications occur only by growth. Five main types of scales are recognized by their shape: symmetrical scutes, asymmetrical scutes, overlapping scutes, tuberculate scales, and elevated asymmetrical scutes (tail verticils). Pigmentation, mainly due to epidermal melanocytes, is visible at embryonic stage 23 and progresses through stages 24 and 25.