A technical improvement for sectioning hard laminated fibrous tissues for electron microscopic studies

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.

New data on the structure and the growth of the osteoderms in the reptile Anguis fragilis L. (Anguidae, Squamata)

Light and electron microscopy shows the osteoderms of Anguis fragilis to be small, flat disks located in the dermis along the adult trunk: microradiography established the extent of the mineralization. Each osteoderm coincides exactly with an epidermal fold forming the keratinized scales characteristic of the skin of reptiles. Sections perpendicular to the surface show two mineralized layers differing in histological and histochemical characteristics and in fine structure, although both contain collagen fibrils. The structure of each layer can be related to that of the surrounding dermis. The outer superficial layer located in the loose dermis contains few collagen bundles that form a discontinuous sheet at the upper surface of the osteoderms. This superficial layer appears to be constituted of units separated by furrows and is composed of woven fibered bone. The basal plate comprises stratified lamellae formed of parallel-oriented collagen fibrils; the fibrils of successive lamellae lie at right angles. The densely packed collagen fibrils of the basal plate are distributed similarly to those of the dense dermis within which it lies. This layer exhibits structural and histochemical characteristics of a lamellar bone. The presence of two different layers in the osteoderms of Anguis fragilis may reflect their mode of formation, which consists of the deposit of mineral crystals in the preexisting dermal tissue. This mineralization process, considered as a "metaplastic ossification," may reflect the potentiality retained by the dermis of reptiles to form mineralized structures.