RESUMO
1. Melanophores and xanthophores are pigment cell derivatives of the NC. In amphibian embryos they migrate from their original position on the neural tube dorsally (into the dorsal fin) as well as laterally (between somites and epidermis) and arrange themselves into typical pigment patterns of the skin. We investigated pigment pattern formation in two species of tailed amphibians, Triturus alpestris (alpine newt) and Ambystoma mexicanum (Mexican axolotl). In larvae of T. alpestris alternating longitudinal stripes or bands of melanophores and xanthophores develop, whereas in larvae of A. mexicanum a barred pattern with alternating transverse bands of melanophores and xanthophores is formed. Iridophores, a third type of pigment cell, are present later in both species and therefore play no role during early larval pigment pattern development. Visibly differentiated melanophores and xanthophores can be distinguished from each other under the light microscope by their contents of black melanins and yellow pterins respectively. With the dopa reaction (indicates tyrosinase in melanophores), and ammonia treatment (stimulates pterin fluorescence in xanthophores), the pigment cell phenotypes can be visualized even before their normal visible differentiation. In the TEM, melanophores and xanthophores can be distinguished from each other by their morphologically distinct pigment organelles and in the SEM by their different surface structure. 2. Because of the NC origin of melanophores and xanthophores and the ease with which these cells can be demonstrated even before they are visible from outside, their different arrangements in Triturus and axolotl embryos offer suitable model systems for studying the migration, interaction and localization of NC derivatives in relation to specific environmental influences. The environment of NC cells are the neural tube, epidermis, somites and lateral plate mesoderm, and the subepidermal ECM, a network of collagen fibrils associated with glycosaminoglycans, proteoglycans and glycoproteins. 3. Development of the pigment pattern in T. alpestris: Melanophores and xanthophores start to leave the NC at stage 28, melanophores slightly earlier than xanthophores. Both cell types become scattered in the dorsolateral trunk. In contrast to melanophores in the axolotl, melanophores in T. alpestris cannot be demonstrated with the dopa reaction before they become visibly black. From stage 29+ onwards, melanophores start to accumulate in zones alongside the dorsal and lateral somite edges, where they form compact stripes later. Xanthophores can be demonstrated from stage 28+ onwards only with the SEM (by means of their specific surface structures) or with the fluorescence microscope (by means of their fluorescing pterins). At state 34, xanthophores become visible externally as yellow cells.(ABSTRACT TRUNCATED AT 400 WORDS)
Assuntos
Ambystoma mexicanum/embriologia , Ambystoma/embriologia , Larva/fisiologia , Pigmentação da Pele , Triturus/embriologia , Animais , Catecolaminas/biossíntese , Diferenciação Celular , Movimento Celular , Fenômenos Químicos , Química , Imuno-Histoquímica , Melaninas/biossíntese , Melanóforos/fisiologia , Crista Neural/embriologia , Pterinas/metabolismoRESUMO
The purpose of the present investigation was to provide and apply a methodological manual with which the distribution, patterning and relationship of melanophores and xanthophores can be analyzed during early amphibian development. For demonstration of the methods, which include ultrastructural, histochemical and biochemical approaches, Triturus alpestris and Ambystoma mexicanum (axolotl) embryos are used. These two species differ conspicuously in their larval pigment patterns, showing alternating melanophore bands in horizontal (T. alpestris) and vertical (axolotl) arrangements. With transmission- and scanning electron microscopy melanophores and xanthophores were distinguished by their different pigment organelles and surface structures. The presence of phenol oxidase (tyrosinase) was used to reveal externally invisible or faintly visible melanophores by applying an excess of 3,4 dihydroxy-phenylalanine (dopa). Xanthophores were made visible in fixed and living embryos by demonstrating their pterin fluorescence. In addition, pterins were analyzed by HPLC in embryos before and after pigmentation was visible.