Biocromo

Pigmentos biológicos ou biocromos são substâncias produzidas por organismos vivos que resultam na impressão de cores devido a absorção seletiva. Muitas estruturas possuem pigmentos produzidos em células especializadas denominadas cromatóforos.

A ocorrência de mudanças fisiológicas de coloração nos animais está relacionada com a presença na epiderme ou na derme de células especializadas conhecidas como cromatócitos. Essas células possuem um grande número de prolongamentos citoplasmáticos que lhes conferem aspecto dendrítico e contêm em seu interior grânulos formados por pigmentos de diversas cores (biocromos) compostos por substâncias como a pteridina (tons amarelos), e vesículas de carotenóides (vermelhos), presentes nos xantóforos; grânulos de purina (brancos) nos leucóforos; melanina (tons pretos ou marrons) que estão presentes em grânulos nos melanóforos .Também são encontrados pigmentos (esquemocromos) que interferem na luz incidente produzindo cores iridescentes. Os mamíferos e as aves perderam a maior parte desses pigmentos durante sua evolução e possuem em sua pele somente um tipo de cromatócito conhecido como melanócito ou melanóforo. Essas células apresentam apenas grânulos nas cores preta, marrom ou amarelada, devido à presença dos pigmentos melanina e feomelanina sintetizados a partir do aminoácido tirosina. Outras cores presentes, por exemplo, nas penas das aves, devem-se à deposição extracelular de outros pigmentos, como porfirinas, cobre, carotenóides e purinas. Os melanócitos, por exemplo, são embriologicamente derivados a partir de uma população germinativa de melanoblastos originários de células da crista neural, pouco tempo após o fechamento do tubo neural. Na maioria das espécies, os melanoblastos começam o processo de melanização imediatamente antes ou logo depois de alcançar seu destino. A única função conhecida da melanina é a proteção contra a radiação UV, sendo essa função fundamental para os animais que vivem nas regiões intertropicais, onde a incidência dessa radiação é significativamente maior que nas regiões ditas temperadas.

Referências

  • OLIVEIRA, Classius de.; ZIERI, Rodrigo. Pigmentação testicular em Physalaemus nattereri (Steindachner) (Amphibia, Anura) com observações anatômicas sobre o sistema pigmentar extracutâneo. Revista Brasileira de Zoologia, Curitiba, v.22, n.2, p.454-460, jun. 2005.
  • RUND, C. R.; CHRISTIANSEN, J. L.; JOHNSON, J. C. In vitro culture of melanomacrophages from the spleen and the liver of turtles: comments on melanomacrophge morphology. Pigment Cell Research. v.11, n.11, p.114 – 119. 1998.
  • SILVA, R. G.; SCALA, N. L.; POCAY, P. L. B. Transmissão de Radiação Ultravioleta Através do Pelame e da Epiderme de Bovinos. Revista Brasileira de Zootecnia. v. 30, n. 6, p. 1939-1946, 2001.
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