Comparative Histolab Padova, Italy; Department of Biology, University of Bologna, Bologna, Italy.
Tissue Cell. 2023 Dec;85:102228. doi: 10.1016/j.tice.2023.102228. Epub 2023 Sep 27.
The evolution of modern reptiles from basic reptilian ancestors gave rise to scaled vertebrates. Scales are of different types, and their corneous layer can shed frequently during the year in lepidosaurians (lizards, snakes), 1-2 times per year in the tuatara and in some freshwater turtle, irregularly in different parts of the body in crocodilians, or simply wore superficially in marine and terrestrial turtles. Lepidosaurians possess tuberculate, non-overlapped or variably overlapped scales with inter-scale (hinge) regions. The latter are hidden underneath the outer scale surface or may be more exposed in specific body areas. Hinge regions allow stretching during growth and movement so that the skin remains mechanically functional. Crocodilian and turtles feature flat and shield scales (scutes) with narrow inter-scale regions for stretching and growth. The epidermis of non-avian reptilian hinge regions is much thinner than the exposed outer surface of scales and is less cornified. Despite the thickness of the epidermis, scales are mainly composed of variably amount of Corneous Beta Proteins (CBPs) that are coded in a gene cluster known as EDC (Epidermal Differentiation Complex). These are small proteins, 100-200 amino acid long of 8-25 kDa, rich in glycine and cysteine but also in serine, proline and valine that participate to the formation of beta-sheets in the internal part of the protein, the beta-region. This region determines the further polymerization of CBPs in filamentous proteins that, together a network of Intermediate Filament Keratins (IFKs) and other minor epidermal proteins from the EDC make the variable pliable or inflexible corneous material of reptilian scales, claws and of turtle beak. The acquisition of scales and skin derivatives with different mechanical and material properties, mainly due to the evolution of reptile CBPs, is essential for the life and different adaptations of these vertebrates.
现代爬行动物从基本的爬行动物祖先进化而来，产生了有鳞的脊椎动物。鳞片有不同的类型，它们的角质层在蜥蜴目动物（蜥蜴、蛇）中可以频繁地脱落，每年 1-2 次在楔齿蜥中脱落，在一些淡水龟中不规则地脱落，在鳄目动物中则在身体的不同部位脱落，或者在海龟中只是表面磨损。蜥蜴目动物具有结节状、不重叠或可变重叠的鳞片，鳞片之间有间片（铰链）区域。后者隐藏在外部鳞片表面之下，或者在特定的身体区域可能更暴露。铰链区域允许在生长和运动过程中伸展，从而使皮肤保持机械功能。鳄目动物和龟类具有扁平的盾状鳞片（甲板），其狭窄的间片区域允许伸展和生长。非鸟类爬行动物铰链区域的表皮比暴露在外的鳞片表面薄得多，角质化程度也较低。尽管表皮较厚，但鳞片主要由角蛋白β蛋白（CBPs）组成，这些蛋白在一个称为 EDC（表皮分化复合物）的基因簇中编码。这些是小蛋白，长 100-200 个氨基酸，8-25 kDa，富含甘氨酸和半胱氨酸，但也富含丝氨酸、脯氨酸和缬氨酸，这些氨基酸参与蛋白质内部β-折叠的形成，即β-区域。该区域决定了 CBPs 在丝状蛋白中的进一步聚合，这些丝状蛋白与中间丝角蛋白（IFKs）和 EDC 中的其他少量表皮蛋白一起形成了爬行动物鳞片、爪子和龟喙的柔韧或不柔韧的角蛋白材料。由于爬行动物 CBPs 的进化，获得具有不同机械和材料特性的鳞片和皮肤衍生物对于这些脊椎动物的生命和不同适应至关重要。
