Osteoderms are mineralised structures consisting mainly of calcium phosphate and collagen. They form directly within the skin, with or without physical contact with the skeleton. Osteoderms, in some form, may be primitive for tetrapods as a whole, and are found in representatives of most major living lineages including turtles, crocodilians, lizards, armadillos, and some frogs, as well as extinct taxa ranging from early tetrapods to dinosaurs. However, their distribution in time and space raises questions about their evolution and homology in individual groups. Among lizards and their relatives, osteoderms may be completely absent; present only on the head or dorsum; or present all over the body in one of several arrangements, including non-overlapping mineralised clusters, a continuous covering of overlapping plates, or as spicular mineralisations that thicken with age. This diversity makes lizards an excellent focal group in which to study osteoderm structure, function, development and evolution. In the past, the focus of researchers was primarily on the histological structure and/or the gross anatomy of individual osteoderms in a limited sample of taxa. Those studies demonstrated that lizard osteoderms are sometimes two-layered structures, with a vitreous, avascular layer just below the epidermis and a deeper internal layer with abundant collagen within the deep dermis. However, there is considerable variation on this model, in terms of the arrangement of collagen fibres, presence of extra tissues, and/or a cancellous bone core bordered by cortices. Moreover, there is a lack of consensus on the contribution, if any, of osteoblasts in osteoderm development, despite research describing patterns of resorption and replacement that would suggest both osteoclast and osteoblast involvement. Key to this is information on development, but our understanding of the genetic and skeletogenic processes involved in osteoderm development and patterning remains minimal. The most common proposition for the presence of osteoderms is that they provide a protective armour. However, the large morphological and distributional diversity in lizard osteoderms raises the possibility that they may have other roles such as biomechanical reinforcement in response to ecological or functional constraints. If lizard osteoderms are primarily for defence, whether against predators or conspecifics, then this ‘bony armour’ might be predicted to have different structural and/or mechanical properties compared to other hard tissues (generally intended for support and locomotion). The cellular and biomineralisation mechanisms by which osteoderms are formed could also be different from those of other hard tissues, as reflected in their material composition and nanostructure. Material properties, especially the combination of malleability and resistance to impact, are of interest to the biomimetics and bioinspired material communities in the development of protective clothing and body armour. Currently, the literature on osteoderms is patchy and is distributed across a wide range of journals. Herein we present a synthesis of current knowledge on lizard osteoderm evolution and distribution, micro- and macrostructure, development, and function, with a view to stimulating further work.

中文翻译：

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蜥蜴的骨皮回顾（爬行动物：有鳞）

骨胚层是主要由磷酸钙和胶原蛋白组成的矿化结构。它们直接在皮肤内形成，与骨骼有或没有物理接触。以某种形式，骨皮对于整个四足动物来说可能是原始的，并且存在于大多数主要生活谱系的代表中，包括海龟、鳄鱼、蜥蜴、犰狳和一些青蛙，以及从早期四足动物到恐龙的灭绝类群。然而，它们在时间和空间上的分布引发了关于它们在各个群体中的进化和同源性的问题。在蜥蜴及其近亲中，骨皮可能完全缺失；仅存在于头部或背部；或以几种排列之一遍布全身，包括不重叠的矿化簇，重叠板块的连续覆盖，或随着年龄的增长而变厚的针状矿化。这种多样性使蜥蜴成为研究骨皮结构、功能、发育和进化的绝佳焦点群体。过去，研究人员的重点主要是在有限的分类群样本中单个骨皮的组织学结构和/或大体解剖。这些研究表明，蜥蜴的骨皮有时是两层结构，在表皮下方有一个玻璃体无血管层，而深层真皮内有一个更深的内层，含有丰富的胶原蛋白。然而，这个模型在胶原纤维的排列、额外组织的存在和/或以皮质为边界的松质骨核心方面存在相当大的变化。此外，对于成骨细胞在成骨细胞发育中的贡献（如果有的话）缺乏共识，尽管研究描述了吸收和替代的模式，这表明破骨细胞和成骨细胞都受累。对此的关键是有关发育的信息，但我们对参与骨皮发育和图案形成的遗传和骨骼形成过程的了解仍然很少。骨皮存在的最常见主张是它们提供了保护性盔甲。然而，蜥蜴骨皮的巨大形态和分布多样性增加了它们可能具有其他作用的可能性，例如响应生态或功能限制的生物力学强化。如果蜥蜴的骨皮主要用于防御，无论是针对掠食者还是同类，那么与其他硬组织（通常用于支撑和运动）相比，这种“骨甲”可能具有不同的结构和/或机械性能。形成骨皮的细胞和生物矿化机制也可能与其他硬组织不同，这反映在它们的材料组成和纳米结构上。材料特性，特别是延展性和抗冲击性的结合，是仿生学和仿生材料社区在防护服和防弹衣开发中的兴趣所在。目前，关于骨皮的文献不完整，分布在各种期刊上。在这里，我们综合了当前关于蜥蜴骨皮进化和分布、微观和宏观结构、发育和功能的知识，