Vertebrate hearts have undergone marked morphological and structural changes to adapt to different environments and lifestyles as part of the evolutionary process. Amphibians were the first vertebrates to migrate to land. Transition from aquatic to terrestrial environments required the ability to circulate blood against the force of gravity. In this study, we investigated the passive mechanical properties and histology of the ventricles of three species of Anura (frogs and toads) from different habitats, Xenopus laevis (aquatic), Pelophylax nigromaculatus (semiaquatic), and Bufo japonicus formosus (terrestrial). Pressure-loading tests demonstrated stiffer ventricles of P. nigromaculatus and B. j. formosus compared X. laevis ventricles. Histological analysis revealed a remarkable difference in the structure of cardiac tissue: thickening of the compact myocardium layer of P. nigromaculatus and B. j. formosus and enrichment of the collagen fibers of B. j. formosus. The amount of collagen fibers differed among the species, as quantitatively confirmed by second-harmonic generation light microscopy. No significant difference was observed in cardiomyocytes isolated from each animal, and the sarcomere length was almost the same. The results indicate that the ventricles of Anura stiffen during adaptation to life on land.

作为进化过程的一部分，脊椎动物心脏经历了显着的形态和结构变化，以适应不同的环境和生活方式。两栖动物是最早迁移到陆地的脊椎动物。从水生环境到陆生环境的过渡需要能够抵抗重力使血液循环。在这项研究中，我们调查了来自不同栖息地的三种无尾动物（青蛙和蟾蜍）心室的被动机械特性和组织学，非洲爪蟾（水生）、黑斑鳖（半水生）和日本蟾蜍（陆生）。压力负荷试验表明P. nigromaculatus和B. j. 的心室较硬。福尔摩斯比较X. laevis心室。组织学分析揭示了心脏组织结构的显着差异：P. nigromaculatus和B. j. 的致密心肌层增厚。B. j.胶原纤维的形成和富集。福尔摩斯 通过二次谐波光学显微镜定量证实，不同物种之间胶原纤维的数量不同。从每只动物分离的心肌细胞中没有观察到显着差异，并且肌节长度几乎相同。结果表明，Anura 的心室在适应陆地生活时会变硬。