Mucus and mucus glands are important features of the amphibian cutis. In tree frogs, the mucus glands and their secretions are crucial components of the adhesive digital pads of these animals. Despite a variety of hypothesised functions of these components in tree frog attachment, the functional morphology of the digital mucus glands and the chemistry of the digital mucus are barely known. Here, we use an interdisciplinary comparative approach to analyse these components, and discuss their roles in tree frog attachment. Using synchrotron micro-computer-tomography, we discovered in the arboreal frog Hyla cinerea that the ventral digital mucus glands differ in their morphology from regular anuran mucus glands and form a subdermal gland cluster. We show the presence of this gland cluster also in several other—not exclusively arboreal—anuran families. Using cryo-histochemistry as well as infrared and sum frequency generation spectroscopy on the mucus of two arboreal (H. cinerea and Osteopilus septentrionalis) and of two terrestrial, non-climbing frog species (Pyxicephalus adspersus and Ceratophrys cranwelli), we find neutral and acidic polysaccharides, and indications for proteinaceous and lipid-like mucus components. The mucus chemistry varies only little between dorsal and ventral digital mucus in H. cinerea, ventral digital and abdominal mucus in H. cinerea and O. septentrionalis, and between the ventral abdominal mucus of all four studied species. The presence of a digital mucus gland cluster in various anuran families, as well as the absence of differences in the mucus chemistry between arboreal and non-arboreal frog species indicate an adaptation towards generic functional requirements as well as to attachment-related requirements. Overall, this study contributes to the understanding of the role of glands and their secretions in tree frog attachment and in bioadhesion in general, as well as the evolution of anurans.

中文翻译：

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树蛙手指粘液腺和分泌物的比较和功能分析

粘液和粘液腺是两栖动物皮肤的重要特征。在树蛙中，粘液腺及其分泌物是这些动物的粘性数字垫的重要组成部分。尽管这些成分在树蛙附着中具有多种假设功能，但手指粘液腺的功能形态和手指粘液的化学性质却鲜为人知。在这里，我们使用跨学科比较方法来分析这些组成部分，并讨论它们在树蛙依恋中的作用。使用同步加速器微型计算机断层扫描，我们在树栖青蛙 Hyla cinerea 中发现腹侧指粘液腺的形态不同于常规的无尾目粘液腺，并形成皮下腺簇。我们在其他几个——不仅仅是树栖的——anuran家族中也显示了这种腺体簇的存在。对两种树栖动物（H. cinerea 和 Osteopilus septentrionalis）和两种陆生非攀爬青蛙物种（Pyxicephalus adspersus 和 Ceratophrys cranwelli）的粘液使用低温组织化学以及红外和和频生成光谱，我们发现中性和酸性多糖，以及蛋白质和类脂粘液成分的适应症。H. cinerea 的背侧和腹侧的粘液、H. cinerea 和 O. septentrionalis 的腹侧和腹侧粘液之间以及所有四个研究物种的腹侧腹侧粘液之间的粘液化学变化很小。在各种无尾动物家族中存在数字粘液腺簇，以及树栖和非树栖青蛙物种之间的粘液化学没有差异表明对一般功能要求以及与附着相关的要求的适应。总体而言，这项研究有助于了解腺体及其分泌物在树蛙附着和一般生物粘附中的作用，以及无尾目动物的进化。