Barnes W Jon P, Baum Martina, Peisker Henrik, Gorb Stanislav N
Centre for Cell Engineering, Institute of Molecular, Cell and Systems Biology, University of Glasgow, Scotland, UK.
J Morphol. 2013 Dec;274(12):1384-96. doi: 10.1002/jmor.20186. Epub 2013 Sep 3.
Cryo-scanning electron microscopy (cryo-SEM) and atomic force microscopy (AFM) offer new avenues for the study of the morphology of tree frog adhesive toe pads. Using these techniques, we compare toe pad microstructure in two distantly related species of tree frog, Litoria caerulea, White (Hylidae) and Rhacophorus prominanus, Smith (Rhacophoridae), in which the toe pads are considered to be convergent. AFM demonstrates the extraordinary similarity of both surface microstructures (largely hexagonal epithelial cells surrounded by deep channels) and nanostructures (an array of nanopillars, ca. 350 nm in diameter, all with a small dimple at the apex). The cryo-SEM studies examined the distribution of the fibrillar cytoskeleton within the different layers of the stratified toe pad epithelium, demonstrating that the cytoskeletal elements (keratin tonofilaments) that lie at an angle to the surface are relatively poorly developed in L. caerulea, clearly so in comparison to R. prominanus. Cryo-SEM also enabled the visualization of the fluid layer that is critical to a toe pad's adhesive function. This was achieved by examination of the frozen fluid residues left behind after removal of a toe within the cryo-SEM's experimental chamber. Such 'toeprints' demonstrated the presence of a wedge of fluid surrounding each toe pad, as well as fluid filling the channels that surround each epithelial cell. Cryo-SEM was used to examine epithelial cell shape. In a sample of 582 cells, 59.5% were hexagonal, the remainder being mainly pentagonal (23.1%) or heptagonal (16.1%). The distribution of differently-shaped cells was not random, but was not associated with either pad curvature or the distribution of mucous pores that provide fluid for the frogs' wet adhesion mechanism. Our main finding, the great similarity of toe pad structure in these two species, has important implications for biomimetics, for such convergent evolution suggests a good starting point for attempts to develop adhesives that will function in wet conditions.
低温扫描电子显微镜（cryo-SEM）和原子力显微镜（AFM）为研究树蛙粘性趾垫的形态提供了新途径。利用这些技术，我们比较了两种亲缘关系较远的树蛙——白氏大泛树蛙（Litoria caerulea，隶属雨蛙科）和史密斯氏显著树蛙（Rhacophorus prominanus，隶属树蛙科）的趾垫微观结构，这两种树蛙的趾垫被认为是趋同进化的产物。原子力显微镜显示，二者的表面微观结构（主要是被深沟环绕的六边形上皮细胞）和纳米结构（直径约350纳米的纳米柱阵列，顶端均有一个小凹坑）都极为相似。低温扫描电子显微镜研究了分层趾垫上皮不同层内纤维状细胞骨架的分布，结果表明，与表面呈一定角度的细胞骨架成分（角蛋白丝）在白氏大泛树蛙中发育相对较差，与显著树蛙相比明显如此。低温扫描电子显微镜还能观察到对趾垫粘附功能至关重要的流体层。这是通过在低温扫描电子显微镜实验室内移除趾后检查留下的冷冻流体残余物来实现的。这种“趾印”显示每个趾垫周围存在一个楔形流体，以及填充每个上皮细胞周围通道的流体。低温扫描电子显微镜用于检查上皮细胞形状。在582个细胞样本中，59.5%为六边形，其余主要为五边形（23.1%）或七边形（16.1%）。不同形状细胞的分布并非随机，但与趾垫曲率或为青蛙湿粘附机制提供流体的粘液孔分布均无关联。我们的主要发现，即这两个物种的趾垫结构极为相似，对仿生学具有重要意义，因为这种趋同进化为开发在潮湿条件下起作用的粘合剂的尝试提供了一个良好的起点。
