Variation in Articular Cartilage Thickness Among Extant Salamanders and Implications for Limb Function in Stem Tetrapods
Variation in Articular Cartilage Thickness Among Extant Salamanders and Implications for Limb Function in Stem Tetrapods
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DOI：
10.3389/fevo.2021.671006
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发表时间：
2021-05-11
影响因子：
3
通讯作者：
Molnar, Julia L.
中科院分区：
文献类型：
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作者：
Molnar, Julia L.
现存脊椎动物四肢关节软骨的大小和形状差异很大，但它们对于在进化背景下理解关节和肢体功能是至关重要的。例如，关于早期四足动物未保存的关节软骨的推断，暗示了四肢动物的肢体长度、关节活动范围和肌肉杠杆在四足动物从水到陆的转变过程中是如何变化的。现存的火蜥蜴通常被用作早期肢体脊椎动物的功能模型，它们的关节软骨比大多数脊椎动物群体要厚得多，但软骨的确切比例以及它在火蜥蜴物种中的变化尚不清楚。我的目标是在13只火蜥蜴样本中量化这种变化，这些样本代表了广泛的大小、生活方式和属。使用增强后的微型CT，对每个标本的肱骨、桡骨、尺骨、股骨、胫骨和腓骨的软骨尺寸和骨长度进行非破坏性测量。软骨矫正系数计算为近端和远端软骨的厚度除以骨干长度。关节软骨平均增加了约30%的长骨长度。水栖火蜥蜴的软骨厚度(肱骨42+/-14%，股骨35+/-8%)明显大于陆生火蜥蜴(21+/-7%的肱骨和股骨)。相对软骨厚度与身体大小或系统发育关系没有一致的关系。除了增加肢体长度外，软骨帽还增加了骨痂的宽度和宽度，其大小在不同的类群中差别很大。为了预测火蜥蜴样软骨矫正因子对肌肉杠杆的影响，建立了泥盆系四足棘骨后肢的简化模型。在这个模型中，与股骨成斜角穿过臀部的肌肉的杠杆臂增加了多达6厘米。通过明确考虑未保存的软骨的可能影响，并基于现有分类群(包括水生和陆生火蜥蜴)的现有数据，可以使茎四足动物和茎两栖动物的骨学活动范围和肌肉杠杆的未来重建变得更加严格。
The size and shape of articular cartilage in the limbs of extant vertebrates are highly variable, yet they are critical for understanding joint and limb function in an evolutionary context. For example, inferences about unpreserved articular cartilage in early tetrapods have implications for how limb length, joint range of motion, and muscle leverage changed over the tetrapod water-land transition. Extant salamanders, which are often used as functional models for early limbed vertebrates, have much thicker articular cartilage than most vertebrate groups, but the exact proportion of cartilage and how it varies across salamander species is unknown. I aimed to quantify this variation in a sample of 13 salamanders representing a broad range of sizes, modes of life, and genera. Using contrast-enhanced micro-CT, cartilage dimensions and bone length were measured non-destructively in the humerus, radius, ulna, femur, tibia, and fibula of each specimen. Cartilage correction factors were calculated as the combined thickness of the proximal and distal cartilages divided by the length of the bony shaft. Articular cartilage added about 30% to the length of the long bones on average. Cartilage was significantly thicker in aquatic salamanders (42 +/- 14% in the humerus and 35 +/- 8 in the femur) than in terrestrial salamanders (21 +/- 7% in both humerus and femur). There was no consistent relationship between relative cartilage thickness and body size or phylogenetic relatedness. In addition to contributing to limb length, cartilage caps increased the width and breadth of the epiphyses by amounts that varied widely across taxa. To predict the effect of salamander-like cartilage correction factors on muscle leverage, a simplified model of the hindlimb of the Devonian stem tetrapod Acanthostega was built. In this model, the lever arms of muscles that cross the hip at an oblique angle to the femur was increased by up to six centimeters. Future reconstructions of osteological range of motion and muscle leverage in stem tetrapods and stem amphibians can be made more rigorous by explicitly considering the possible effects of unpreserved cartilage and justifying assumptions based on available data from extant taxa, including aquatic and terrestrial salamanders.