Department of Biological Sciences, Purdue University, West Lafayette, Indiana, USA.
Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts, USA.
J Morphol. 2024 Sep;285(9):e21764. doi: 10.1002/jmor.21764.
As they grow, sharks both replace lost denticles and proliferate the number of denticles by developing new (de novo) denticles without prior denticle shedding. The loss and replacement of denticles has potential impacts on the energetic cost of maintaining the skin surface, the biomechanical functions of shark skin, as well as our ability to predict shark abundance from fossil denticle occurrence in sediment cores. Here, we seek to better understand patterns of denticle loss and to show how denticles are being replaced in mature sharks. We illustrate shark skin surfaces with missing denticles and quantify both within-species and between-species patterns of missing denticles using images from across regions of the body for two species and images at similar body regions for 16 species of sharks. Generally, sharks are missing similar numbers of denticles (0%-6%) between species and regions. However, there are exceptions: in the smooth dogfish, the nose region is missing significantly more denticles than most posterior-body and fin regions, and the common thresher shark is missing significantly more denticles than the smooth dogfish, leopard shark, angel shark, bonnethead, and gulper shark. Denticle regrowth starts with crown development and mineralization beneath the epidermis, followed by eruption of the crown, and finally the mineralization of the root. The pulp cavity of replacement denticles is initially large and surrounded by a thin shell of enameloid upon eruption of the denticle. After eruption of the denticle, the deposition of dentine continues internally after the denticle reaches its final position. Replacement of missing denticles, representing less than 6% of the skin surface at any one time, may not compromise hydrodynamic function, but by constantly updating the skin surface throughout life, sharks may reduce surface fouling and maintain a functional complex skin surface by repairing local damage to individual denticles.
随着鲨鱼的生长，它们不仅通过形成新的（从头）齿鳞来增殖齿鳞数量，而且还会替换已丢失的齿鳞，从而对维持皮肤表面的能量成本、鲨鱼皮肤的生物力学功能以及我们从沉积物核心中化石齿鳞的出现预测鲨鱼丰度的能力产生潜在影响。在这里，我们试图更好地了解齿鳞脱落的模式，并展示成熟鲨鱼的齿鳞是如何被替换的。我们用缺失齿鳞的鲨鱼皮肤表面来说明问题，并使用来自两个物种的全身各部位的图像以及 16 个鲨鱼物种的相似身体部位的图像来量化同种和种间缺失齿鳞的模式。一般来说，鲨鱼在种间和区域间缺失的齿鳞数量相似（0%-6%）。然而，也有例外：在平滑长尾鲨中，鼻子区域缺失的齿鳞明显多于大多数后体和鳍部区域，而普通长尾鲨缺失的齿鳞明显多于平滑长尾鲨、豹鲨、天使鲨、墨西哥湾蝠鲼和宽咽鲨。齿鳞的再生始于表皮下的冠发育和矿化，随后是冠的萌出，最后是根的矿化。替换齿鳞的牙髓腔最初很大，并且在齿鳞萌出时被一层薄薄的釉质壳包围。在齿鳞萌出后，牙本质的沉积在内部继续进行，直到齿鳞到达最终位置。替换缺失的齿鳞，代表任何时候皮肤表面不到 6%的缺失，可能不会影响流体动力功能，但通过在整个生命过程中不断更新皮肤表面，鲨鱼可以减少表面污垢并通过修复单个齿鳞的局部损伤来维持功能复杂的皮肤表面。
