Myelin sheaths wrapping axons are key structures that help maintain the propagation speed of action potentials in both central and peripheral nervous systems (CNS and PNS). However, noninvasive, deep imaging technologies visualizing myelin sheaths in the digital skin in vivo are lacking in animal models. 3-photon fluorescence (3PF) imaging excited at the 1700-nm window enables deep imaging of myelin sheaths, but necessitates labeling by exogenous fluorescent dyes. Since myelin sheaths are lipid-rich structures which generate strong third-harmonic signals, in this paper, we perform a detailed comparative experimental study of both third-harmonic generation (THG) and 3PF imaging in the mouse digital skin in vivo. Our results show that THG imaging also enables visualization of myelin sheaths deep in the mouse digital skin, which shows colocalization with 3PF signals from labeled myelin sheaths. Besides its superior label-free advantage, THG does not suffer from photobleaching due to its 3PF property.

包裹轴突的髓鞘是帮助维持中枢和外周神经系统（CNS 和 PNS）动作电位传播速度的关键结构。然而，动物模型缺乏在体内可视化数字皮肤中的髓鞘的无创、深度成像技术。在 1700 nm 窗口激发的 3 光子荧光 (3PF) 成像能够对髓鞘进行深度成像，但需要使用外源荧光染料进行标记。由于髓鞘是产生强烈三次谐波信号的富含脂质的结构，在本文中，我们对小鼠数字皮肤中的三次谐波产生 (THG) 和 3PF 成像进行了详细的比较实验研究. 我们的结果表明，THG 成像还能够可视化小鼠数字皮肤深处的髓鞘，这显示了与来自标记髓鞘的 3PF 信号的共定位。除了其卓越的免标记优势外，由于其 3PF 特性，THG 不会受到光漂白的影响。