Multiple aspects of cornea development, including the innervation of the cornea by trigeminal axons, are sensitive to embryonic levels of thyroid hormone (TH). Although previous work showed that increased TH levels could enhance the rate of axonal extension within the cornea in a thyroxine (T4)-dependent manner, details underlying the stimulatory effect of TH on cornea innervation are unclear. Here, by examining the effects throughout all stages of cornea innervation of the two main THs, triiodothyronine (T3) and T4, we provide a more complete characterization of the stimulatory effects of TH on corneal nerves and begin to unravel the underlying molecular mechanisms. During development, trigeminal axons are initially repelled at the corneal periphery and encircle the cornea in a pericorneal nerve ring prior to advancing into the corneal stroma radially from all along the nerve ring. Overall, exogenous T3 led to pleiotropic effects throughout all stages of cornea innervation, whereas the effects of exogenous T4 was confined to timepoints following completion of the nerve ring. Specifically, exogenous T3 accelerated the formation of the pericorneal nerve ring. By utilizing in vitro neuronal explants studies we demonstrated that T3 acts as a trophic factor to directly stimulate trigeminal nerve growth. Further, exogenous T3 caused disorganized and precocious innervation of the cornea, accompanied by the downregulation of inhibitory Robo receptors that normally act to regulate the timing of nerve advancement into the Slit-expressing corneal tissues. Following nerve ring completion, the growth rate and branching behavior of nerves as they advanced into and through the cornea were found to be stimulated equally by T3 or T4. These stimulatory influences of T3/T4 over nerves likely arose as secondary consequences brought on by TH-mediated modulations to the corneal extracellular matrix. Specifically, we found that the levels of nerve-inhibitory keratan- and chondroitin-sulfate containing proteoglycans and associated sulfation enzymes were dramatically altered in the presence of exogenous T3 or T4. Altogether, these findings uncover new roles for TH on corneal development and shed insight into the mechanistic basis of both T3 and T4 on cornea innervation.

中文翻译：

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三碘甲状腺素（T3）和甲状腺素（T4）对鸡角膜感觉神经的独特且重叠的影响。

角膜发育的多个方面，包括三叉神经轴突对角膜的神经支配，对甲状腺激素（TH）的胚胎水平很敏感。尽管以前的工作表明TH水平升高可以以甲状腺素（T4）依赖的方式提高角膜内轴突延伸的速率，但尚不清楚TH对角膜神经支配的刺激作用的细节。在这里，通过检查两个主要TH（三碘甲状腺素（T3）和T4）在角膜神经支配的所有阶段的作用，我们提供了TH对角膜神经刺激作用的更完整特征，并开始阐明潜在的分子机制。在开发过程中，三叉神经轴突最初在角膜周围被排斥，并在沿整个神经环径向进入角膜基质之前环绕角膜周围神经环中的角膜。总体而言，外源性T3会在角膜神经支配的所有阶段导致多效性作用，而外源性T4的作用仅限于神经环完成后的时间点。具体而言，外源性T3加速了角膜周围神经环的形成。通过利用体外神经元外植体研究，我们证明T3充当直接刺激三叉神经生长的营养因子。此外，外源性T3导致角膜的混乱和早熟神经支配，伴随着抑制性Robo受体的下调，这些受体通常会调节神经进入表达Slit的角膜组织的时间。在完成神经环后，发现当它们进入和穿过角膜时，神经的生长速率和分支行为受到T3或T4的同等刺激。T3 / T4对神经的这些刺激性影响可能是TH介导的对角膜细胞外基质的调节所带来的次要结果。具体而言，我们发现在存在外源性T3或T4的情况下，含有神经抑制性角质素和软骨素硫酸盐的蛋白聚糖和相关的硫酸化酶的水平发生了显着变化。共，