Immunomodulation Stimulates the Innervation of Engineered Tooth Organ

The sensory innervation of the dental mesenchyme is essential for tooth function and protection. Sensory innervation of the dental pulp is mediated by axons originating from the trigeminal ganglia and is strictly regulated in time. Teeth can develop from cultured re-associations between dissociated dental epithelial and mesenchymal cells from Embryonic Day 14 mouse molars, after implantation under the skin of adult ICR mice. In these conditions however, the innervation of the dental mesenchyme did not occur spontaneously. In order to go further with this question, complementary experimental approaches were designed. Cultured cell re-associations were implanted together with trigeminal ganglia for one or two weeks. Although axonal growth was regularly observed extending from the trigeminal ganglia to all around the forming teeth, the presence of axons in the dental mesenchyme was detected in less than 2.5% of samples after two weeks, demonstrating a specific impairment of their entering the dental mesenchyme. In clinical context, immunosuppressive therapy using cyclosporin A was found to accelerate the innervation of transplanted tissues. Indeed, when cultured cell re-associations and trigeminal ganglia were co-implanted in cyclosporin A-treated ICR mice, nerve fibers were detected in the dental pulp, even reaching odontoblasts after one week. However, cyclosporin A shows multiple effects, including direct ones on nerve growth. To test whether there may be a direct functional relationship between immunomodulation and innervation, cell re-associations and trigeminal ganglia were co-implanted in immunocompromised Nude mice. In these conditions as well, the innervation of the dental mesenchyme was observed already after one week of implantation, but axons reached the odontoblast layer after two weeks only. This study demonstrated that immunodepression per se does stimulate the innervation of the dental mesenchyme.牙间充质（dental mesenchyme）的感觉神经支配对于牙齿的功能与防护至关重要。牙髓（dental pulp）的感觉神经支配由源自三叉神经节（trigeminal ganglia）的轴突介导，且具有严格的时间调控特性。将胚胎第14天小鼠磨牙解离后的牙上皮与间充质细胞进行体外重聚培养，再移植至成年ICR小鼠皮下后，可形成牙齿结构。但在此实验条件下，牙间充质的神经支配并未自发发生。为深入探究该问题，研究人员设计了补充性实验方案。将体外培养的细胞重聚体与三叉神经节共同植入受体小鼠体内，培养1至2周。尽管可常规观察到轴突从三叉神经节向正在形成的牙齿周围延伸，但在移植2周后的样本中，仅不到2.5%的样本可检测到轴突存在于牙间充质内，这表明轴突进入牙间充质的过程存在特异性障碍。在临床场景中，研究发现使用环孢素A（cyclosporin A）的免疫抑制疗法可促进移植组织的神经支配。具体而言，将细胞重聚体与三叉神经节共同植入经环孢素A处理的ICR小鼠体内后，可在1周后于牙髓中检测到神经纤维，甚至可抵达成牙本质细胞（odontoblasts）层。但环孢素A具有多种生物学效应，包括对神经生长的直接作用。为验证免疫调节与神经支配之间是否存在直接功能关联，研究人员将细胞重聚体与三叉神经节共同植入免疫缺陷裸鼠（Nude mice）体内。在此实验条件下，同样可在移植1周后观测到牙间充质的神经支配，但轴突仅在移植2周后才可抵达成牙本质细胞层。本研究证实，免疫抑制本身即可促进牙间充质的神经支配。