Supplementary Material for: On the Enigma of the Human Neurenteric Canal

Existence and biomedical relevance of the neurenteric canal, a transient midline structure during early neurulation in the human embryo, have been controversially discussed for more than a century by embryologists and clinicians alike. In this study, the authors address the long-standing enigma by high-resolution histology and three-dimensional reconstruction using new and historic histological sections of 5 human 17- to 21-day-old embryos and of 2 marmoset monkey embryos of the species Callithrix jacchus at corresponding stages. The neurenteric canal presents itself as the classical vertical connection between the amniotic cavity and the yolk sac cavity and is lined (a) craniolaterally by a horseshoe-shaped “hinge of involuting notochordal cells” within Hensen’s node and (b) caudally by the receding primitive streak epiblast dorsally and by notochordal plate epithelium ventrally, the latter of which covered the (longitudinal) notochordal canal on its ventral side at the preceding stage. Furthermore, asymmetric parachordal nodal expression in Callithrix and morphological asymmetries within the nodes of the other specimens suggest an early non-cilium-dependent left-right symmetry breaking mode previously postulated for other mammals. We conclude that structure and position of the mammalian neurenteric canal support the notion of its homology with the reptilian blastopore as a whole and with a dorsal segment of the blastopore in amphibia. These new features of the neurenteric canal may further clarify the aetiology of foetal malformations such as junctional neurulation defects, neuroendodermal cysts, and the split notochord syndrome.

人胚胎早期神经胚形成（neurulation）过程中出现的暂时性中线结构——神经肠管（neurenteric canal）的存在及其生物医学相关性，百余年来一直是胚胎学家与临床医师共同争论的议题。本研究通过对5枚17至21日龄人类胚胎、以及对应发育阶段的2枚普通狨猴（Callithrix jacchus）胚胎的全新与历史组织学切片开展高分辨率组织学分析与三维重建，旨在解决这一长期悬而未决的谜题。神经肠管表现为羊膜腔与卵黄囊腔之间的经典垂直连通结构，其内衬结构分为两部分：(a) 颅侧缘为亨森结（Hensen’s node）内呈马蹄形的「内卷脊索细胞铰链」；(b) 尾侧则为背侧退缩的原条上胚层（primitive streak epiblast）与腹侧的脊索板上皮（notochordal plate epithelium），后者在发育的前一阶段，于腹侧覆盖了（纵向）脊索管（notochordal canal）。此外，普通狨猴中出现的不对称脊索旁节点表达，以及其余样本的节点内形态不对称现象，提示了一种此前在其他哺乳动物中提出的、不依赖纤毛的早期左右对称性打破（left-right symmetry breaking）模式。本研究认为，哺乳动物神经肠管的结构与位置整体上支持其与爬行类胚孔（blastopore）的同源性，同时也与两栖类胚孔的背侧片段存在同源关系。这项关于神经肠管的新发现，可进一步阐明连接性神经胚形成缺陷、神经内胚层囊肿（neuroendodermal cysts）、分裂脊索综合征（split notochord syndrome）等胎儿畸形的发病机制。