Development of the posterior neural tube in human embryos

Abstract

Development of the posterior neural tube (PNT) in human embryos is a complicated process that involves both primary and secondary neurulation. Because normal development of the PNT is not fully understood, pathogenesis of spinal neural tube defects remains elusive. To clarify the mechanism of PNT development, we histologically examined 20 human embryos around the stage of posterior neuropore closure and found that the developing PNT can be divided into three parts: 1) the most rostral region, which corresponds to the posterior part of the primary neural tube, 2) the junctional region of the primary and secondary neural tubes, and 3) the caudal region, which emerges from the neural cord. In the junctional region, the axially-condensed mesenchyme (AM) intervened between the neural plate/tube and the notochord at the stage of posterior neuropore closure, while the notochord was directly attached to the neural plate/tube in the most rostral region. A single cavity was found to be formed in the AM as the presumptive luminal surface cells were radially aligned in the junctional region prior to the formation of the neural cord. The single cavity was continuous with the central cavity of the primary neural tube. In contrast, multiple or isolated cavities were frequently observed in the caudal region of the PNT. Our observation suggests that the junctional region of the PNT is distinct from other regions in terms of the relationship with the notochord and the mode of cavitation during secondary neurulation.

Comparison of 3 Different Types of Spinal Arteriovenous Shunts below the Conus in Clinical Presentation, Radiologic Findings, and Outcomes

“…These cells, termed the "caudal cell mass," coalesce and then epithelialize to form a separate neural tube that will connect with the neural tube formed by the primary neurulation. 21,22 The exact level where the 2 neural tubes meet is still debatable, ranging from the upper conus 23 to the lowest sacral level of the conus 21 ; however, it is agreed that the caudal cell mass will give rise to the filum terminale and the cauda equina. Thus, the embryologic origin of FTAVFs and rAVSs below the conus is associated with secondary neurulation, differing from the remainder of intradural spinal vascular shunt origins that are associated with the primary neurulation.…”

New classification of spinal lipomas based on embryonic stage

“…4 Presence of the junctional region between the primary and secondary neurulation has also been reported. 24 The complex process of caudal neural tube formation is also implicated in the development of various types of spinal lipomas. It should be borne in mind, however, that the process of neurulation described here is mainly based on animal models, and the human process could be different.…”

Junctional Neurulation: A Unique Developmental Program Shaping a Discrete Region of the Spinal Cord Highly Susceptible to Neural Tube Defects

“…In avian and mammalian embryos, neurulation takes place along the axis through two consecutive processes involving radically different morphogenetic events and referred to as primary and secondary neurulations. Primary neural tube forms rostrally by rolling, folding, or bending of the neural plate, a flat epithelial sheet, whereas secondary neural tube is generated caudally by elongation, epithelialization, and cavitation of the tail bud, a rod-shaped mesenchyme (Colas and Schoenwolf, 2001;Copp et al, 2003;Saitsu et al, 2004). Classically, open NTDs are attributed to failure of neural tube closure during primary neurulation, whereas closed NTDs concern mostly the lower trunk issued from secondary neurulation.…”

“…Importantly, the majority of spinal dysraphisms involve the lower spine, at the thoracolumbar and sacral levels (Welch and Winston, 1987). This anatomical region corresponds to the spinal cord situated at the boundary between the portions generated by primary and secondary neurulations (O'Rahilly and Muller, 1994;Saitsu et al, 2004). Specific local morphogenetic events during neurulation have then to be invoked to account for the high incidence of NTDs in this region.…”