Secondary neurulation mechanisms likely arose in vertebrates during mesenchymal tailbud extension

The central nervous system (CNS) is an essential organ system common to all vertebrates and is one of the defining features of the Chordata phylum. In Chordates, neural tubes may form via either primary neurulation, secondary neurulation, or a mixture of the two. During primary neurulation, there is folding of an already-polarized epithelium. In contrast, secondary neurulation involves mesenchymal to epithelial transition, cell interdigitation and sometimes de novo cavitation from an initially solid rod of mesenchymal cells. Current studies only show secondary neurulation in a subset of chordates, and it is unclear why this should be the case.

This article uses published literature to reevaluate the mode and the relative timings of neurulation and tailbud extension in chordate species. It also uses micro-computerized tomography (microCT) and immunohistochemistry (IHC) to investigate the cellular mechanisms of neurulation within Actinopterygii fish, in which so far only teleosts have been shown to undergo secondary neurulation.

Our literature review suggests that early branching chordates likely neurulate via exclusively primary neurulation and do not have a mesenchymal tailbud. In contrast, in the Vertebrate species in which secondary neurulation has been previously documented, there is a strong positive correlation between the onset of secondary neurulation and of a mesenchymal tailbud extension. The mechanisms of neurulation have not been well characterised across the Chordates, resulting in a patchy reported distribution of secondary neurulation. MicroCT images demonstrated that tropical gar fish from the Holostei clade neurulate via secondary-like mechanisms along most of their axis, whilst Senegal bichir (Cladistia clade) and sterlet sturgeon (Chondrostei clade) exhibit primary neurulation in anterior regions and have an undifferentiated tailbud at the posterior end of the embryo. Higher resolution IHC data demonstrated that the sterlet embryo neural tube forms via primary mechanisms until a neuroenteric canal, whilst proliferative, Sox2-positive cells in the tailbud intercalate into the extending neural tube via secondary mechanisms of neurulation, without de novo lumen formation.

Together, our findings suggest that the cellular mechanisms of secondary neurulation likely evolved in the Vertebrata subphylum to enable cells from an extending mesenchymal tailbud to be incorporated into an already polarised posterior spinal cord. However, it remains unclear why different cellular mechanisms of secondary neurulation exist between species (e.g. de novo polarisation and cell interdigitation vs. only cell interdigitation) and why some species undergo secondary neurulation along the majority of their axis whilst in others it is limited to the tailbud. We suggest that varying geometrical constraints due to differences in relative rates of body axis elongation, mesenchymal cell proliferation and MET are likely key to which specific mechanisms of neurulation are adopted.