Origin of the dorsal surface of the neural tube by progressive delamination of epidermal ectoderm and neuroepithelium: implications for neurulation and neural tube defects.

Knowledge of the morphogenetic events involved in the development of the dorsal portion of the neural tube is important for understanding neural tube closure, neural crest cell formation and emigration, and the origin of neural tube defects. Here, I characterize the progressive development of the tips of the neural folds during fold elevation in the trunk of mouse and chick embryos and the events leading to formation of the dorsal portion of the neural tube as the epidermal ectoderm (EE) and neuroepithelium (NE) separate from each other. The nature and timing of appearance of collagen IV, laminin and fibronectin were analysed by immunofluorescent and immunogold labelling, and ruthenium red and tannic acid were used to enhance staining for proteoglycans and glycosaminoglycans. As the neural folds elevate, the NE and EE delaminate progressively beginning at the basal surface of the lateral extremes of the neural plate. Nevertheless, the two epithelia remain connected across the zone of delamination by their previously existing basal laminae. In each fold, proteoglycan granules appear at the interface between the NE and EE before delamination begins, and then an (interepithelial) space begins to open and propagate dorsally. Other extracellular matrix (ECM) molecules appear within the space a short distance behind its tip and basal lamina deposition begins shortly thereafter. As fusion occurs, the interepithelial spaces of the two folds coalesce and the final separation of the EE from the NE is accomplished. These observations suggest that the previously recognized delay in deposition of ECM and basal lamina on the dorsal portion of the neural tube and on the overlying EE is a direct consequence of the delamination of the two epithelia and the establishment of two new basal surfaces. The observation that the surface of the dorsal third of the neural tube forms by delamination rather than by juxtaposition of previously existing basal surfaces of the two epithelial is discussed in terms of possible implications for models of neurulation and the origin of neural tube defects.