Schoenwolf G C
Department of Neurobiology and Anatomy, University of Utah, School of Medicine, Salt Lake City 84132.
Ciba Found Symp. 1994;181:25-38; discussion 38-50. doi: 10.1002/9780470514559.ch3.
Formation of the neuraxis is dependent on cell-cell interactions and cell movements beginning during stages of gastrulation. Cell movements bring together new combinations of cells, allowing sequential inductive interactions to occur and leading to the specification of the neural plate and to its ultimate mediolateral (subsequently dorsoventral) and rostrocaudal patterning. Formation of the neural plate involves changes in the shape of its constituent cells and the first appearance of neural-specific cell markers. Shortly after the neural plate forms it undergoes 'shaping', in which the pseudostratified columnar epithelium constituting the neural plate thickens apicobasally, narrows transversely and extends longitudinally. Shaping is driven by three principal intrinsic types of cell behaviour: changes in cell shape, position and number. The next stage of neurulation begins while shaping is underway--bending of the neural plate. Bending involves two main processes, furrowing and folding. Furrowing of the neural plate is associated with the formation of the hinge points; these are localized, longitudinal areas where the neuroepithelium is attached to adjacent tissues and where wedging of neuroepithelial cells occurs. Cell wedging in the median hinge point occurs as a result of inductive interactions with the notochord; such wedging drives furrowing, thereby facilitating subsequent folding. Folding of the neural plate requires extrinsic forces generated largely by the surface ectoderm. Types of cell behaviour that could provide such forces include changes in cell shape, position and number. As a result of shaping and bending of the neural plate, the neural folds are brought into apposition in the dorsal midline. Final closure of the neural groove is mediated by cell surface glycoconjugates coating the apical surfaces of the neural folds. Patterning of the neuraxis begins during shaping of the neural plate and continues throughout stages of neurulation and into early postneurula stages. Patterning probably involves inductive interactions with adjacent tissues and the expression of putative positional identity genes such as homeobox-containing genes.
神经轴的形成依赖于原肠胚形成阶段开始的细胞间相互作用和细胞运动。细胞运动使细胞形成新的组合，从而允许依次发生诱导性相互作用，并导致神经板的特化及其最终的内外侧（随后是背腹侧）和头尾模式的形成。神经板的形成涉及其组成细胞形状的变化以及神经特异性细胞标志物的首次出现。神经板形成后不久便会经历“塑形”过程，在此过程中，构成神经板的假复层柱状上皮在顶基方向上增厚，横向变窄并纵向延伸。塑形由三种主要的内在细胞行为类型驱动：细胞形状、位置和数量的变化。神经胚形成的下一阶段在塑形过程中开始——神经板弯曲。弯曲涉及两个主要过程，即沟化和折叠。神经板的沟化与铰链点的形成有关；这些是神经上皮附着于相邻组织的局部纵向区域，也是神经上皮细胞发生楔入的地方。正中铰链点处的细胞楔入是与脊索诱导性相互作用的结果；这种楔入驱动沟化，从而促进随后的折叠。神经板的折叠需要主要由表面外胚层产生的外在力。能够提供这种力的细胞行为类型包括细胞形状、位置和数量的变化。由于神经板的塑形和弯曲，神经褶在背中线处并置。神经沟的最终闭合由覆盖神经褶顶端表面的细胞表面糖缀合物介导。神经轴的模式形成在神经板塑形过程中开始，并在神经胚形成的各个阶段以及神经胚后期早期持续进行。模式形成可能涉及与相邻组织的诱导性相互作用以及假定的位置身份基因如含同源框基因的表达。
