The mitotic pattern in the neural tube of Ambystoma maculatum



In stages H18–H22 the neural tube was composed of a partially stratified epithelium, reflecting its origin from epidermal and sensory layers of ectoderm. Mitotic figures were distributed over the whole neural wall. Fifty-four to 91% were peripheral to the luminal zone. There was no evidence of nuclear migration attending mitosis. After integration of epidermal and sensory cells into a single epithelium by stage H24, mitotic figures were mostly near the lumen and for some time prophase stages were distributed peripherally. During this period nuclear migration to and from the lumen accompanied mitosis as in higher vertebrates. By stage H46 nuclei of the neural epithelium were restricted to a narrow zone near the lumen. Most mitotic figures and all other phases of mitosis were in this zone. There was a consequent lack of appreciable nuclear migration. A few peripheral mitotic figures remained in neural cells through stage H46. Some were in epithelial cells whose nuclei did not migrate to the lumen. Others were in the mantle zone, perhaps in neuroblasts. These observations support the view that the neural epithelium, whatever its extent, is germinal. Neither a “germinal zone” nor a special strain of “germinal cells” could be distinguished at any stage.