Surface modifications of neural epithelial cells during formation of the neural tube in the rat embryo.

The apical (juxtaluminal) ends of the neural epithelial cells of rat embryos were examined using light and electron microscopy during varying stages of neural tube formation. At the neural-plate stage the apical surfaces exhibit numerous microvilli. At the presomite neurula stage the microvilli are longer and more irregular. Filaments of approximately 40-60 A diameter appear in the apical cytoplasm. By the neural-groove stage, cytoplasmic protrusions containing various organelles have begun to appear. Apical filaments are present. At the beginning of closure the apical surfaces are characterized by large, irregular protrusions that are still associated with apical filaments. Finally, at the time of neural closure, the apical protrusions as well as the apical filaments have disappeared and the apical surfaces of the neural epithelial cells are relatively smooth. These observations bear out the proposal that contraction of the apical filaments is responsible for the folding of the neural plate and the production of apical protrusions.