Quantitative analyses of changes in cell shapes during bending of the avian neural plate

It is widely believed that changes in cell shapes play important roles in the bending or folding of epithelial sheets, but few studies have actually examined cell shapes in such systems. We have determined the percentages of four types of neuroepithelial cells (i.e., spindle, flask, inverted flask, and globular) present during bending of the avian neural plate. Serial transverse plastic sections through seven craniocaudal levels of the neuroepithelium were examined. Four distinct periods of bending were chosen based on the morphology of the neuroepithelium: period I, flat neural plate; period II, midline furrow without elevation of the neural folds; period III, midline furrow with elevation; and period IV, bilateral furrows with convergence of the neural folds. We compared statistically the percentages of different cell types in bending (furrowed) and nonbending regions of the neuroepithelium, as well as changes in cell shapes with time. Our results demonstrate that dramatic changes in cell shapes occur in the midline and bilateral furrows during bending of the neural plate, such that as many as 70% of the neuroepithelial cells in the midline and 55% in the bilateral furrows are wedge shaped by the end of bending. In contrast, less than 35% of the neuroepithelial cells are wedge shaped outside of the three morphological loci of bending. These results support the hypothesis that localized changes in cell morphologies have roles in bending and shaping of the neural plate, but exactly how cells change shapes and what precise roles such changes play in bending remain to be determined.