The chick epiblast: a model for examining epithelial morphogenesis

The epiblast of early chick embryos is an important model system for examining morphogenesis. Five major morphogenetic processes can be readily examined by scanning electron microscopy of the epiblast: thickening of epithelial sheets by cell elongation, folding of epithelial sheets, fusion of epithelial sheets, cavitation of epithelial cords and dispersal of cell sheets during cell migration. The purpose of this paper is to describe these morphogenetic processes, showing examples of each type. Thickening of epithelial sheets occurs by cell elongation during formation of the neural plate. Folding then ensues to form the neural groove, which is flanked laterally by the neural folds. Fusion of the neural folds closes the neural groove and separates the incipient neural tube from the overlying surface ectoderm. The caudal part of the neural tube develops much differently. Cells derived from the tail bud cluster together as an epithelial medullary cord, the peripheral cells of this cord elongate, and, simultaneously, several small cavities appear at the inner ends of the peripheral cells. All of these cavities eventually coalesce, forming a single lumen. Thus, the neural tube can be formed by markedly different morphogenetic processes depending upon its particular craniocaudal level of origin. Cell migration is exhibited by neural crest cells. In chick embryos, these cells originate from the roof of the closed neural tube. They then migrate laterad to take up residence in a variety of new locations. Scanning electron microscopy has served as an important tool, aiding us greatly in visualizing complex spatial changes that occur during the morphogenesis of epithelia.