An experimental analysis of the role of bottle cells and the deep marginal zone in gastrulation of Xenopus laevis

Earlier work suggested that the change in shape or the active migration of the bottle cells in the amphibian blastoporal region results in an invagination that comprises a major part of gastrulation. In the present study of gastrulation in Xenopus laevis, microsurgical extirpation and rearrangement experiments, analyzed with time-lapse cinemicrography and scanning electron microscopy, show that the bottle cells have a lesser role in gastrulation. Gastrulation is not a process of invagination but of involution of the deep and superficial layers of the marginal zone. Involution is dependent on unique properties of the cells in the deep marginal zone. In contrast, the superficial layer, including the bottle cells, does not have properties essential for involution but is passively moved inside to form the lining of the archenteron by what is probably an active migration of the underlying cells of the deep marginal zone. Bottle cells form by the shrinking and thickening of the superficial layer in the late blastula and early gastrula. They are moved inside, largely because of their attachment to the underlying deep cells, and then they spread and flatten in the latter half of gastrulation to form a large area of the lining of the periphery of the archenteron. The formation of the initial blastoporal groove by bending of the superficial cells sheet during bottle cell formation and the extension of the periphery of the archenteron during spreading of the bottle cells is the extent of the active contribution of bottle cells to the depth of the archenteron. The bulk of the depth is generated by the vegetal extension of the marginal zone and the movement of the involuted deep cells toward the animal pole.