Surface changes during development and involution of the cement gland of Xenopus laevis.

The cement gland was studied from stage 17, when the anlage is established, to stage 49, shortly before its disappearance. At early stages, the apical membrane is covered by small microvilli that are more abundant than in the surrounding epiblast cells. Vesicular protrusions along the cell boundaries are also more numerous in the gland cells. When the gland reaches maturity, the apical membranes of gland cells differentiate into two regions. In the cranial, kidney-shaped region, the membranes are very narrow and protrude above the level of cell boundaries. Long and slender villi raise from the surface adjacent to cell boundaries. Apical surfaces in the caudal portion are larger and flattened. Cell boundaries are lined with shorter and thicker surface projections. At these stages, the bordering cells are covered with secretion vesicles. During involution the number of cells is progressively reduced. The area of the caudal portion increases relative to the area of the cranial portion. Apical surfaces become more flattened. Surface projections become much shorter and invade the whole of the apical surface. Bordering cells lose their secretion vesicles and their apical surface becomes ruffled with numerous short wrinkles. The significance of the apical structures and their evolution is discussed.