Figure 3

(A-B): The nuclei of cells within the secondary invagination sites are located basally. Confocal micrographs of flat preparations of embryos double-stained with phalloidin-rhodamine (red) and YOYO (green) (A,A') and single stained with phalloidin-rhodamine (B). (A,A') The apical optical section at 250 hours of development shows that the secondary invagination sites (arrow) are still attached to the apical surface. The nuclei of the secondary precursors are located basally, as revealed by the absence of nuclei staining in the apical cell layer. The asterisks in A' indicate the positions of the cell processes of the secondary invagination sites (compare to A). (B) The basal optical section shows the distinct morphology of the sheath cells (arrows) that subdivide the individual invagination sites.