Ultrastructure of cleft contacts. Cells after 20 min of reaggregation. (A) Pair of mesoderm (large yolk platelets) and ectoderm cells (small platelets). (B) Alternating large gaps and narrow contacts between cells of ectoderm–mesoderm pair. (C and D) High-magnification views of regions of close (C) and intermediate contacts (D), respectively, of a cell pair. (E and F) Cleft contact ECM stained with lanthanum/alcian blue. ECM stretching across gaps (small arrows) or in patches on cell surface and in large gaps (dashed arrows) is shown. (G and H) Contacts between normal ectoderm and PAPC-MO–injected prechordal mesoderm cells, ECM stained with lanthanum/alcian blue. e, ectodermal BCR; m, prechordal mesoderm; l, large gaps; i, intermediate contacts; c, close, adherens junction–like contacts; sc, super-close contacts; large arrows, points of divergence of membranes at end of intermediate contacts.