||Figure 1. (A) Xenopus embryos at three stages during HPC (red) movement to the ventral midline. hpf, hours post-fertilization.
(B) Transverse schematic of HPCs and their microenvironment (blue, ectoderm; red, mesoderm; yellow, endoderm; brown, fibronectin; purple, fibrillin; orange, aPKC; green, ZO-1; dark red, tropomyosin).
(C) Cell shapes by β-catenin localization and cardiomyocytes by tropomyosin expression. Note: red spot (asterisk) in stage 28 is non-specific staining outside the embryo.
(D) aPKC appears on the apical surface of HPCs (white arrows) with fibrillin at the HPC basal surface.
(E) Tight junction protein ZO-1 appears perinuclearly and nascently on the apical surface by stage 23 and strongly at HPC apical intercellular junctions by stage 28. Fibronectin appears at all germ-layer interfaces.
(F–I) Apical aPKC (F and G) and ZO-1 (H and I) during early heart-tube formation. Immunofluorescence shows epithelial individual markers and tropomyosin (F and H, upper panels) and normalized intensity with pseudocolor lookup table (LUT) (F and H, lower panels).
(G and I) Quantification of apical polarity from five or six embryos per time point over two clutches.
Scale bars, 50 μm. ∗p < 0.05, ∗∗p < 0.01. See also Figure S1.