Figure 1.

Evidence for long-range GJC in chick and frog embryos determining left-right asymmetry

Chick embryos cultured as whole blastoderms (A) exhibit the normal left-sided expression of the markers Nodal (B) and Sonic hedgehog in Hensen’s node (C), indicating normal asymmetry. In contrast, when the distal left side of the embryo is cut off before culture (D), cells on the far right side begin to express Nodal (E) and Sonic hedgehog expression in the node is bilateral (F).

Panels A and D are schematics of st. 2 chick embryos (primitive streak stage); the region encompassed by the square indicates the part that was cultured. The purple stain in panels B-F is signal, indicating expression of the relevant gene marker via in situ hybridization. Red arrowheads indicate expression; white arrowheads indicate absence of expression.

In frog embryos, small molecule fluorescent probes are able to penetrate several cell diameters through gap junctions (G). Panel G shows four large blastomeres of the 16-cell embryo. The left-most cell (indicated by red arrow) was injected with a fluorescent small molecule tracer dye, which is seen spreading to all 4 of its neighbors in a clock-wise direction. Panel H shows, in section, induced GJC (via injection of C×26 mRNA) in early frog embryos. The transfer of Lucifer Yellow (< 1kD) but not rhodamine-labeled dextran (RLD, =10 kD) in section demonstrates true gap junctional communication and rules out cytoplasmic bridges and incomplete cleavages.