Electrical coupling of blastomeres in early embryos of ascidians and sea urchins

In the early embryos of ascidians and sea urchins, blastomeres are in electrical communication; however, the type and extent of interaction is related to the basic characteristics of the embryo. In the mosaic-like structure of the ascidian embryo, blastomeres have a coupling ratio of about 1 throughout the division cycle. Coupling is facilitated by the extremely low conductance of the non-junctional membrane and possibly mediated via specialized low resistance junctions. Sea urchin embryos do not have specialized low resistance junctions; however, blastomeres are electrically coupled, probably via cytoplasmic bridges, during the first half of the division cycle. The coupling ratio in sea urchins, initially about 0.3, progressively decreases, together with the conductance of the nonjunctional membrane. During the latter half of the division cycle blastomeres are uncoupled; however, a structural junction appears at the periphery, which may play a role in their destiny.