New membrane formation and intercellular communication in the early Xenopus embryo.

The ionic permeability of the nonjunctional and newly formed junctional membranes was investigated in embryos of Xenopus laevis up to the onset of the fifth cleavage. Continuous measurements were made of the equivalent nonjunctional (R'o) and junctional resistances (R'i) in different pairs of adjacent cells separated by one of the four cleavage membranes formed in that period. The specific resistance of the nonjunctional membranes (ro) and of each cleavage membrane (ri) as a function of time were derived using a generally applicable computer simulation model. ro decreased from about 40 komega cm2 in the in the uncleaved egg to about 10 komega cm2 at the 16-cell stage, due to the insertion of a small fraction of the relatively permeable newly formed cleavage membranes into the outer surface. Superimposed on this overall decline, a transient decrease of ro was observed during each cycle, caused by a temporary partial separation of the peripheral parts of adjacent blastomeres. The changes in followed the same pattern. R'1 increased stepwise during each cleavage cycle. At the onset of each cleavage there were no significant differences in R'i as measured between different pairs of cells. After an initial phase of membrane formation ri of all cleavage membranes remained constant at about 400 omega cm2. In the states investigated the coupling ratio ranged from 0.8 to 1. It is argued that this close coupling could be the result of the highly impermeable outer surface even in the absence of specialized junctions in the intercellular membranes.