Large Ionic Currents Leave the Primitive Streak of the 7.5-Day Mouse Embryo

During mouse gastrulation, underlying mesoderm cells contribute to the future embryonic endoderm, though the mechanism of this morphogenetic process is not well understood. We have utilized the two-dimensional vibrating probe to investigate the relationship between transembryonic ionic currents and the underlying cellular morphogenetic rearrangements. We were able to detect strong outward currents along the midline of the embryo, with inward currents completing the current loop via the extraembryonic endoderm. The average maximum current density for 21 embryos was 21.2 µA/cm² situated over the developing foregut region. This outward current sometimes reached magnitudes of 60 µA/cm² and could be detected superior to Reichardt's membrane when it was left intact during the dissection. The outward current gradually decreased as the probe was moved posterior to the foregut and turned inward just beyond the hindgut and rostral to the foregut over the extraembryonic endoderm. These currents were stable over several hours and were temperature-sensitive, gradually decreasing as the temperature was lowered. Neither amiloride nor ouabain blocked or decreased these currents, though the lack of an effect may have been due to restricted access of these drugs to internal epithelia. The outward current was dramatically increased if an artificial leak was created. This suggests that the observed currents are due to a passive leak of ions through leaky junctions along the midline. Such leaky junctions would be expected to occur at regions in which extensive cellular rearrangements were occurring due to breakage and reformation of cellular junctions as would occur during gastrulation in this region.