Ion currents and membrane domains in the cleaving Xenopus egg.

Abstract

We used an extracellular vibrating probe to measure ion currents through the cleaving Xenopus laevis egg. Measurements indicate sharp membrane heterogeneities . Current leaves the first cleavage furrow after new, unpigmented membrane is inserted . This outward current may be carried by K+ efflux . No direct involvement of the Na+,K+-ATPase in the generation of this outward current is detected at first cleavage. Inward current enters the old, pigmented membrane ; however, it does not enter uniformly. The inward current is largest at the old membrane bordering the new membrane . This suggests a heterogeneous ion channel distribution within the old membrane . Experiments suggest that the inward current may be carried by Na' influx, Ca" influx, and CI-efflux . No steady currents were detected during grey crescent formation, the surface contraction waves preceding cleavage, or with groove formation at the beginning of cleavage .The cleavage process in amphibian eggs includes the addition of new membrane at the cleavage furrow that forms blastomeres with two distinct membrane domains . Each blastomere retains some of the original egg surface with pigment granules underlying the membrane and each gains new, unpigmented membrane . In addition to being morphologically distinct, these membrane domains appear to have different electrical properties . Woodward (45) and de Laat and co-workers (7,8) have suggested that the membrane hyperpolarization observed during first cleavage is due to the addition of new, unpigmented membrane that has a high potassium permeability . We investigated the electrical properties of these two membranes in Xenopus laevis embryos, using the extracellular vibrating probe that is used to measure steady ion currents which cells drive through themselves .Ion currents have been shown to be correlated with growth and pattern formation in a number of developing systems (5,19,22,23,28,44) . In a previous study of immature Xenopus oocytes, Robinson (31) detected an animal-vegetal current which falls to nearly zero in response to a variety of maturation-promoting factors. No steady transmembrane currents have been detected in the mature, unfertilized egg or in precleavage fertilized eggs aside from a ring-shaped wave of inward current which spreads over the egg for 4 min following activation (D. Kline and R. Nuccitelli, manuscript in preparation) . Here we report on electrical measurements made beginning 15 min after fertilization and on through second cleavage . Large steady currents are found after new membrane

Implications of a poroelastic cytoplasm for the dynamics of animal cell shape

“…We hypothesize it is also be driven, in part, by local shrinkage of the cytoplasm at the equator due to K + (and/or Cl − ) efflux at the equator. K + efflux has been detected in furrowing frog eggs [63]. In both cases, for water influx to cause swelling locally, rather than globally, requires that pressure not equilibrate globally on the time-and length-scales of motility.…”

“…If, instead, ion flows promote local swelling as proposed in figure 7, we would instead see water influx at the front. Detecting the predicted ion flows might be easier, indeed outflow of ions at the cleavage furrow in frog eggs, probably driven by K + efflux, was detected using vibrating electrodes [63]. Perturbation of both the osmotic and cytoskeletal players in figure 7 is possible using specific small molecule inhibitors.…”

Epithelial cell polarity in early Xenopus development

“…Although transcellular currents were indeed measured, their role in the maintenance of apical/basolateral polarity is still unclear (Kline et al, 1983;Nuticelli and Wiley, 1985). Holtfreter (1943aHoltfreter ( , 1948 originally inferred that polarity of superficial blastomeres is maintained because of the presence of extracellular deposits, the "surface coat."…”

Early embryonic expression of ion channels and pumps in chick and Xenopus development

“…Epithelial cell sheets often consist of highly polarized cells that drive current through the embryo much like the plasma membrane drives ion flux through the single cell (Nuccitelli, 1986). Strong standing membrane potentials and ion currents have been found in many organisms (Nuccitelli, 1988) and have been specifically described in various tissues in chick and frog embryos (Oconnor et al, 1977;Kline et al, 1983;Hotary and Robinson, 1990;Levin et al, 2002).…”