Membrane potential and resistance were measured in eggs, cleavage stages and blastulae of the South African toad Xenopus laevis, using intracellular microelectrodes.
The membrane potential increased from −6·5 ± 2mV in eggs to −57 ± 8·0mV at the mid-blastula stage.
The input resistance of fertile eggs ranged from 0·5 MΩ to 5·0 MΩ corresponding to a specific resistance of 20–200kΩcm2. During the first two or three division cycles the input resistance usually decreased by a factor of 2–10 and then subsequently rose during the blastula stages from a mean value of 600 ± 100kΩ at stage 5 to 2·0 ± 0·5 MΩ at stage 8.
At all developmental stages examined, point polarization of a surface cell in the embryo by rectangular current pulses of 0·5−6 × 10−8 A produced voltage deflexions in other surface cells. This was seen even when several (7–8) cell junctions intervened between the current passing and voltage recording microelectrodes at distances of more than 1 mm. These measurements suggest that the junctional resistance is low compared with that at the surface, though the geometrical arrangement of cells is not favourable for calculation of absolute values of membrane resistance.
Current spread between cells occurred apparently less easily during mid-blastula stages than at earlier stages in development, perhaps indicating an increase in junctional resistance during development.
A comparison has been drawn between the present measurements and similar ones made in another amphibian, Triturus.