Electrophysiological studies on the egg of Fucus serratus: The membrane potential

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Abstract
The transmembrane electric potential difference (PD) between cytoplasm and external medium has been measured on unfertilized eggs and zygotes of the marine brown alga, Fucus serratus. The technique devised to record the PD of each of a large number of small cells (Ø 60 μ) with a microelectrode is described (cf. Figs. 1, 2). While the unfertilized egg displays a stationary PD of around—19 mV (cytoplasm negative), the zygote develops a PD of around—78 mV within 12 hours after fertilization. The exponential time-course of this PD is steady except for a remarkable transient depolarisation of about 13 mV, which occurs at 9 hours after fertilization (Fig. 3). Experiments with different external media, in particular different Na+/K+-ratios, imply that the observed PD rise is paralleled by a gain in the plasma membrane's K+-selectivity (Fig. 4). Hence the permeability ratio α=PNa/PK drops substantially, i.e. from about 0.1 to almost 0.001 (Fig. 5). The data indicate that the origin of the developing PD is essentially a passive K+-diffusion potential. This, in turn, requires the cytoplasmic K+-concentration to increasd drastically during the development of the egg. There is clear evidence from the literature for such an increase. Ultimately, this study is to help analyze the morphogenetic responses of the. egg. In Fig. 6, these have been put on the same time-scale together with the PD. The plot grossly reveals two phases of developmental activity, i.e., from 2 to 8 and from 9 to 14 hours after fertilization. This evidence is considered to justify the electrophysiological approach as a tool to analyze the development of the egg.