MEMBRANE POTENTIAL AND RESISTANCE OF THE STARFISH EGG BEFORE AND AFTER FERTILIZATION

Abstract

1. The paper describes electrical characteristics of the egg of the starfish Asterias forbesii as measured with a microelectrode penetrating the surface. The study included the effects of fertilizing the egg while the latter was impaled on the electrode. 2. It has been confirmed that penetration of the egg membrane cannot be indicated solely by the seeming visualization of the microelectrode at the center of the egg. 3. A method involving use of two microelectrodes is described for ascertaining penetration of the egg surface. One internal electrode delivers a current pulse and the other records the time course of the resultant membrane IR drop. 4. Contrary to the reports of many earlier investigations on echinoderm and other eggs a potential difference is found upon penetration of the unfertilized egg. 5. The potential difference at the time of penetration amounts to about 60 mv., inside negative, but this soon decreases to lower steady values ranging from – 10 to – 50 mv. in different eggs. 6. Upon insemination of the impaled egg the membrane potential abruptly decreases by 5 to 10 mv. when sperm are seen to have reached the egg, then rises during the ensuing 1 to 2½ minutes, as the fertilization membrane is raised, reaching a new steady value 5 to 20 mv. greater than that of the unfertilized egg. 7. The possible basis of these changes is discussed. 8. The internal K⁺ of unfertilized Asterias eggs is from 17 to 21 times higher than that of sea water. The sodium determinations, while subject to larger error, indicate a concentration less than 5% that of sea water. 9. The membrane potential of either unfertilized or fertilized eggs decreases when the external K⁺ is raised and returns to the original value when the excess K⁺ is removed. 10. As in many other kinds of cells the potential is evidently a consequence primarily of the high internal concentration of, and permeability to, K⁺, but the magnitude appears less than predicted by the Nernst equation. 11. The membrane resistance of the unfertilized egg averages 3100 ohm-cm.², but might be higher on the assumption of possible leaks around the microelectrode. 12. The measured resistance is unchanged after fertilization, but, on the assumption of the formation of a tighter electrical seal the actual membrane resistance would be lower. 13. The membrane capacity is of the order of 0.5 µF/cm.²