Bioelectric Responses of the Echinoderm Egg to Fertilization

Abstract

The fertilization reaction of eclihiiodernm eggs (Lytechinus pictus, a sea urchin, and Dendraster excentricus, a sand dollar) was followed with intracellular electrodes. Membrane potential and K+ activity were recorded.The unfertilized egg of Lytechinus has a membrane potential of -8 mV, inside negative. Within 5 sec after the addition of sperm, a fertilization action potential develops, going to +10 mV, inside positive. The time from the initial depolarization to a return to the original -8 mV is 120-150 see. The repolarization continues until a potential of -10 to -14 mV is reached, at which point it pauses for 3-4 min. At 6-8 min after fertilization, a further and relatively rapid hyperpolarization begins, going to -60 to -65 mV by 15-25 min after fertilization and remaining constant at these values.The membrane potential of the unfertilized egg appears to depend on a general permeability to anions. The fertilization action potential seems to reflect a prolonged influx of sodium. The final depolarization to -60 nmV is attributable to the development of potassium conductance.Simultaneous measurements with a K+ ion-selective electrode gives constant readings of about 240 mM K+ iil the unfertilized eggs throughout the fertilization process.Similar results were obtained with Dendraster eggs. The resting potential of the unfertilized eggs was -7 mV; the action potential on activation attained +18 mV; the repolarization paused at -16 to -24 mV and the final potential attained was -70 mV. The electrical cliatuges after fertilization with spermatozoa or activation with Pronase were identical.This report deals with alterations iii inembrane potentials accompanying the fertilization of echinoderm eggs. The problem has a long history, associated with the intuition that the activation of the egg should have some analogies with the responses of recognized types of excitable cells to stimulation. An electrical response at fertilization was predicted by Lillie (1); the difficulty of recording it was discussed by Rothschild (2) and, in more recent times, there has appeared some evidence that the membrane potentials of animal eggs do chang(e during the activation of the eggs (3). We now report and interpret a definite sequence of bioelectric changes during the activation of sea urchin and sand dollar eggs, including a 'response analogous to an action potential.Previous studies had been hampered by the difficulty of penetrating the extremely flexible plasma membranes with microelectrodes. The solution of various technical piroblemis permitted us to make more than 150 successful recordings under conditions where the ionic medium surrounding the eggs could be varied; thus, the permeability characteristics of the membranes at various stages of the fertilization process could be analyzed. Our interpretations of the membrane changes have also been reinforced by simultaneous measurement of intracellular K + ion activity and membrane potential. MATERIALS AND METHODSHandling of gametes Eggs and sperm of the sea urchin Lytechin...

Intracellular calcium release at fertilization in the sea urchin egg

“…Sperm was added to dishes in the undiluted form as described below. Eggs were stuck down by the protamine method (Steinhardt et al, 1971) to permit microinjection of the aequorin solution. The dish was placed in a water bath the temperature of which was maintained at 18 ± 0.5°C throughout these experiments.…”

Bioelectric responses at fertilization: Separation of the events associated with insemination from those due to the cortical reaction in sea urchin, Lytechinus variegatus

“…The early changes include an action potential [Okamoto et al, 1977;Chambers and de Armendi, 19791 and a Na+-dependent reversal of the membrane potential [Steinhardt et al, 19711, which both together may be responsible for the fast block to polyspermy [Jaffe, 19761. The repolarization of the membrane is accompanied by an increase in K+-conductance and by intracellular changes in [Ca++], which trigger the metabolic activation of the egg [Steinhardt et al, 1971;Grainger et al, 1979;Steinhardt and Winkler, 1979;Shen and Steinhardt, 1980;Winkler et al, 19801.…”

Studies of the mechanism of the electrical polyspermy block using voltage clamp during cross-species fertilization.

“…4 and Table I) . Thus, the amplitude of the fertilization potential is a characteristic of the species of egg, as expected since the fertilization potential channels are not introduced by the sperm but are in the egg before fertilization (2,11,14,24,25) . On the other hand, the duration of the fertilization potential was longer when sea urchin eggs were fertilized with Urechis sperm than when they were fertilized with homologous sperm (Fig.…”