Structural organization of actin in the sea urchin egg cortex: microvillar elongation in the absence of actin filament bundle formation

Abstract

The relationship between the formation of actin filament bundles and the elongation of microvilli (MW) after fertilization in sea urchin eggs is studied. The morphology of microvilli in eggs activated with and without the release of fertilization acid was compared. Activation of eggs in normal sea water with the Ca ionophore A23187 causes the release of fertilization acid and the elongation of MW containing core bundles of actin filaments. Eggs activated with A23187 in Na+-free water do not undergo normal fertilization acid release but develop elongated, flaccid MV. These MV contain an irregular network of actin filaments rather than the parallel bundles of filaments found in normal MV. The addition of 40 mM NaCl to these eggs results in the release of H+ and the concomitant conversion of flaccid MV to erect MV containing typical core bundles of actin filaments. Identical results are obtained when 10 mM NH4Cl is substituted for NaCl. The induction of cytoplasmic alkalinization in unactivated eggs with NH4Cl does not cause either MV elongation or the formation of actin filament bundles. The elongation of MV is stimulated by a rise in intracellular free Ca2+ concentration. Actin filament bundle formation is triggered by an increase in cytoplasmic pH. The formation of actin filament bundles is not necessary for MV elongation but is required to provide rigid support for MV.