Extracellular Mg2+ induces a loss of microvilli, membrane retrieval, and the subsequent cortical reaction, in the oocyte of the prawn Palaemon serratus

Surface changes induced by sea water were analyzed in the ovulated oocyte of the prawn Palaemon serratus. They depended on the presence of external Mg2+ but not on external Ca2+ alone. Increasing external Mg2+ from 0 mM to 30 mM stimulated first a progressive disappearance of preexisting microvilli, which was over within 30 min of incubation. This is correlated with membrane removal via internalization of coated vesicles, ascertained by observations of endocytosis of an extracellular fluid-phase marker and by measurement of a diminution in membrane capacitance (Cm). Thirty-five minutes after sea water contact, the prawn oocyte underwent a cortical reaction independent of fertilization. It consists in a heavy exocytosis of ring-shaped elements, leading to the deposition of a thick capsule, and requiring a threshold Mg2+ concentration of greater than or equal to 10 mM and at least a 3-min incubation with Mg2+. Concurrently, the values of the membrane capacitance (Cm) and conductance (Gm) increased about 2 and 10 times their initial values, respectively. The calcium ionophore ionomycin, added to Mg(2+)-free artificial sea water, stimulated the cortical reaction with requirement of external Ca2+. Other divalent cations (Mn2+, Zn2+, Co2+, Ni2+, Cd2+) instead of Mg2+, induced the cortical reaction, but Ba2+, Sr2+, and La3+ did not. When eggs are fertilized, the cortical reaction takes place in two steps, the first being a discrete exocytosis of a foamy material and the second always involving ring-shaped elements.