Control of polar lobe formation in fertilized eggs of Ilyanassa obsoleta stimpson

Abstract

Mechanisms of polar lobe formation in fertilized eggs of Ilyanassa obsoleta are discussed. Data are presented in the context of a model involving a contractile ring of microfilaments in the cortical cytoplasm of the vegetal hemisphere. The polar lobe neck diameter decreases at two distinct rates during formation of second, third, and fourth polar lobes. During the second, more rapid phase of lobe constriction, eggs contain a band of microfilaments arranged circumferentially in the cortical cytoplasm apposed to the plasma membrane at the base of the polar lobe constriction. These microfilaments disappear and lobe constrictions regress in cytochalasin B, but not in colchicine. Colchicine prevents eggs from beginning the second, more rapid phase of lobe constriction. Eggs require the presence of potassium ions but not sodium ions for normal polar lobe formation and cleavage. When eggs are placed in isotonic solutions of CaCl2, within 10-15 min they form cytoplasmic blebs which enlarge into lobes. This calcium-induced blebbing is inhibited by cytochalasin B but not by colchicine. Blebbing occurs in the calcium concentration range of 0.17 M-0.34 M in the presence of NaCl, MgCl2, or MnCl2. Potassium ions actively inhibit the calcium-induced blebbing, however. © 1973 by the American Society of Zoologists.