The hormone that induces maturation of starfish oocytes is 1-MA. When oocytes are exposed to 1-MA, a series of cytological changes occur.Scanning EM and immunofluorescence microscopy have established that 1-MA
immediately stimulates the transient appearance of prominent microvilli on the oocyte surface caused by the rapid assembly and disassembly of the filamentous actin bundles in their inner cores. This is a fast response that occurs within 1 min after the addition of 1-MA. An equally rapid change in response to 1-MA is
the quick release of intracellular ca. Following these early events, 1-MA then induces more extensive reorganization of cytoplasmic actin network and drastic changes in the phosphorylation state of numerous proteins. In parallel,intracellular organelles such as the endoplasmic reticulum(ER) undergo structural changes. The final event of the maturation process is the breakdown of the nuclear envelope of the GV. owing to these cytoplasmic changes, theAlternatively, phalloidin itself have stabilized and thereby increased the amount of F-actin. Although it is difficult to distinguish between these two possibilities, phalloidin-induced actin polymer-ization is known to take amorphous shapes and require more time. It is worth emphasizing that the site of our interest is in the cortical region where the effect of phalloidin on the actin cytoskeleton is minimal or absent. Furthermore, even in the
presence of microinjected phalloidin, live oocytes underwent normal maturation in response to 1-MA (not shown) implying that the phalloidin probe does not interfere with meiotic maturation.