FORMATION OF THE SEA-URCHIN MALE PRONUCLEUS IN-VITRO - MEMBRANE-INDEPENDENT CHROMATIN DECONDENSATION AND NUCLEAR ENVELOPE-DEPENDENT NUCLEARSWELLING

We demonstrate that complete sea urchin male pronuclear development in vitro is a two-step process involving membrane-independent chromatin decondensation and nuclear envelope-dependent pronuclear swelling. In the absence of cytoplasmic membrane vesicles (MVs), permeabilized sper m chromatin decondenses into a spherical nucleus of approximate to 4 m u m in diameter. Pronuclear swelling to approximate to 7 mu m requires an intact nuclear envelope, and the degree of swelling is limited by the amount of MVs assembled on the chromatin. Furthermore, after a nuc lear envelope is formed, swelling can occur in the absence of addition al cytoplasmic MVs. Nuclear swelling also requires ATP hydrolysis, Ca2 + and cytosolic factors, some of which are sensitive to heat and to th e sulfhydryl alkylating agent, N-ethylmaleimide. The requirement for a nuclear envelope and the rate of pronuclear swelling are consistent w ith previous in vivo observations. (C) 1995 Wiley-Liss, Inc.