Formation of the sea urchin male pronucleus in vitro: Membrane‐independent chromatin decondensation and nuclear envelope‐dependent nuclear swelling

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 sperm chromatin decondenses into a spherical nucleus of ≈4 μm in diameter. Pronuclear swelling to ≈7 μm requires an intact nuclear envelope, and the degree of swelling is limited by the amount of MVs assembled on the chromatin. Furthermore, after a nuclear envelope is formed, swelling can occur in the absence of additional cytoplasmic MVs. Nuclear swelling also requires ATP hydrolysis, Ca 2+ and cytosolic factors, some of which are sensitive to heat and to the sulfhy‐dryl alkylating agent, N ‐ethylmaleimide. The requirement for a nuclear envelope and the rate of pronuclear swelling are consistent with previous in vivo observations. © 1995 wiley‐Liss, Inc.