Development of human male pronucleus: ultrastructure and timing.

The process of human male pronuclear formation was studied using an experimental model based on in vitro inseminated human zona-free eggs prepared from oocytes that failed to fertilize in a clinical in vitro fertilization program. The main ultrastructural changes in penetrated sperm nuclei transforming pronuclei were used to define four stages of pronuclear development. The first two stages, representing partial (Stage 1) and total (Stage 2) sperm chromatin decondensation, appeared as early as 1 hr after mixing of gametes. This rapid initial phase was followed by a more lengthy array of events leading to transformation of decondensed sperm nuclei into fully developed male pronuclei (Stages 3 and 4). Stage 3 was characterized by reformation of the nuclear envelope, reorganization of chromatin, and the assembly of nucleolar precursors. It was not completed until 12 hr after in vitro insemination when fully developed male pronuclei (Stage 4) were first observed. In some eggs pronuclei did not reach Stage 4 at all. The results of this study provide a morphological background for further research into molecular aspects of human male pronuclear development and its regulation.