NUCLEAR AND CYTOPLASMIC DYNAMICS OF SPERM PENETRATION, PRONUCLEAR FORMATION AND MICROTUBULE ORGANIZATION DURING FERTILIZATION AND EARLY PREIMPLANTATION DEVELOPMENT IN THE HUMAN

This report describes spatial and temporal aspects of sperm penetratio n and intracytoplasmic migration, pronuclear evolution and the specifi city of presyngamic opposition, stage-specific changes in cytoskeletal organization and the relative contribution of maternal and paternal c omponents to mitotic spindle formation, These studies involved observa tions of living human oocytes during conventional insemination in vitr o and after intracytoplasmic deposition of spermatozoa, analysis of ch romatin organization and distribution during pronuclear evolution, and detection of actin and alpha-, beta- and gamma-tubulin by confocal im muno-fluorescence microscopy, Immature and mature oocytes, penetrated but unfertilized oocytes, fertilized but arrested eggs, and cleavage-s tage embryos from normal and dispermic fertilizations were examined, T he results demonstrate that sperm nuclear migration to the maternal pe rinuclear region is rapid and linear, occurs in the absence of a detec table cytoskeletal system and appears to be assisted by an unusual con figuration of the sperm tail principal piece which results from either retained intracytoplasmic motility or the process by which the sperm tail is progressively incorporated into the oocyte, Our findings also show a specificity of pronuclear alignment that is associated with a p olarized distribution of both maternal and paternal chromatin, and wit h the position of the sperm centrosome and the presence of microtubule s nucleated from this structure, The results also indicate that a mate rnal microtubule nucleating capacity is present in the immature oocyte but is apparently inactive until spindle formation, The poles of the first mitotic spindle appear to be derived from the sperm centrosome, although some maternal contribution cannot be excluded, The sperm tail and centrosome persist in a single cell through the cleavage stages, and the latter serves as a prominent site of cytoplasmic microtubule n ucleation, The results provide a detailed understanding of the cellula r and nuclear morphodynamics of the human fertilization process and in dicate subtle defects that may be responsible for early developmental failure.