During maturation of most animal sperm cells, sperm-specific nuclear proteins replace somatic histones, DNA synthesis, RNA synthesis, and cell division cease, and the chromatin becomes highly compacted in preparation for the sperm cell's journey to the egg. After fertilization, the dormant sperm nucleus must become activated in order to participate in normal cellular events such as replication, transcription, and mitosis. The deactivation of the male germ cell nucleus during spermatogenesis and its reactivation following fertilization can serve as model systems for the study of gene activity, replication, chromatin and nuclear envelope assembly, and higher order chromatin structure.

We use cultures of sea urchin testis fragments, polyspermically fertilized eggs, microinjection, and cell-free systems to study at the biochemical level transitions in nuclear composition, architecture, and activity during spermatogenesis and fertilization. Using techniques such as gel electrophoresis, in situ hybridization, fluorescence microscopy, flow cytometry, organ culture and subcellular fractionation, we are attempting to understand the molecular basis of sperm nuclear differentiation and male pronuclear formation.

We have been able to reconstruct many of the pronuclear transformations in a test tube, thus making possible much more detailed biochemical investigations. Most recently, we have focused on formation of the male pronuclear envelope, emphasizing the role of proteins and lipids in membrane fusion events. Our work is currently centered on the role of a membrane phospholipid which when hydrolyzed facilitates membrane fusion leading to nuclear envelope formation.