Ultrastructural aspects of mammalian fertilization: new discoveries and inspirations from the work of Daniel Szöllösi.

Our current level of knowledge on mammalian fertilization would not be attained without the contribution of ultrastructural studies. The late Daniel Szöllösi, to whose memory this review is dedicated, was one of the most enthusiastic explorers of this fascinating field. In his landmark electron microscopic studies, he revealed the importance of nuclear envelope breakdown for oocyte maturation and its reconstitution after fertilization, and predicted the era of cloning by publishing articles on the remodeling of a somatic cell, thymocyte nucleus fused with an oocyte. His challenge of Boveri's hypothesis on the paternal inheritance of centrosomes spurred further research on this subject that ultimately led to the definition of biparentally contributed mammalian zygotic centrosomes, for which the only exception is found in rodents. Very early, Szöllösi and his colleagues devoted their interest to the studies of the fate of sperm accessory structures after fertilization, an area that has yet to be explored at the molecular level, but which may have profound implications for the swiftly advancing field of assisted human and animal reproduction. These studies contributed a great deal to our current understanding of mammalian fertilization and still serve as an inspiration for present studies on involved mechanisms.