This volume contains the Proceedings of the Serono Symposium on Pre implantation Embryo Development, held in Newton, Massachusetts, in 1991. The idea for the symposium grew out of the 1989 Serono Symposium on Fertilization in Mammals* at which preimplantation development was the predominant suggestion for a follow-up topic. This was indeed a timely subject in view of the recent resurgence of interest in this funda mental phase of embryogenesis and its relevance to basic research and applied fertility studies in humans, food-producing animals, and endangered species. The symposium brought together speakers from a broad range of disciplines in order to focus on key regulatory mechanisms in embryo development, using a wide variety of animal models, and on representative topics in human preimplantation embryogenesis. The culmination of preimplantation development is a blastocyst con taining the first differentiated embryonic tissues and capable of initiating and sustaining pregnancy. The central objective of the symposium was to throw light on the regulation of cellular and molecular events underlying blastocyst formation. It was particularly appropriate that the date of the symposium marked the 20th anniversary of the publication of the classic volume Biology of the Blastocyst, the proceedings of an international workshop held in 1970. This book, which summarized most of the information then available on this topic in mammals, was edited by the pioneer in blastocyst research, Dr. Richard B1andau, who was the guest speaker at the symposium."

The use of human in vitro fertilization in the management of infertility is the outgrowth of years of laboratory observations on in vitro sperm-egg interaction. "The editors of this work have themselves contributed significantly to basic knowledge of the mammalian fertilization process. The observations of Don Wolf on sperm penetration, the block to polyspermy and, most recently, sperm hyperactivation in the monkey and human, Gregory Kopf's elucidation of the mechanisms of sperm activation during penetration and the reciprocal dialogue between sperm and egg, and Barry Bavister's definition of culture conditions and requirements necessary for in vitro oocyte maturation, fertilization and development in model mammalian systems including nonhuman primates have contributed greatly to our understanding of the mammalian fertilization process. Wolf, Kopf and Gerrity have enjoyed substantial interaction with clinicians in Departments of Obstetrics and Gynecology and have been directly involved with successful IVF programs. Both Wolf and Kopf have served as research scientists in the Division of Reproductive Biology at the University of Pennsylvania, which, for more than 22 years, has fostered co-mingling of clinically oriented and basic science faculty. It is through such interaction, which clearly exists at many institutions including the University of Wisconsin, that the process of technology transfer is best served. Without an exquisitely coordinated laboratory, there can be no consistent success in human in vitro fertilization. Quality control is pivotal, but close collaboration between the laboratory and the clinic is also essential as information is shared and correlated.

With a few notable exceptions, mammalian preimplantation embryos grown in vitro are likely to exhibit sub-optimal or retarded development. This may be manifested in different ways, depending on the species and on the stage(s) of embryonic development that are being examined. For example, bovine embryos often experience difficulty in cleaving under in vitro conditions, and usually cease development at about the 8-cell stage (Wright and Bondioli, 1981). The block to development is stage-dependent; embryos cultured for 24 hr from the I-cell stage are much more capable of developing into viable blastocysts after transfer to oviducts than embryos cultured for 24 hr from the 4-cell stage prior to transfer (Eyestone et oZ. , 1985). Similar problems with in vitro embryo development are encountered in other species. Pig embryos can be grown up to the 4-cell stage in vitro but usually no further (Davis and Day, 1978). In the golden hamster, in the rat and in many outbred strains of mice, development of zygotes in vitro is blocked at the 2-cell stage (Yanagimachi and Chang, 1964; Whittingham, 1975). Even with some inbred mouse strains, embryo development is reduced if very early cleavage stages are used as the starting point for in vitro culture (Spielmann et oZ. , 1980). A common finding is that embryos grown in vitro have reduced cell counts (Harlow and Quinn, 1982; Kane, 1985) and their viability is reduced (Bowman and McLaren, 1970; Papaioannou and Ebert, 1986) compared to equivalent developmental stages recovered from mated animals.