In vitro enhancement of early-stage embryos with co-culture.

Over the years, a great deal of effort has been made to maintain the viability of mammalian embryos once they have been removed from the female's uterus. Early embryo research studies used the simplest of medium (eg, physiological saline) in an effort to maintain embryo homeostasis in vitro, but little progress on embryo viability was made. The addition of heat-treated serum, buffers, and essential amino acids to culture media have shown evidence that improvements could be made in culturing mammalian embryos in vitro; however, it has become clearly evident that this was not going to be a simple task. In recent years, the most notable progress made in this research area has been made with the use of "helper" cells to co-culture early-stage embryos to hatched blastocysts. This article reviews the classic works that have contributed to the development of embryo culture systems that are now used in genetic engineering research and commercial embryo transplant units in the livestock industry. The development of trophoblastic vesicle culture methods and uterine cell co-culture systems were basic contributors to the development of the mammalian embryo co-culture systems as we know them today. Recent studies with follicular granulosa cells and oviduct epithelial cells have added much to our understanding of embryo development in vitro. Novel co-culture systems, such as culturing mammalian embryos in the amnion of a developing chick embryo, certainly offer encouragement that research efforts should continue until an optimal culture system is developed for each mammalian species. The potential uses of embryo culture systems for both humans and animals have yet to be fully understood.