Abstract
Development of mammalian embryos in vitro is functionally and temporally inferior to embryo development as it occurs inside the female reproductive tract. The deficiencies of cultured embryos range from slow cleavage rates to complete developmental arrests or blocks, occurring at particular stages in many species. A variety of approaches have been used to overcome the blocks, including most extensively the coculture of preimplantation embryos with various somatic cells. However, even with coculture, development of embryos in vitro is still not equivalent to that in vivo. In most laboratories, only 25-40% of inseminated oocytes develop into morulae and blastocysts in spite of numerous variations on the basic technique. A better understanding of the factors governing embryonic growth is required before we can hope to achieve results comparable with those occurring in vivo.