[Co-culture of embryos: influencing factors and mechanisms of action].

In comparison with their in vivo counterparts, the in vitro produced mammalian embryos had markedly lower rates of morula/blastocyst development and pregnancy after transfer to the recipients. Things became even worse in the cloned embryos. This necessitates improvement of the embryo culture system. Co-culture of embryos with different types of somatic cells was found beneficial for embryo development in vitro and many studies have been conducted in this area in recent years. In this paper, recent developments and the authors' own work in studies of co-culture of early mammalian embryos with somatic cells were reviewed, with emphasis on the effects of cell type, stage of estrous cycle and number of passages of somatic cells and supplement of serum on embryo development, and the mechanisms by which co-culture promote embryo development. The recent developments are summarized as follows: 1. Somatic cells of both homogeneous and heterogeneous origins can be used for co-culture of mammalian embryos, with similar developmental rates. 2. Supplementation of animal serum at appropriate concentrations improved the somatic cell growth and consequently the development of embryos in co-culture. 3. The estrous cycle stages of oviduct epithelial cells used for co-culture had no effect on the development of embryos. 4. Over-passaging of somatic cells reduced their efficiency in promoting development of the co-cultured embryos. In conclusion, studies have shown that co-culture overcame the block of embryo development in vitro and improved embryo quality with increased rates of implantation and pregnancy, but many problems remain to be solved on its influencing factors and mechanisms of action.