Biological factors in culture media affecting in vitro fertilization, preimplantation embryo development, and implantation.

Optimal culture conditions are of paramount importance for in vitro fertilization of gametes, preimplantation embryo development, and implantation for all species. Water is the basis of all culture media, and ultrapure water should be employed. The main energy sources of a medium are lactate, pyruvate, and glucose. The concentrations of the first two vary in different media, whereas the latter is necessary mainly for the later stages (morula to blastocyst) of development. A fixed nitrogen source is essential for implantation embryo development whether this is provided by amino acids, albumin, or serum. Suboptimal culture conditions can block development. Pronuclear zygotes of most species (but not human) arrest at some point between the two-cell and the 16-cell stage. Modifying culture conditions can lead the embryos to develop through this block. Hypoxanthine also causes a two-cell block to mouse pronuclear zygotes, and this again depends largely on culture conditions. Simple culture media are bicarbonate-buffered systems with pyruvate, lactate, and glucose. Complex media, such as Ham's F-10, contain in addition amino acids and other elements found in serum. Human tubal fluid simulates the fallopian tube microenvironment. EDTA, gonadotropins, growth factors, and other substances can be included in the media to stimulate development. Coculture of embryos with oviductal cells has shown promising results.