Metabolic control during preimplantation mammalian development.
Metabolic control during preimplantation development may be either intrinsic to the embryo or extrinsic, i.e. mediated via the environment within the female tract. Intrinsic control is exhibited by early preimplantation embryos, which are undifferentiated, largely autonomous and show no growth. The best characterized intrinsic metabolic control mechanism is the switch from pyruvate to glucose as major energy substrate during mouse preimplantation development. A shift to extrinsic control occurs during the later preimplantation stages, when differentiation into trophectoderm and inner cell mass occurs, net growth begins and the embryo prepares for implantation. Thus, the environment provided by the oviduct, in which the embryos of most mammalian species spend their first 3 days, may be considered as facilitatory rather than obligatory for early embryo development, whereas the uterine environment is required to provide a more complex milieu. The 'facilitators' provided by the oviduct include physical factors, such as ciliary and muscular action, and chemical factors, including oxygen, nutrients, ions and macromolecules. Oviduct fluid secretion is under adrenergic influence, defects in which could contribute to impaired mucosal defence and tubal blockage in women. Preimplantation embryos exhibit metabolic adaptation, whereby their metabolism responds to changes in the external environment. Such adaptation explains the ability of early embryos to develop in a variety of culture media, a situation similar to that which they will experience in the female tract. Thus, the provision of a variety of media in vitro is likely to ensure the development of embryos in a physiological manner.