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Comparative studies of embryo metabolism in early pregnancy.

Embryo metabolism during early pregnancy has been investigated, mainly in the mouse and rabbit, using a variety of catabolic and anabolic parameters. The rate of metabolic activity of the conceptus continues to increase markedly with the approach of implantation. However, in species in which the embryo enters a period of delayed implantation the diapause is associated with relative metabolic quiescence. Mouse, rabbit, ovine and bovine preimplantation embryos can be cultured in a simple defined medium. Under such conditions energy substrates in the medium represent a major source of carbon for anabolism. Glucose carbon is incorporated into macromolecules during in-vitro culture of cleaving mouse embryos and implanting and diapausing mouse blastocysts. In particular, both acid-soluble glycogen and desmoglycogen are rapidly synthesized from glucose presumably to act as a source of energy at implantation. The pattern of glucose accumulation by cleaving sheep embryos and embryonic discs and early somites excised from sheep and cattle embryos between Days 14 and 18 or pregnancy is basically similar to that in the mouse except that very little glycogen is synthesized by ruminant embryos. During embryonic diapause in the tammar wallaby there is a cessation of cell division and glucose uptake appears to be relatively low. After removal of pouch young in the breeding season to reactivate embryo development, glucose accumulation by the tammar blastocyst increases. The increase is related to the start of blastocyst expansion. Furthermore, the pattern of glucose metabolism in the reactivated wallaby blastocyst is similar to that in ruminant species, approximately 30% of the glucose carbon being incorporated into macromolecules such as protein, lipid and nucleic acid but with little synthesis of glycogen.