Regulation of nutrient uptake and metabolism in pre-elongation ruminant embryos

Our current understanding of pre-elongation embryo metabolism and its regulation by factors both intrinsic to the embryo and present in its immediate environment is limited mainly to studies in rodents and of ruminant embryos that have been cultured in vitro. Energy metabolism in such embryos is initially low and dependent on oxidative phosphorylation for the generation of ATP. The embryo exhibits substrate preference for carboxylic acids, such as pyruvate, during this period. Glucose uptake is limited initially but increases after compaction, and it is metabolized mostly to lactate. Glucose uptake is facilitated by a number of transporters, but the presence and function of these, and their regulation by growth factors, such as insulin, are not well characterized. Even less is known about the metabolic fate of amino acids and lipids. Approximately 50% of the lipid fraction in the mature oocyte is in the form of triglyceride, much of which is oxidized during fertilization and the early cleavage stages. Immunoreactivity of the growth hormone receptor is detectable from day 3 after fertilization, and so growth hormone acting in either a paracrine or endocrine manner may serve to regulate glucose, glycogen and lipid metabolism. The metabolic and mitogenic actions of insulin and insulin-like growth factor (IGF) I and II are thought to be mediated mainly by the IGF-I receptor. The actions of leptin in the ruminant embryo are less well understood. Circulating concentrations of these growth factors, together with nutrients supplied to the follicle and oviduct, can be modified by diet, but in ways that are not fully understood. The present review discusses these issues and highlights areas for future research endeavour where emphasis is directed on to combining thoughtfully designed whole animal studies with in vitro culture experiments.