Placental transfer of fatty acids.

Information derived from animal investigations using labelled thtty acid infusions and measurements of the fatty acid concentrations found in the various fractions i n maternal and foetal umbilical artery and venous blood and tissues have shown that the net flux of fatty acid from mother t o foetus across the placenta varies greatly from species to species. In general, in those species with maternal as well as foetal layers in the placenta, e.g. sheep, pig and cat, fatty acid transfer is small, and the profile of fatty acids in the foetal circulation does not match that in the maternal circulation (Elphick ct al., 1979, 1980: Leat & Harrison. 1980: Elphick & Hull, 1985), whereas in those species with only foetal layers in the placenta, e.g. rabbits, rats and guinea pigs, tlie net flux can be high and the fatty acid mixture entering the foetal circulation from the placenta rcflccts maternal free fatty acid concentrations of tlie various fatty acids (Elphick & Hull, 1977; Hunimel cf al., 1075: Herslifield & Nemeth, 1968; Thomas & Lowy, 1982). Arachidonic acid is a comnion exception. The human placenta like the rabbit placenta does not have maternal layers. The human foetus. i n contrast to other species, lays down sizeable stores of fat in white adipose tissue in the last trimester of pregnancy. In tlie guinea pip and rabbit the fat is mainly found in brown adipose tissue. Fatty acids have been shown to cross from tlie maternal spaces into the foetal vessels of tlie isolated perfused human placenta and the fatty acids appear to cross in an unselected manner (Booth et al., 1981). However, during tlie brief period when this preparation is viable, arachidonic acid leaves the placenta into both the maternal and foetal compartments. So human placental tissue is permeable to fatty acids. Measurement of fatty acid mixtures in the various plasma compartments in umbilical cord artery and vein blood taken easily and simultaneously during elective Caesarean section at term is the closest we have come t o obtaining data which reflect tlie physiological state in man (Hendrickse et al., 1985). Even so the anaesthesia and surgery provoked a rise in maternal circulating free fatty acid concentrations and n o doubt handling the cord affected placental perfusion. Fatty acid concentrations in the human maternal circulation. umbilical vein. umbilical artery and the umbilical venous-arterial difference are shown in Fig. 1. There was a significant flow of fatty acids to tlie foetus. I n o 40 r (dl