Placental Transfer

Essentially all maternal fetal transfer occurs via the placenta. The chorioallantoic placenta has the same transfer mechanisms found in other epithelial systems. These include passive diffusion, facilitated diffusion, active transfer, and receptor-mediated endocytosis. Most substances present in fetal or maternal blood can cross the placenta via one or more of these mechanisms.

Numerous substances traverse the placenta by simple diffusion. In general, diffusion permeability varies directly with solubility and varies inversely with molecular weight, degree of polarity, and electrical charge. Placental tissues are relatively permeable to water, some electrolytes, and lipophilic substances such as oxygen, carbon dioxide, and ethanol. These compounds equilibrate between maternal and fetal blood. Transfer of this type is limited by maternal and fetal perfusion of the placenta, and is most strongly related to the lesser flow (generally umbilical blood flow). Both fetal and maternal perfusion of the placenta, as well as placen-tal vascularization, increase dramatically as pregnancy advances. The increased uterine and umbilical blood flows, in combination with increased placental vascu-larity, facilitate greater rates of transfer as fetal needs increase.[1]

For flow-limited clearance, orientation of maternal and fetal blood flow in the placenta affect the efficiency of placental transfer. Arrangements of maternal and fetal microvasculatures in placental exchange areas include countercurrent, concurrent, crosscurrent, and multivillous. The countercurrent arrangement, in which the maternal and fetal blood flows in opposite directions (horse, guinea pig), is thought to be the most efficient. Trans-placental clearance of highly diffusible substances is approximately equal to umbilical blood flow. The multi-villous arrangement, in which maternal blood bathes the villous branches in which fetal blood flows to, then away from the villous tips (hemochorial villous pla-centation; human, primates), appears to be the next most efficient. Vascular relationships in the sheep, goat, and cow are somewhat controversial, but it is generally agreed that all have relatively inefficient exchange systems. It is thought that sheep have a crosscurrent vascular arrangement. Efficiency of transfer in the cow is somewhat less than would be predicted by either concurrent or crosscurrent arrangement, suggesting significant shunting away from areas of exchange. Efficiency of transfer increases as pregnancy advances, possibly due to decreased shunting and greater vascular development, especially in the umbilical microcirculation.

Diffusion of hydrophilic molecules is lower than observed with lipophilic substances. Transfer of these substances, such as urea, is diffusion limited. Transfer is not greatly affected by fetal or maternal blood flows and there is incomplete equilibrium between maternal and fetal blood. There are large differences among species in placental permeability to substances having diffusion-limited clearance.

Glucose is an example of substances transferred across the placenta by carrier-mediated facilitated diffusion. Rate of transfer is considerably greater than could be accomplished by simple diffusion, but rate and direction of transfer are dependent on concentration gradient. The predominant glucose transporter protein isoforms in sheep placenta are GLUT-1 and GLUT-3.[2] Ontogenic change in GLUT-3 activity is thought to account for much of the fivefold increase in glucose transport capacity of the sheep placenta between mid and late gestation. Glucose transport is insulin and sodium independent, is stereospecific, and can be competitively inhibited by glucose analogues.

Glucose entry into gravid uterine tissues is largely determined by maternal arterial concentration, whereas transport to the fetus is determined by the transplacental concentration gradient. This concentration gradient is directly related to placental and fetal glucose metabolism, which are in turn influenced by fetal arterial glucose concentration. Thus, as fetal glucose concentration changes relative to that of the mother, placental transfer of glucose to the fetus varies inversely with placental glucose metabolism.

Placental tissues utilize oxygen and glucose at a high rate, reflecting the very high metabolic rate of those tissues. In addition to its impact on placental transfer of glucose, placental glucose metabolism has a major impact on the pattern of carbohydrates delivered to the fetus. In the sheep and cow, uteroplacental tissues utilize 65 to 80% of the glucose taken up from maternal circulation. Lactate 35%), alanine and other nonessential amino acids 30%), CO2 20%), and fructose and various polyols 5%) are major products of placental glucose metabolism. During midgestation, virtually all of the lactate produced by the ovine placenta is released into maternal circulation. At that time, about 70% of fetal lactate is oxidized by both fetal (60%) and fetal placental (40%) tissues to CO2, with the remaining 30% of the carbon appearing primarily in nonessential amino acids, especially glutamate, glutamine, serine, and glycine. During late gestation, the fetal placenta becomes a major net source of fetal lactate, and a negligible contributor to fetal lactate disposal.

Fructose is a major form of carbohydrate in fetal blood. Fructose, as well as several polyols, is produced in fetal placental tissues from glucose in all ungulates and

Cetacea,[3] and large fetal/maternal concentration ratios are maintained. The large concentration gradient is maintained, in part, by very low placental permeability.

Amino acid concentrations are higher in fetal than maternal blood, although the magnitude of the difference varies among amino acids. Observations that most amino acids taken up by the placenta are transported against a fetal/maternal concentration gradient imply the use of energy-dependent, active transport processes. Transfer of amino acids across the placenta involves mediated transport mechanisms at the microvillus and basal membrane, and perhaps diffusion. Both Na-dependent (e.g., system A, ASC, N) and Na-independent (e.g., system L, y+, bo,+) transporter systems have been identified.[4,5] Concentrations of amino acids in maternal and fetal blood, as well as metabolism within placental tissues, are also intimately involved in regulation of amino acid supply to the developing fetus.[6]

Iron and calcium are also transferred by active transport mechanisms. Knowledge of lipid transport and metabolism within placental tissues is limited, in part, because there are wide species differences. Lipid transport in epitheliochorial placentae appears to be much more limited than in hemochorial placentae.

Immunoglobulins and lipoproteins are transported across the placenta of some species, including primates, by endocytosis. This process serves to provide passive immunity to the fetus. In rodents, rabbits, and guinea pigs, transfer occurs via the yolk sac placenta. In ruminants, offspring receive maternal immunoglobulins only after birth via colostrum.

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