Advances in understanding permeability in fetal capillaries of the human placenta: a review of organization of the endothelial paracellular clefts and their junctional complexes.

A review is presented of the evidence that the capillaries of the fetal-placental circulations of man and the guinea-pig are typical members of class of continuous non-brain capillaries. Their permeability is similar to that of muscle capillaries, and there is much evidence that the main permeation route for hydrophilic solutes in capillaries of this class is through the paracellular clefts of the endothelium. The properties of this paracellular route are based on discontinuous tight junctions which may serve to restrict the fraction of the paracellular cleft available to solutes, and on a molecular sieve which may be based on arrays of adhesion molecules in the zonula adhaerens of the junctional complex. The characteristics of this paracellular route and of overall microvascular permeability in these types of placenta make it clear that fetal capillaries form a significant component of the total transplacental permeability restriction.