[Placental passage of macromolecules: transport pathway and mode]

In vivo and in vitro data have recently shown that macromolecules cross the hemochorial placenta. This feature contrats with the size selectivity of the sheep placenta which exclude molecules of 0.45 nm radius. Macromolecules moves by diffusion or endocytosis. In addition to size selectivity, the electrical charges of the macromolecules might be involved in placental transfer. The main resistance to placental transfer lie in the trophoblastic layer and the pathways of specific or unspecific transcytosis are not elicited. Membrane or fluid phase markers (peroxydases), carrier molecules (LDL, transferrin...), hormones and growth factors (insuline, EGF) are internalized by endocytosis into the trophoblast and further degraded or recycled. Immunoglobulins G, which are protected from degradation, reach the fetal circulation. The endothelial layer looks less selective and permits passage either by interendothelial diffusion or transcytosis.