Extracellular matrix components of the placental extravillous trophoblast: immunocytochemistry and ultrastructural distribution.

Invasive extravillous trophoblast cells of the human placenta are embedded in a self-secreted extracellular matrix, the matrix-type fibrinoid. The ultrastructure and molecular composition of the matrix-type fibrinoid of the term human placenta were studied by transmission electron microscopy and immunogold labelling. We used antibodies directed against different matrix proteins such as collagen type IV, laminin, vitronectin, heparan sulfate, various fibronectin isoforms, and against the oncofetal blood group antigen, "i". Immunogold labelling patterns of matrix proteins are the basis for the subdivision of the trophoblast-derived matrix-type fibrinoid into mosaic-like patches of structurally and immunocytochemically different compartments. Firstly, fine granular patches with structural similarities to basal lamina material are composed solely of collagen type IV and laminin. Secondly, an ultrastructurally amorphous glossy substance shows reactivity with antibodies against heparan sulfate and vitronectin. A third type of patches, fine fibrillar networks embedded in the above-mentioned glossy matrix, are reactive with antibodies against normal fibronectin isoforms (IST-4, IST-6, IST-9) and oncofetal isoforms (BC-1, FDC-6). The blood group precursor antigen "i" was not only expressed on the surfaces of the extravillous trophoblast cells but was associated with the fibronectin-positive fibrils. In conclusion, within this extracellular matrix, clear compartments of different composition can be distinguished from each other. Glycosylation with "i" in this matrix may be involved in immunological masking, thus preventing rejection of placenta and fetus.