EXTRACELLULAR-MATRIX COMPONENTS OF THE PLACENTAL EXTRAVILLOUS TROPHOBLAST - IMMUNOCYTOCHEMISTRY AND ULTRASTRUCTURAL DISTRIBUTION

Invasive extravillous trophoblast cells of the human placenta are embe dded in a self-secreted extracellular matrix, the matrix-type fibrinoi d. The ultrastructure and molecular composition of the matrix-type fib rinoid of the term human placenta were studied by transmission electro n microscopy and immunogold labelling. We used antibodies directed aga inst different matrix proteins such as collagen type IV, laminin, vitr onectin, heparan sulfate, various fibronectin isoforms, and against th e oncofetal blood group antigen, ''i''. Immunogold labelling patterns of matrix proteins are the basis for the subdivision of the trophoblas t-derived matrix-type fibrinoid into mosaic-like patches of structural ly and immunocytochemically different compartments. Firstly, fine gran ular patches with structural similarities to basal lamina material are composed solely of collagen type IV and laminin. Secondly, an ultrast ructurally amorphous glossy substance shows reactivity with antibodies against heparan sulfate and vitronectin. A third type of patches, fin e fibrillar networks embedded in the above-mentioned glossy matrix, ar e reactive with antibodies against normal fibronectin isoforms (IST-4, IST-6, IST-9) and oncofetal isoforms (BC-1, FDC-6). The blood group p recursor antigen ''i'' was not only expressed on the surfaces of the e xtravillous trophoblast cells but was associated with the fibronectin- positive fibrils. In conclusion, within this extracellular matrix, cle ar compartments of different composition can be distinguished from eac h other. Glycosylation with ''i'' in this matrix may be involved in im munological masking, thus preventing rejection of placenta and fetus.