IMMUNOHISTOCHEMISTRY OF 2 DIFFERENT TYPES OF PLACENTAL FIBRINOID

The structure and composition of human placental fibrinoid were studie d on cryostat and paraffin sections and by transmission electron micro scopy as well as immunohistochemistry using antibodies directed agains t fibrin, fibronectin isoforms, collagens IV and VI, laminin and tenas cin. The findings suggest two structurally and immunohistochemically d ifferent subtypes of fibrinoid: fibrin-type fibrinoid and matrix-type fibrinoid. Fibrin-type fibrinoid was characterized by immunoreactivity for fibrin and cellular fibronectin, including the ED-A sequence. Imm unostaining for all other extracellular matrix molecules was negative. Ultrastructurally, this fibrinoid subtype consisted of a meshwork of fibers with 20-nm cross striation typical of fibrin. Fibrin-type fibri noid never contained extravillous trophoblast cells. It is therefore p rimarily a blood clot product derived from maternal and fetal blood. I n contrast, matrix-type fibrinoid showed virtually no evidence of fibr in; it was immunopositive for extracellular matrix molecules such as t he fibronectins, particularly oncofetal fibronectin (containing the ED -B sequence), collagen IV, laminin and tenascin. Oncofetal fibronectin , which was neither expressed in fibrin-type fibrinoid nor in the vill ous stromal core, seemed to be a specific marker for matrix-type fibri noid. Single or clustered nonproliferative extravillous trophoblast ce lls were embedded within the matrix molecules. It is very likely that these cells secrete the matrix in a nonpolarized fashion. Fibrin-type fibrinoid would appear to be involved in shaping the intervillous spac e and in replacing damaged syncytiotrophoblast acting as a transport a nd immune barrier. Matrix-type fibrinoid, as a secretory product of th e extravillous trophoblast, should be discussed in context with the in vasive properties of this cell population.