THE EXTRAVASCULAR CONTRACTILE SYSTEM IN THE HUMAN PLACENTA - MORPHOLOGICAL AND IMMUNOCYTOCHEMICAL INVESTIGATIONS

In the human placenta, besides the fetal blood vessel system a second extravascular contractile system exists. It is localized in the chorio nic plate and runs in a longitudinal direction and adjacent to fetal b lood vessels into the stem villi, where it forms perivascular contract ile sheaths. Characteristically, cells of the extravascular contractil e system are extremely long and spindle-shaped and give rise to fine c ell processes, by which they obviously contact each other or insert in to the basement membrane of the trophoblast. They show immunoreactivit y with desmin, vimentin, alpha-actin, myosin, nitric oxide synthase ty pe I (brain form) and dipeptidyl peptidase IV. The ultrastructure sugg ests that cells of the extravascular contractile system are related to smooth muscle cells, including subpopulations with myofibroblastic fe atures. In stem villi a few cells are nitric oxide synthase type I imm unoreactive. These cells are thought to be specialized smooth-muscle-l ike cells of the extravascular contractile system or cells of the extr avascular contractile system related to paraneurons that generate nitr ic oxide, which, in turn, may modulate the tone of perivascular contra ctile sheaths. The high dipeptidyl peptidase IV activity suggests that modulation of the extravascular contractile system may also occur by substance P.