THE UROKINASE PLASMINOGEN-ACTIVATOR (U-PA) AND ITS INHIBITOR (PAI-1) IN EMBRYO-FETAL BONE-FORMATION IN THE HUMAN - AN IMMUNOHISTOCHEMICAL STUDY

The role of urokinase plasminogen activator and plasminogen activator inhibitor-1 in human embryo-fetal bone formation between the 9th and t he 20th week of gestation has been studied immunohistochemically. Whil e mature osteocytes of the secondary spongiosa and resting chondrocyte s of the bone epiphyses were negative for both antigens in each develo pmental stage, metabolically active parts of the osseocartilaginous sy stem showed a strong immunoreactivity. Until the end of the 10th week of gestation urokinase plasminogen activator and plasminogen activator inhibitor-1 could not be demonstrated in the shaft of the preexisting cartilaginous models of bones, which correlates with the morphologica l developmental stage of the embryos. Later, osteoblasts and chondrocy tes in the areas of enchondral ossification, and the perivascular chon drocytes of the epiphyseal secondary ossification centres, showed simi larly high concentrations of urokinase plasminogen activator and plasm inogen activator inhibitor-1. Moreover, the individual ossification st ages of the different bones in embryo-fetal development could be demon strated immunohistochemically. While humeri and femora showed diaphyse al immunoreactivities at an early stage, positive reactions in the pha langes were found only much later. Thus, the enzymes of the fibrinolyt ic system studied are clearly involved in the desmal and enchondral os sification process in the osseocartilaginous compartment.