Localization of urokinase-type and tissue-type plasminogen activator mRNA during organogenesis in the mouse.

Cryostat sections of E8.5 to E9.5 mouse embryos were hybridized with S-labelled RNA probes to urokinase-type and tissue-type plasminogen activator (uPA and tPA, respectively) mRNA. The sections were screened for expression of either gene. Two main features emerge from the results. (a) The stage of initial detection is different for each transcript. The uPA mRNA is first detected in cephalic mesenchyme at E8.5 when tPA mRNA is already widely distributed in tissues derived from ectoderm and mesoderm; later, the uPA mRNA transcripts were found throughout most mesodermic tissues, (b) Each gene presents a different pattern of expression. The uPA is restricted to single cells or to small groups of cells within tissues; this distribution suggests its involvement in cell migratory mechanisms. On the other hand, tPA was detected in most tissues, with variable intensities. Its expression gains complexity while organogenesis proceeds. This pattern supports the hypothesis that regulatory mechanisms other than a direct gene regulation are involved. In extraembryonic tissues, uPA and tPA genes are constantly expressed at a high level in trophoblastic giant cells and parietal endoderm, respectively. Our results confirm the presence of plasminogen activator during embryonic development and provide detailed picture of the plasminogen activator gene expression in mouse organogenesis.