The rat endometrium during pregnancy was used as a model system to study fibronectin in vivo. Fibronectin distribution on stromal fibroblasts, as determined by indirect immunofluorescence staining, was studied in relationship to cell shape during decidual transformation. Fibroblasts of the estrus endometrial stroma were elongated cells with a fibrillar pattern of fibronectin on their surfaces. During days 1-6 of pregnancy, as these elongated cells acquired a round morphology, fibronectin changed first to a patched distribution on the cells'a surfaces and then disappeared. The change in fibronectin was specific for the fibroblasts since over the same time period there was no decrease in fibronectin found associated with blood vessels or in the epithelial-stromal basement membrane. These results support the proposed relationship between cell surface fibronectin and cell shape that has been inferred from in vitro experiments. After implantation, fibronectin distribution was studied in relationship to the position of the conceptus. In the stroma proximal to the implanting conceptus, fibronectin was absent except around blood vessels, which may help explain how decidual tissue could act as a barrier to trophoblast invasion. Finally, fibronectin distribution was studied in the uterus after parturition. Debris in the uterine lumen was coated with fibronectin, which may be important in the rapid removal of this material by phagocytic cells. Also, fibronectin associated with the epithelial-stromal basement membrane was reorganized after reepithelialization had occurred.

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