Epithelio-mesenchymal interface and fibronectin in the differentiation of the rat mesonephric and paramesonephric ducts.

The distribution of fibronectin and the morphological differentiation of the genital ducts was studied in rat fetuses at ages from 15 to 21 days. Fibronectin was localized with the peroxidase-antiperoxidase and avidin-biotin method at the electron- and light-microscope level. In 15-day-old male and female fetuses, fibronectin was localized as a continuous lamella around the mesonephric duct and as a discontinuous lamella around the paramesonephric duct. During the differentiation of the female paramesonephric duct, the fibronectin layer became continuous and remained so after the age of 16 days. The fibronectin layer of the male mesonephric duct remained continuous at all ages. The accumulation of mesenchymal cells on the outer surface of the female mesonephric duct and the concomitant detachment of the fibronectin layer around the duct suggests that mesenchymal regulation plays a role in the regression of the mesonephric duct. In the regressing male paramesonephric duct fibronectin was simultaneously lost in the condensed periductal mesenchyme, the places of epithelio-mesenchymal contact, and the epithelial cytoplasmic protrusions towards the mesenchyme. Ultrastructurally, fibronectin was localized in the basal laminae, on the cell membrane in contact with the extracellular material, and on the surface of the fibrillar and flocculent extracellular material. In addition to auto- and heterophagy, epithelio-mesenchymal interactions seem to play an important role in the regression of the genital ducts, although in different ways in males and females. The present results give additional support to the theory of the possible migration of epithelial cells into the surrounding mesenchyme during the regression of the paramesonephric duct.