An ultrastructural study of the developing urogenital tract in early human fetuses

Objectives: This study examines the gonoducts during the ambisexual stage of human fetal development to define their ultrastructural characteristics, including the origin and anatomic relationship of the mesonephric and paramesonephric ducts during gonoductal development. Study Design: The reproductive tracts from five fetuses ranging in gestational age from 35 to 45 days were processed for ultrastructural examination. The developing mesonephric ducts, paramesonephric ducts, and their surrounding mesenchyme were studied with a Phillips 300 transmission electron microscope. Results: The mesonephric ducts and paramesonephric ducts have distinctive cytoplasmic and cell surface ultrastructurai characteristics, as well as different morphologic patterns of epithelial-mesenchymal interaction. Cephalad portions of mesonephric ducts and paramesonephric ducts are separated by mesenchyme, but more caudal aspects move progressively closer until they are juxtaposed but separate. Conclusions: Early mesonephric ducts and paramesonephric ducts may be distinguished because of their distinctive ultrastructural features; epithelial-mesenchymal interaction may be important in their differentiation and maintenance; both gonoducts retain their morphologic identity throughout, supporting their separate origins.