Establishment of the urogenital junction in the male bovine embryo: an ultrastructural study

The ultrastructure of the developing extratesticular rete testis, the effer ent ductules and the establishment of the urogenital junction were studied in bovine embryos and fetuses of 41 through 95 days post conceptionem. The efferent ductules originate as a new set of secondary mesonephric tubules f rom the dorsal aspect of the nephric giant corpuscle and grow in the direct ion of the Wolffian duct. Cytological differentiation of the efferent ductu les proceeds in a proximo - distal direction. At about 50-60 days, the simp le columnar epithelium of the proximal portions of the efferent ductules al ready consists of the two typical cell types, i.e. reabsorptive principal c ells with an endocytotic apparatus and a brush-border and ciliated cells. T he lumen of the proximal portion is temporarily filled with intraductular b lood vessels and perivascular tissue which may represent vestigial rudiment s of glomeruli associated with the efferent ductules. At 50 to 60 days, the extratesticular rete still has a blastema - like appearance and consists o f irregular cells with abundant glycogen. Extensions of the extratesticular rete come into contact with the efferent ductules and create the first end -to-side anastomoses with the latter. Somewhat later, the separating basal laminas vanish and invading rete cells intermingle with the epithelium of t he efferent ductules, thus establishing the urogenital junction.