Morphogenesis of the bovine rete testis: extratesticular rete, mesonephrosand establishment of the definitive urogenital junction

The development of the extratesticular rete, the regression of the mesoneph ros and the establishment of the urogenital junction between rete testis an d efferent ductules were investigated in 67 bovine embryos and fetuses coll ected in the period from day 29 through day 250 post conception. The result s were obtained by immunohistochemistry and by the study of semithin sectio ns. At about day 30, the large mesonephros contains a peculiar Malpighian b ody in its cranial part, generally referred to as the mesonephric giant cor puscle, which is connected to the Wolffian duct by a series of well-develop ed and functioning mesonephric tubules. This set of primary mesonephric tub ules, however, will not participate in the formation of the definitive urog enital junction, but will regress and soon disappear completely. The effere nt ductules in the bovine are represented by another set of secondary meson ephric tubules that grow out from the dorsal aspect of the mesonephric gian t corpuscle at about day 50. Transiently, the lumina of the sprouting effer ent ductules are plugged by invading intraductular blood vessels, probably representing rudimentary glomeruli. The proximal portions of the newly-form ed efferent ductules establish side-to-end contacts with extensions of the extratesticular rete that has bypassed the regressing giant corpuscle. At 8 5 days, the efferent ductules have reached the Wolffian duct and open into it. At 150 days, the channels of the extratesticular rete display a patent lumen and now form end-to-end anastomoses with the efferent ductules. The p roliferating mesenchymal cells surrounding the epithelia of the efferent du ctules have arranged in several concentric layers at about 85 days. These m esenchymal cells are the precursors of the periductular musculature and are reached by the first nerve fibers at about day 130.