Morphological studies on the role of the periductal stroma in the regression of the human male Müllerian duct.

The regression of the male Müllerian duct has been studied in human embryos and fetuses by means of the semi-thin light microscopic technique and by electron microscopy. After completion of the duct's differentiation during week 7, a periductal stroma is formed by two types of mesenchymal-like cells: light, epitheloid cells originating from the coelomic epithelium and dark, fusiform cells of mesonephric origin. During week 9 these cells condense to a compact cuff in which the light cells occupy the inner core. The duct is entirely sealed by an epitheloid stratum of the periductal stroma. At the same time, the basal lamina thickens up to 300 nm by apposition of extracellular material. During weeks 10 to 12, the inner stromal core is invaded by dark fusiform cells from the peripheral stratum which intermingle with the light cells. The basal lamina dissolves, the epithelio-stromal interface becomes indistinct and finally disappears. During week 13 remnants of the Müllerian duct can be observed. They result from the complete merging of the ductal into the periductal compartment. Müllerian duct regression is divided into two functional steps: First the duct is prevented from growth by the epitheloid cells of the stromal cuff. This process lasts for 2 to 3 weeks. In the second place the basal lamina breaks up under the influence of the dark stromal component. This event launches the regression proper and lasts for another 2 to 3 weeks. Necrosis of cells or programmed cell death does not play a decisive role in the regression of the human Müllerian duct. In the upper, nonregressing part of the duct, light epitheloid cells are scarce and do not seal the duct. A periductal extracellular space is preserved during the entire period and the periductal stroma does not fuse with the duct's epithelium. The epithelio-stromal interface is maintained along this section.