Early development of the Mullerian duct in avian embryos with reference to the human - An ultrastructural and immunohistochemical study

In vertebrates, the female reproductive system arises from the Mullerian (paramesonephric) duct which develops in both sexes under the influence of the Wolffian (mesonephric) duct. For a better understanding of the interactions between the Mullerian duct and its adjacent tissues, we present a systematic scanning and transmission electron microscopic investigation of early stages of avian Mullerian duct development. This starts within the cranial part of the Mullerian ridge from a placode-like thickening and deepening of the coelomic epithelium containing nephrostomes as remnants of the last pronephric and first mesonephric tubules. Groups of cells detach from this placode and rapidly expand caudally as a solid cord. This becomes canalized, but the tip region remains mesenchymal and is found enclosed within the basal lamina of the Wolffian duct. Immunostaining reveals that the Mullerian duct migrates within a matrix rich in laminin and entactin. When the canalized duct has opened into the coelomic cavity, one or more secondary ducts are found immediately caudal of the main funnel, for a short period only, possibly to supply material to the expanding duct. BrdU-anti-BrdU reaction reveals a high proliferation of the duct epithelium. The thickened epithelium of the Mullerian ridge dissolves to form the mesenchymal layers of the duct. Immunostaining with vimentin argues against a cellular contribution of Wolffian duct cells to the Mullerian duct. Comparing the data from avian embryos with those of human indicates that the modalities of early Mullerian duct development are similar in both species.