Mesonephric tubules expressing estrogen and androgen receptors remain in the rete ovarii of adult mice.

The rete ovarii and epoophoron in females are homologous structures of the rete testis and efferent/epididymal duct in males and are derived from the developing rete cells and mesonephric tubules, respectively. Sex steroid hormones play a critical role in reproductive function for both sexes, and we recently reported expression patterns of sex steroid receptors in developing male reproductive tracts. However, their expression patterns in females remain unclear. We, therefore, investigated the three-dimensional structure and expression patterns of sex steroid receptors in the rete ovarii and epoophoron of fetal and adult female mice. In adult females, the epoophoron was not adherent to the rete ovarii. The rete ovarii had a bursa-like structure, with its extra-ovarian region protruding toward the epoophoron. A marker for mesonephric tubules, PAX2 (Paired box 2), was detected in the epoophoron and a small population of epithelial cells in the extra-ovarian rete ovarii. These epithelial cells expressed estrogen receptor and androgen receptor. During development, mesonephric tubules were adherent to the rete ovarii at first, but as the development proceeded, the continuity was lost due to the interruption of the tubule rather than separation between the tip of the tubule and rete ovarii. These findings suggest that epithelial cells, originating from the mesonephric tubules, persist even in the adult rete ovarii with maintained expressions of receptors for estrogen and androgen.