09-P081 Experimental evidence for the ectodermal origin of the epithelial anlage of the bursa of Fabricius

The prostate develops in the urogenital sinus (UGS) under the influence of androgens. Whilst free androgens can enter the cell to activate the androgen receptor and induce prostate development, approximately 98.5% of androgens in plasma are bound to binding proteins such as sex hormone binding globulin (SHBG). There is increasing evidence that an alternative pathway for androgens to enter target cells is physiologically significant. SHBG-bound steroids can bind to Megalin, which mediates internalization. The steroid cargo is released into cytoplasm where it can bind to and activate the androgen receptor. This internalization process is inhibited by another endogenous protein, Receptor Associated Protein (RAP), which binds to Megalin, preventing binding of steroid–SHBG complexes. This study cloned the tammar orthologues of Megalin, SHBG and RAP and examined their expression of these genes throughout early prostate development in both tammar wallaby and mouse by RT-PCR and immunohistochemistry. Tammar SHBG, Megalin and RAP are highly similar to their murine counterparts (81.8%, 89.4% and 87.9%, respectively). Transcripts were detected in the murine and tammar liver, kidney, testis, ovary and developing male and female UGS from before the start of prostate differentiation to adulthood. SHBG, Megalin and RAP were strongly localized to the urogenital epithelium in both the mouse and the tammar. The presence of these three key proteins in the urogenital sinus, and our preliminary data on their expression patterns is consistent with a conserved role of Megalin in mediating steroid uptake into the developing prostate in both eutherian and marsupial mammals.