surrounds primitive urogenital sinus mesenchyme urogenital sinus mesenchyme Androgen receptor (AR) activation releases instructive signals from UGM that acts on UGS epithelium (UGE) to stimulate cell proliferation, form prostate ductal progenitors (prostatic buds), and regulate cell adhesion dynamics to permit prostatic bud outgrowth EMAPA:31500 EMAPA:31519 Mesenchyme that surrounds primitive urogenital sinus. UBERON:0009845 UGM uberon