surrounds surrounds primitive urogenital sinus mesenchyme urogenital sinus mesenchyme EMAPA:31500 Mesenchyme that surrounds primitive urogenital sinus. UGM UBERON:0009845 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