Sulf1 transcripts were detected in the developing mesenchyme (mes) but not the epithelium (epi) at E16 (A). A Sulf1 sense probe was used as a negative control (B). At E18, Sulf1 transcripts were detected in the muscularis propria (mp in C and D), and in the peri-mesenchymal epithelium (C, arrow). Expression was not detected in the tips of developing prostatic buds (indicated by *) or in the peri-epithelial mesenchyme (C, arrowhead). A high magnification view (D), shows Sulf1 expression was detected in the most proximal region of the epithelial buds (arrowheads) but not in the bud tips (indicated by *). At P1, Sulf1 expression was localized to the peri-mesenchymal epithelium of the proximal ducts (E arrow; * indicate epithelial ducts). At P3, Sulf1 was expressed in the distal stroma (str) but not in the distal epithelium (F; * indicate epithelial ducts). A longitudinal section through a P3 duct (G) further illustrates the pattern of Sulf1 expression (str, stroma; arrows indicate branch points; P, proximal; D, distal). At P14, Sulf1 expression was restricted to the stroma (H, arrow) surrounding epithelial ducts.