(A) Quantitative reverse transcription PCR detects Tbx18 mRNA as early as E14.5, with expression peaking around embryonic day 16.5 (for each stage n ≥ 4). (B, D, F) TBX18 immunohistochemistry shows TBX18 is expressed in the dorsal aspect of the UGS in the region of the forming anterior prostate buds at E16.5 (B), P0 (D), and P3 (F). (C, E, G) Smooth Muscle Actin (SMA) IHC on sections adjacent to TBX18 stains showing the proximity of these two expression domains; TBX18 is detected in only a small subset of SMA-positive cells. (H-K) Lineage tracing analysis of Tbx18 expressing cells using the Gt(ROSA)26Sortm4(ACTB-tdTomato,-EGFP)Luo/J reporter. Green cells express or are descended from Tbx18 expressing cells (in the images presented here, the Tomato signal was excluded for clarity). (H-I) P0. (J-K) P35 sections show Tbx18 descendants primarily occupy the anterior prostate stroma (J), and dorsal prostate stroma (K). M is Mesenchyme, and E is Epithelium. Scale bars are 100μm B and C, 50μm in D-K.