Immunofluorescence and histology.

Paraffin sections were deparaffinized and rehydrated and subjected to antigen retrieval in Antigen Retrieve Solution (HK086-9K, Biogenex) at 95°C for 20 minutes. Sections were blocked with 5% horse serum for 1 hour at room temperature. Mouse tissues were incubated with anti-CD31 (1:100, catalog DIA-310, dianova), anti-tdTomato (1:100, catalog MBS448092, MyBioSource), anti–Ki-67 (1:100, catalog AB9260, MilliporeSigma), anti–NG-2 (1:100, catalog AB5320, MilliporeSigma), anti–α-SMA (1:100, catalog A5228, MilliporeSigma), and anti-eMHC (catalog F1.652, DSHB) antibodies overnight at 4°C. For Matrigel plug assay, sections were incubated with anti-CD31 antibody overnight at 4°C. Sections were stained with Alexa Fluor 488–conjugated (catalog A-21202, catalog A-11006, catalog A-11034, Thermo Fisher Scientific), 568–conjugated (catalog 11079, catalog A-11004, Thermo Fisher Scientific), and 647–conjugated (catalog 405416, BioLegend) appropriate secondary IgGs (1:500) for 1 hour at room temperature. Images were acquired with an Axio Imager fluorescence microscope (ZEISS). Images were quantified by ImageJ software (NIH). For histological study, sections were stained with H&E and imaged with an Axio Lab microscope (ZEISS).