For immunohistochemical analysis, fixed serial sections of mouse tissues were mounted onto slides and deparaffinized with xylene, rehydrated in a series of ethanol solutions (100%, 95%, 70%, 50%), and washed with deionized water. TUNEL Staining to detect apoptosis was preformed using the "DeadEnd™ Colorimetric TUNEL System" (Promega USA, Madison, WI). Sections were pretreated with Pro K enzyme (provided in the kit, diluted 1:500 with PBS). Strepavidin (also provided in the kit) was diluted at 1:500 in PBS.