depots in situ based on their ability to retain DNA nucleotide label (BrdU). Neonate and mature male C57BLG/J mice were injected intraperitoneally with BrdU- and label-retaining cells (LRC) were quantified in fat depots by immunohistochemical, immunofluorescent, and flow cytometric methods. In neonates, LRC Constituted 27% of the cells in inguinal fat (MAT) and 65% in interscapular brown fat (BAT) after Day 10 and 26% of the cells in epididymal fat (eWAT) after Day 28. After 52 days, the LRC accounted for 0.72% of iWAT, 0.53% of eWAT and 1.05% of BAT, respectively. The BrdU-labeled cells localized to two areas: single cells distributed among adipocytes or those adjacent to the blood vessels wall. in mature C57BL6/J mice, flow cytometric analysis determined that a majority of the LRC were also positive for stem cell antigen-1 (Sca-1). (C) 2008 Elsevier Inc. All rights reserved.