Maternal tamoxifen treatment expands the macrophage population of early mouse embryos.

Several different transgenic tamoxifen-inducible cre reporter lines have been used to analyse the contribution of embryonic precursors to the development of the mononuclear phagocyte system in mice. Here we show that tamoxifen treatment of the mother at 8.5dpc with doses commonly-used in lineage trace studies produces a 4-5-fold expansion of the embryonic leukocyte populations by 10.5dpc, detected in whole mounts of embryos using a Csf1r reporter gene or separately by expression of Csf1r, Itgam (CD11b), Adgre1 (F4/80) or Ptprc (CD45) mRNA. These findings indicate that tamoxifen cannot be considered a neutral agonist in macrophage lineage trace studies.