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Lineage tracing using inducible genetic labelling has emerged to be a powerful method for interrogating the developmental fate of cells in intact tissues. A common induction mechanism is the use of tamoxifen-dependent Cre recombinase (CreER and CreERT2), but the effects of tamoxifen at doses normally used in lineage tracing studies on normal adult mammary gland homeostasis are not known.MethodsWe used flow cytometry and immunostaining of intact glands to determine if varying doses of tamoxifen skews the distribution and the apoptosis and proliferation status of different types of mammary epithelial cells in vivo. We also examined how tamoxifen influences the number of progenitor and mammary repopulating units (MRUs).ResultsOur results indicate that ?5?mg/25?g body weight of tamoxifen induces a transient increase in cell proliferation and in the number of basal cells in the adult mammary epithelium up to seven days after tamoxifen administration. However, in the medium term (three weeks), all doses of tamoxifen ?1?mg/25?g body weight results in a decrease in the number of basal and EpCAM+CD49b? luminal cells and a decrease in progenitor cell function. Tamoxifen at doses ?5?mg/25?g body weight induced a transient increase in caspase-3 mediated apoptotic cell death within the mammary epithelium. However, mammary epithelial cell numbers in all subpopulations were restored to their original levels by eight weeks. No long lasting effects of tamoxifen on mammary repopulating unit numbers or on pubertal ductal development were observed.


Tamoxifen can skew the distribution of mammary cell types in a dose dependent manner, and thus caution must be taken when interpreting lineage tracing studies using high doses of tamoxifen, particular when short duration analyses of a quantitative nature are being performed.


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