Lineage tracing of hair follicle stem cells in epidermal whole mounts.

Lineage tracing of tissue stem cells represents a powerful tool to address fundamental questions of deve-lopment, differentiation and cellular renewal in a natural tissue environment. The Cre/lox site-specific recombination system is increasingly used to genetically label specific cell populations to perform cell lineage tracing or fate mapping experiments in sophisticated mouse models. Here we describe a method of labeling and subsequent tracking stem cells of the hair follicle bulge region in mouse skin. Hair follicle stem cells are specifically labeled by expressing the Cre recombinase under control of keratin15 (K15) regulatory sequences and by crossing the Cre-containing animals with Cre-sensitive Rosa26R (R26R) reporter mice. To achieve a temporal control of recombinase activity in stem cells, Cre is fused to a modified estrogen receptor (CreER(G)T2). In the K15CreER(G)T2/R26R mouse model, hair follicle stem cells (HFSCs) are specifically labeled after Cre activation upon treatment of mice with tamoxifen. By analyzing the skin tissue at different time points following genetic labeling, important information on stem cell behavior and contribution of labeled stem cells to epidermal structures during tissue homeostasis and hair follicle regeneration are obtained. Combining the lineage tracing approach with the whole mount technique allows examining large areas of the epidermis containing many hair follicles and sebaceous glands and reveals the complex three-dimensional relationship of labeled stem cell clones within the tissue.