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Functional Characterization of Mammary Stem Cells

The most common epidermal tumours are basal cell carcinomas and squamous cell carcinomas in humans, and papillomas and squamous cell carcinomas in mice. However, the full range of epidermal tumour types reflects the full range of differentiated lineages in the epidermis. There are tumours that contain differentiated sebocytes, tumours with aberrant hair follicles and tumours with characteristics of apocrine and eccrine glands. The differentiated characteristics of a tumour will depend on the nature of the founding cell, the specific genetic lesions sustained and influences from the tumour microenvironment. One key regulator of both normal epidermal lineage commitment and epidermal tumour type is the level of β-catenin signalling (Niemann and Watt, 2002). When β-catenin levels are elevated, new hair follicles form in postnatal IFE. Conversely, when β-catenin is absent, or its activity is blocked, hair follicles are converted into cysts of interfollicular epidermis with associated sebocytes. Transgenic mice that overexpress β-catenin in the epidermis develop hair follicle tumours. Conversely, mice that express N-terminally truncated Lef1, which blocks β-catenin signaling, develop tumours with sebaceous and squamous differentiation.