Two-photon microscopy for intracutaneous imaging of stem cell activity in mice.

The adult skin is a typical example of a highly regenerative tissue. Terminally differentiated keratinocytes are shed from the external layers of the epidermis or extruded from the skin as part of the growing hair shaft on a daily basis. These are effectively replenished through the activity of skin-resident stem cells. Precise regulation of stem cell activity is critical for normal skin homoeostasis or wound healing and irregular stem cell proliferation or differentiation can lead to skin disease. The scarcity and dynamic nature of stem cells presents a major challenge for elucidating their mechanism of action. To address this, we have recently established a system for visualizing stem cell activity, in real time or long term, in the intact skin of live mice using two-photon microscopy. The purpose of this review was to provide essential information to researchers who wish to incorporate two-photon microscopy and live imaging into their experimental toolbox for studying aspects of skin and stem biology in the mouse model. We discuss fundamental principles of the method, instrumentation and basic experimental approaches to interrogate stem cell activity in the interfollicular epidermis and hair follicle.