Stem cells are cells that continue to divide in the adult, but they divide in a very particular manner. At each division cycle, one daughter cell remains a stem cell, while the other goes on to divide further and differentiate. In part, this is due to the environment in which the stem cell finds itself, which is known as the stem cell niche. For example, in mammals, the stem cells that lead to the continuous regeneration of the skin and hair are located in a region of the hair follicle, known as the bulge. These cells divide rarely, with one daughter migrating away from the bulge and the other remaining in place. The migrating daughter cell comes to colonize the basal layer of the epidermis, where it continues to divide a number of times. Again, this is a stem cell-like division; the cells that remain attached to the extracellular matrix layer remain stem cell like, while those that leave the “basal cell layer” begin the process of differentiation that leads, eventually, to their death (you are constantly shedding dead skin cells.) In normal skin the process of cell birth and death is balanced. Hyperplasia occurs when cell birth occurs more frequently than cell death. Typically the non-stem cell products of a stem cell division are committed to differentiate and have a finite proliferative life span - they can divide only a limited number of times before they senesce (that is, stop dividing). Terminally differentiated cells no longer divide. The process of cellular senescence is thought to be an internal defense mechanism against cancer; often cancer cells accumulate mutations than enable them to circumvent the effects of senescence.