Back from the Tip of the Nose

About 130 years ago, Giulio Bizozzero, then in Pavia, made a seminal observation [1]. He divided the tissues of the vertebrate body into three categories: those that divide constantly (labile), such as blood and skin, those that never divide, such as striated muscle and brain (perennial), and those that normally do not divide but can do so if injured (stable). As a consequence, diseases that perturb cell division, such as cancer, affect labile tissues, while degenerative diseases affect perennial tissues where repair is inefficient. Epithelia and blood possess a reservoir of cells that divide and maintain a progenitor pool throughout life (the stem cells) whereas striated muscle and brain were supposed not to contain stem cells. Furthermore, stem cells were supposed to generate only the cells of the tissue where they belong.