1 Introduction: Stem Cell Biology

Abstract

STEM CELLS: AN OVERVIEW
There is still no universally acceptable definition of the term stem cell, despite a growing common understanding of the circumstances in which it should be used. According to this more recent perspective, the concept of “stem cell” is indissolubly linked with growth via the multiplication rather than the enlargement of cells. Various schemes for classifying tissues according to their mode of growth have been proposed, one of the earliest of which is that of Bizzozero (1894). This classification, which relates to the situation in the adult rather than in the embryo, recognizes three basic types of tissues: renewing, expanding, and static. Obvious examples of the first are intestinal epithelium and skin, and of the second, liver. The third category was held to include the central nervous system, although recent studies have shown that neurogenesis does continue in adulthood, for example, with regard to production of neurons that migrate to the olfactory bulbs (Gage 2000). There are various problems with such schemes of classification including, for instance, assignment of organs like the mammary gland which, depending on the circumstances of the individual, may engage in one or more cycles of marked growth, differentiation, and subsequent involution.

Any attempt to find a universally acceptable definition of the term stem cell is probably doomed to fail. Nonetheless, certain attributes can be assigned to particular cells in both developing and adult multicellular organisms that serve to distinguish them from the remaining cells of the tissues to which they belong. Most...