Normal stem cells

Definition

The need to continuously replace mature cells in the blood requires the production of about 1011 new cells daily in a normal adult, and even more in response to hemopoietic stress. It is known that all these cells are derived from a common ancestor population, the pluripotential stem cells (Lajtha, 1983). The usually accepted definition of stem cells is based on three characteristics: first, their marked capacity for proliferation, as just illustrated, and, second, their potential to undergo differentiation to produce all the lymphohemopoietic mature cell types (Metcalf, 1988). Third, and classically, stem cells are also defined by their reported capacity for self-renewal, i.e., the capacity to generate new stem cells, with the implication that they are able to regenerate their own population (Lajtha, 1983; Metcalf, 1988). As we will discuss later, it is mainly this latter concept that has to be discussed in the context of aplastic anemia (AA).

Regulation

Stem cells comprise only a small minority, between 0.01 and 0.05%, of the total cells found in bone marrow. At least 95% of the hemopoietic cells fall into morphologically recognizable types. The remaining, with nonspecific morphological features, require phenotypic and functional characterization. They encompass not only the stem cells but also their immediate progeny, the progenitor cells which were first characterized by their ability to develop in vitro in response to the colony-stimulating factors (CSF) (Metcalf, 1988).