The recognition of early developmental stages in haemopoiesis

Almost all tissues of multicellular organisms contain cells which have the capacity to change their proliferative activity according to the demand. Some tissues show little or no cellular turnover under normal steady state conditions, but they can switch to a regeneration process in response to perturbation (e.g., mechanical injury). In other tissues, there is continuous cell production to compensate for cell loss due to the continuous utilization of functional cells even under normal conditions. Variation in demand is met by variation in the rate of cell production. The cells which generate offspring throughout life in the continuously renewing tissues are usually designated as stem cells. Stem cells are capable of extensive proliferation which results in new stem cells as well as differentiating cells. The most extensively studied stem cell systems in vertebrates are those of the epidermis, the intestinal epithelium, the testis and the haemopoietic tissues. These systems are commonly used for investigations on the mechanisms of cellular differentiation. In comparison to differentiation processes during embryogenesis, the organization of the stem cell systems in the adult is relatively simple. In adulthood, differentiation is restricted to one or to a limited number of cell types, while, in embryogenesis, differentiation into a large variety of tissues takes place