[The stem cell system of hematopoiesis: physiological and pathophysiological concepts]

It is the purpose of this review to describe the physiological as well as the pathophysiological principles of the hematopoietic stem cell system. The concept of hemopoietic stem cells has a long history which is now understood on the basis of its embryogenesis and after collecting extensive experimental and clinical experience using stem-cell transplantations as a means to restore hematopoietic function of the bone marrow after appropriate conditioning. The hemopoietic stem cells cannot be distinguished by light microscopy from "lymphocytes" considered to be a heterogeneous group of mononuclear cells. These stem cells can respond to specific regulatory factors with specific differentiation and proliferation, and are very radiosensitive and resistant to cryopreservation. The system responds to perturbations in a manner characteristic for feed back regulation and is bound in its physiology to an intact stromal matrix.