Genetic resistance to bone marrow transplantation

The homeostasis of several tissues is maintained by a process of proliferation and differentiation to replace the cells which are constantly lost. The most undifferentiated cells of a certain tissue, capable of proliferation and differentiation, are stern cells. Stem cells have "the capacity for extensive proliferation, resulting in renewal of its own kind as well as giving rise to fully differentiated cells" (Caffrey-Tyler and Everett, 1966). In multicellular animals various types of stem cells can exist in a more or less differentiated form. In the most primitive multicellular organisms, the parazoa, any cell type is capable of proliferation and differentiation into the different cell types which form the organism. In these organisms each cell can be considered as a stem cell. Among higher classified animals, Planarians (Platyhelminthes) are known for their enormous capacity to regenerate. These flatworms carry an omnipotent stem cell called neoblast. Different techniques were used to prove that neoblast population forms a stem cell pool which can differentiate into any cell type of the animal in case of a spontaneous fissure of the animal or a healing process after a trauma (cf. BrQ\ndsted, 1969). Recent radiobiological studies produced suggestive evidence that the survival of planarians after irradiation is dependent on the presence of one single cell population (Lange, 1968a, b). This evidence is in favour of the neoblast concept, but could also be in favour of the concept which attributes the regeneration to dedifferentiation of differentiated cells. In vertebrates various types of stem cells have been found. Cell renewal systems in these animals are generaUy restricted to tissues with relatively short living cells. Cell renewal systems are also found in other tissues and have a function in repair of local injuries. This latter type of repair seldom results in remodeling of the original form of the injured tissue. Among the vertebrates only the Urodele amphibians (Caudata) show a .complete regeneration of an injured or amputated limb or taiL Most authors assume that this regeneration is not dependent on stem cells1 but on dedifferentiation of differentiated cells (cf. de Both, 1969). The most active cell renewal systems are those which produce ceHs, that have a relatively short life time when compared with the life span of the individual. Examples of these cell renewal systems are among others the hemopoietic system, the epithelia of the skin, the gastro-intestinal tract and the urogenital system.