This computer model simulates the cell population kinetics of the development and later degeneration of the thymus. Nutritional factors are taken into account by the growth of blood vessels in the simulated thymus. The stem cell population is kept at its maximum by allowing some stem cells to divide into two stem cells until the population reaches its maximum, thus regenerating the thymus after an insult such as irradiation. After a given number of population doublings the maximum allowed stem cell population is gradually decreased in order to simulate the degeneration of the thymus. Results show that the simulated thymus develops and degenerates in a pattern similar to that of the natural thymus.
This simulation is used to evaluate cellular kinetic data for the thymus. The results from testing the internal consistency of available data are reported. The number of generations which most represents the natural thymus includes seven dividing generations of lymphocytes and one mature, nondividing generation of small lymphocytes. The size of the resulting developed thymus can be controlled without affecting other variables by changing the maximum stem cell population allowed. In addition, recovery from irradiation is simulated.
Computer simulation can be a valuable adjunct to laboratory studies of problems which involve a large number of variables such as cell population kinetics of the thymus. The function of the thymus appears to be related to the maturation of large lymphoblasts which appear to migrate to the thymus from the bone marrow. These large lymphoblastic cells progress through a series of cell divisions and ultimately become mature small immunocompetent lymphocytes which leave the thymus and populate the other lymphoid tissues such as the spleen and lymph nodes.
Laboratory data which are obtained from the literature include generation times of the different cell size classes, number of generations in the thymus, total numbers of lymphocytes in the thymus at various stages of development and distribution of cells into cell size classes. A preliminary form of a computer simulation has been reported (I). This report describes refinements of that simulation (a) to allow the model to regenerate after irradiation, (b) to account for nutritional limitations of growth and (c) to allow the model to degenerate in later life. Normal development and recovery from x-irradiation are simulated and compared to laboratory data.