Ionizing radiation and the thymus : effects of whole-body irradiation with fast fission neutrons and X-rays on the murine thymus

As described in Chapter I of this thesis, the thymus is an extremely complex lympho-epithelial organ in which bone marrow-derived lymphoid precursor cells, i.e. prothymocytes, differentiate and mature in a stromal matrix. During their differentiation in this specialized microenvironment, thymocytes are selected on the basis of tolerance to self-MHC gene products and they acquire the capacity to recognize foreign antigens in the context of self-MHC antigens. Furthermore, during this differentiation, the thymocytes acquire a number of cell surface differentiation antigens. It is generally accepted that the thymic stromal cells are involved in the process of differentiation and maturation of T cells- When animals are subjected to whole-body irradiation, severe effects develop in the thymus. As shown by many authors, irradiation with X-rays or gamma-rays, i.e. low LET radiation types, leads to a severe depopulation of the thymus and, subsequently, thymus recovery has been shown to follow a biphasic pattern (reviewed by Sharp & Crouse, 1980 and Watkins et al., 1980)- The initial phase in this biphasic thymic recovery is brought about by radioresistant intrathymic precursor cells which are not derived immediately from bone marrow stem cellsLimited proliferative capacity and the resulting exhaustion of these intrathymic precursor cells as well as an impaired production of thymus precursor cells in the bone marrow are responsible for a second thymus involution- The final recovery of the thymus is due to its replenishment from extrathymic precursors in the regenerated bone marrow.