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

Sensitivity of murine haemopoietic stem cell populations to X-rays and I MeV fission neutrons in vitro and in vivo under hypoxic I. Conditions

The radiosensitivity of primitive haemopoietic stem cells that repopulate the bone marrow with precursors of granulocytes and macrophages (MRA[CFU-C]), mature stem cells capable of forming spleen colonies in lethally irradiated recipients (CFU-S-7) and colony-forming units in culture (CFU-C) were determined in vitro and under hypoxic conditions in vivo for 1 MeV fission neutrons and 300 kV X-rays. The obtained D0's were compared with previously observed D0's after irradiation in vivo under normal oxic conditions. With 1 MeV fission neutron irradiation no significant difference in radiosensitivity of the cell populations was observed between normal in vivo irradiation and in vitro irradiation. With 300 kV X-rays a lower radiosensitivity for all three cell populations was observed after in vitro compared to in vivo irradiation. In vivo irradiation with fission neutrons under hypoxic conditions led to a small decrease in radiosensitivity. The obtained oxygen enhancement ratio (OER) for fission neutrons varied from 1.2 for MRA[CFU-C] to 1.5 for CFU-C. After in vivo irradiation with 300 kV X-rays under hypoxic conditions much higher OERs were observed. An OER= 1.8 was obtained for CFU-S and for MRA[CFU-C] and for CFU-C OER 3.0 and 2.9 were observed. These results indicate that the radioresistance of primitive haemopietic stem cells (MRA[CFU-C]) compared to mature stem cells (CFU-S-7) is mainly due to intrinsic factors and not to differences in localization or oxygenation between primitive and mature stem cells

Het optreden van cavity spot in peenThe occurence and control of cavity spot of carrots

Onderzoek toont aan dat de condities waaronder cavity spot (een schimmelziekte) kan ontstaan als volgt kunnen worden samengevat: 1. Er dient Pythium aanwezig te zijn in de bodem. 2. De vochttoestand van de bodem dient gedurende enige tijd teminste op veldcapaciteit te zijn. 3. De peen dient minimaal in het stadium van diktegroei te verkeren, voordat de aantasting kan optrede