Experimental chimeras were produced by aggregating morulae from congenic strains of PVG rats differing in the major histocompatibility complex (RT1). Monoclonal antibodies against variant class I antigens of the two strains were directly conjugated to iodine-125 and applied to tissue sections. Autoradiograms allowed examination of most internal tissues. The proportion of PVG-RT1 a cells in the erythrocyte populations of the chimeras varied from 8 to 70 percent, as determined with fluorescence-activated flow cytometry. Digital analysis of autoradiograms demonstrated that the contribution of PVG-RT1 a cells to the livers of the chimeras ranged from 34 to 86 percent. Patches of cells of each genotype in the liver were geometrically complex, with large variations in size. The thymus, but not the spleen, showed evidence of oligoclonal development. The adrenal cortex revealed a radially striped pattern, suggestive of clonal expansion of stem cells. With this approach it is possible to measure cell distribution in chimeras through direct histological visualization, which may prove useful in the study of rat organogenesis.
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