CELLULAR COMPOSITION OF MOUSE THYMUS AFTER X-RAY EXPOSURE

The composition of the cell population in mouse thymus was studied quantitatively using cell volume as a particular cellular marker characteristic determined by electronic particle size analysis. In unirradiated mice, an exponential increase of the cell population was found after birth, followed by a gradual decrease during the age involution of the organ. Small size cells increased in frequency with increasing age. In the thymus of X-irradiated mice, the number of small size cells decreased to a smaller extent than large size cells after X-ray doses lower than 200 R but, in contrast, to a greater extent after larger doses. Repopulation of the irradiated organ followed a cyclic pattern, the repopulation by large size cells preceding, as a rule, that by small size cells. The relevance of the observations in indicating a differential sensitivity of thymus cells of different size, and the origin of small size cells from large size cells was discussed. © 1968.