Histologic analyses following irradiation. (A) Histology of hematolymphoid tissue over time following sublethal irradiation. Compared with unirradiated controls (top left panel), kidney interrenal cells decrease daily following a dose of 20 Gy until reaching a nadir on day 8. Thymic sections (bottom panels) show similar decreases in thymocytes over time. By day 8, most thymi are involuted and difficult to detect. After a dose of 20 Gy, both the kidney and thymus recover cellularity by day 20 (right panels). T indicates renal tubule. (B) Histology of hematolymphoid tissue following lethal irradiation. Compared with unirradiated controls (left panels), kidney and thymic hematopoietic cells are largely depleted by day 12 (right panels). Unlike the sublethal time course, hematopoietic cells do not recover after a nadir on day 8 following a dose of 40 Gy. Tips of red chevrons point to erythrocytes, and green chevrons point to leukocytes. (C) TUNEL staining of kidney and thymus sections at 10 hours after irradiation. Both tissues showed a dose-dependent increase in the number of dying cells. (D) Effect of 40 Gy on nonhematopoietic tissues. Compared with unirradiated controls (top panels), only the testis exhibited gross alterations, with spermatogonial progenitor cells (arrowheads) disappearing by day 12 (bottom left panel). Other tissues receiving a 40-Gy dose, including the intestine, gill, liver, and pancreas, did not show gross abnormalities (bottom panels).