Histologic and Transplantation Studies on Preleukemic Thymus of the AKR Mouse2

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Abstract
A histologic survey of 264 thymuses from high-leukemia strain AKR mice, age 2–14 months, revealed that abnormal morphologic changes preceded the development of thymic lymphomas by at least several months. These changes were narrowing of the cortex with a selective depletion of cortical small lymphocytes and enlargement of the medulla by follicular aggregates of lymphoid cells, frequently with germinal centers. In its extreme form (cortical inversion), small lymphocytes were almost completely absent from the thymus cortex. Similar changes were only rarely observed in thymuses from low-leukemia strain mice of the same age. Transplantation tests failed to reveal the presence of lymphoma cells in AKR thymuses showing preleukemic changes. Neonatal or 1-week-old AKR thymus grafts in preleukemic mice showed normal morphology 1 month after grafting and exhibited normal Coulter Counter cell-distribution patterns despite the abnormal morphology of the host thymus. Neonatal AKR thymus grafts did develop cortical thinning and medullary lymphoid aggregates if left in host animals for 6 months. Preleukemic thymuses grafted to young hosts gave rise to thymus grafts which reproduced the morphologic abnormalities of the donor thymus. These results implicate the thymic epithelial and reticulum cells as being responsible in part for the preleukemic changes in the thymus.