REGENERATION OF THYMUS GRAFTS. I. HISTOLOGICAL AND CYTOLOGICAL ASPECTS

Abstract

The regeneration of thymus tissue in a subcutaneous site or under the renal capsule has been studied histologically and cytologically in mice. The pattern of regeneration, under normal circumstances, was arbitrarily divided into four stages. Necrosis affected all cell types in the centre of the implant during the first 24 to 48 hours after grafting and only a peripheral rim of viable tissue containing both epithelial and lymphoid elements was left (stage I). During the next 2 days intense mitotic activity became evident in the lymphoid rim (stage II), and by 5 to 6 days the typical structure of the thymus cortex was restored (stage III). By day S normal thymus architecture with characteristic cortico-medullary lobules became evident (stage IV). For about the first 2 weeks the cells dividing in the graft were entirely derived from donor cells surviving in the peripheral rim of the original implant. During the 3rd week, host cells were found dividing in the graft and by 21 days the dividing donor cell population had been entirely replaced by host cells. The only significant difference between the regeneration of subcutaneous and subcapsular implants was an earlier invasion by host cells in the former. Pretreatment of the host by thymectomy, irradiation, or both, caused some delay in the regeneration pattern of some syngeneic implants. Thymus tissue grafted to normal allogeneic hosts lost the lymphocytes surviving the initial transplantation injury and failed to undergo lymphoid differentiation. Only an epithelial-reticular framework with myelopoietic foci and scattered small darkly staining dense round cells, referred to as “small lymphocyte-like cells,” were present between the 2nd and 7th day. Between 7 and 11 days the grafts were heavily infiltrated with lymphocytes and histiocytes and they were subsequently rejected. Preirradiation of the host allowed some lymphoid differentiation in some allogeneic grafts but these too were sub