Comparisons of thymic cells in the developing and involuting rat thymus in vitro

Graduation date: 1979In the rat thymus, four stages of development have been distinguished:\ud (1) a newborn stage (Stage I), (2) an accelerated growth\ud stage (Stage II), (3) an involuting stage (Stage III), and (4) an involuted\ud stage (Stage IV) . There were three major types of thymic\ud lymphoid cells: small (STL), medium (MTL), and large (LTL) thymic\ud lymphoid cells. By Ficoll- Hypaque gradient centrifugation, MTL- and\ud LTL-rich populations were isolated for comparative analysis.\ud Growth kinetics studies revealed that the MTL were the most\ud active in proliferation and the most responsive to Concanavalin A\ud (Con A) activation. The LTL had lower Con A responsiveness, and the\ud STL were the least active in both categories. Stage II cell populations\ud were found to be the most proliferative and Con A reactive. Stage I\ud and III cell populations were second in both categories of cellular\ud response, while Stage IV cells were the least active in both proliferation\ud and Con A activation. Stage III and IV cells also appeared to be more tolerant of high serum concentration than Stages I and II cells.\ud Increases in cell concentration had synergistic effects on cellular\ud proliferation and responsiveness to Con A activation. The suggested\ud mechanism involves cell-to-cell mediation requiring intimate cell\ud contact and possibly the formation of gap-junctions. The newborn\ud MTL can tolerate a limited range of low temperature, and this tolerance\ud is lost rapidly as a rat matures. On the other hand, an older\ud rat has a higher proportion of long-lived MTL in its thymus.\ud The proportion of reticuloepithelial cells (REC) decreased with\ud age in the rat thymus, while the proportion of free macrophages remained\ud unchanged. The number of lipoid cells increased with age,\ud and was the result of REC and macrophage differentiation. Nonlymphoid\ud cell and lymphoid cell rosettes in thymus explants were\ud interpreted to be interacting macrophages and thymic lymphoid cells\ud leading to the differentiation of the latter. Macrophages also served\ud to remove debris and to restructure the involuting thymus