Thymic lymphopoiesis: protected from, or influenced by, external stimulation?

Abstract

The mechanisms regulating thymic lymphopoiesis are still a matter of debate. Intracortical proliferation and differentiation of thymocytes are thought to be controlled by locally produced humoral factors and close contact with epithelial, possibly also phagocytic, cells, and restricted by products of the major histocompatibility complex. The observation of a translocation of intraabdominally introduced PVP-coated silica particles (Percoll) via parathymic lymph vessels and through the thymic capsule into the cortical parenchyma demonstrates that the thymic cortex is accessible to materials carried with the transcapsular flux of interstitial fluid, and that this barrier is less effective than the blood-thymus barrier. The proliferative activity of cortical thymocytes following an intraabdominal injection of particulate tetanus toxoid was compared in sites adjacent to, and distant from, parathymic lymph nodes. Absolute numbers of DNA-synthesizing thymocytes were found to be much higher in cortical areas close to the lymph nodes, where lymphatic vessels are most numerous, than on the opposite sides of the thymic lobes. Taken together, these findings indicate that--in addition to intrinsic control mechanisms--cortical thymocyte production may be influenced by peripheral stimulation to some extent, and that materials from sites which are drained by parathymic lymph nodes may be important in this respect.