Parathymic lymph node: oriented proliferative response of the murine thymic cortex to intraperitoneal stimulation.

This study was designed to answer the question if a primary immune response elicited in parathymic lymph nodes would influence the proliferative activity in the neighboring thymic cortex. Young adult mice were stimulated by an injection of aluminum phosphate-adsorbed tetanus toxoid into the peritoneal cavity which is known to be drained predominantly via parathymic lymph nodes. Animals were given an intravenous (i.v.) injection of deoxy(5-3H)cytidine and were killed 1 h later at various time points after priming. Combined morphometry, counts of cell numbers per unit area and radioautography revealed that a proliferative response of cortical thymocytes was confined to the vicinity of parathymic lymph nodes and peaked around day 14 after stimulation. It was also shown that the enhanced DNA synthetic activity took place in the subcapsular, outer zones rather than in the inner portions of the cortex. Results are discussed in the light of recent observations which demonstrated a translocation of small particles injected intraperitoneally (i.p.) into parathymic lymph nodes and the surrounding lymphatics, and from there, in small amounts, into the cortical parenchyma of the thymus. We conclude that cortical thymocytopoiesis can be enhanced in the segment adjacent to aprathymic lymph nodes as a result of i.p. injection of stimulants.