Human parathymic lymph node: morphological and functional significance.

Parathymic lymph nodes (PTLNs) have been identified in several species, but in humans they have been noted only once before in a study 90 years ago using fetal material. We now report their occurrence in children. Human PTLNs are small but distinctive lymphatic organs located on the surface of the thymus (or sometimes between the upper and lower lobes of the thymus) and covered with the thymic capsule. Histologically, the medullary cords of these lymph nodes were found to be thin, with only small numbers of plasma cells. In addition, they had a well-developed paracortical area rich with high endothelial venules (HEV), but a thin cortex, including only a few undeveloped follicles. Flow cytometric analysis of PTLNs revealed that the ratios of T:B cells (14.6+/-9.3) and of CD4+:CD8+ T cells (4.9+/-1.4) in PTLNs were much higher than in other peripheral lymphoid tissues and in peripheral blood. Because of these characteristics of the human PTLNs, we propose that the human PTLNs might influence the functional differentiation of T cells.