Role of parathymic lymph nodes in metastatic tumor development

Parathymic lymph nodes as potential sites of tumor progression have been neglected in humans. We have established a rat renal capsule-parathymic lymph node model to study in vivo metastasis. Epithelial liver carcinoma (HeDe) and mesenchymal mesoblastic nephroma (NeDe) cell lines have been established after inducing chemical carcinogenesis in newborn Fisher 344 inbred rats by N-nitrosodimethylamine. Implanting the exact number of tumor cells (HeDe, NeDe) under the renal capsule allowed the standardization and timing of metastatic development. Tumor cells released from the primary tumor in the peritoneal cavity were drained to the parathymic lymph nodes (PTNs) as sentinel lymph nodes. Similarly, tumor cells injected i.p. were engulfed by macrophages, drained through the transdiaphragmatic channels, and transported to the thoracal lymphatics, primarily to PTNs. Tumor cells after transdiaphragmic drainage can enter both anterior mammary and parathymic sentinel lymph nodes. The potential common origin can shed new light on the metastatic cell progression of PTNs and mammary tumors.