Mast cell types and cell-to-cell interactions in lymph nodes of the opossum Didelphis albiventris

Previous light-microscopic studies have shown a unique population of mast c ells in lymphatic sinuses of lymph nodes located in the head, neck, axillar y fossa and inguinal region of the opossum. In the present work, scanning a nd transmission electron-microscopic studies in the opossum mandibular and superficial axillary lymph nodes have strengthened the differences between connective-tissue mast cells (CTMC) and the lymphatic-sinus mast cells (LSM C). Further, close appositions of mast cells to other cells were described. At the nodal capsule, CTMC contacted fibroblast and granulocytes. In the l ymphatic sinuses a few CTMC contacted LSMC, macrophages and reticular cells . The LSMC contacted macrophages, reticular cells and other LSMC. A few LSM C could be located in the medullary cord in close contact with plasma cells or other lymphoid cells, keeping the same ultrastructural features of thos e found in the lymphatic sinuses. An important new finding was provided by light-microscopic studies in nine abdominal lymph nodes. Most of them (para -aortic, common iliac, cardial, cecocolic and those of the body and tail of the pancreas) displayed numerous LSMC with the same distribution and histo logical features described herein. However, the mesenteric, pyloric and hea d-of-pancreas lymph nodes were virtually devoid of LSMC. Instead, their mas t cells occurred mainly at the medullary cords and were very similar to the CTMC. Ultrastructural studies at the mesenteric lymph nodes confirmed the CTMC character of the mast cells located at both medullary cords and sinuse s, and disclosed interactions with macrophages and lymphoid cells.