Lymph nodes (rat)

Stain: Hematoxylin & pyronin. Pyronin was formerly used to demonstrate young blast cell types rich in polysomes and packed rough endoplasmic reticulum (RER) resulting in a reddish stain of the cytoplasm. A: Part of the medulla with medullary cords (1) and medullary sinuses (2) close to the hilum of the lymph node. After antigenic stimulation (e.g. infection) an increase in amount of activated B lymphocytes (migrated from the cortex) is found in the medulla. The arrows (-->) point to pyroninophilic activated B cells, i.e. plasma cells in the medullary cords. These cells are migrated from the follicle area. Within the sinuses free circulating lymphocytes (small, dark) and macrophages with a light-yellowish cytoplasm (*) are shown. Note the small flattened reticular cells lining the medullary cords and the sinuses (3). B: The infected medulla shows a tremendous increase in activated, pyroninophilic lymphocytes and plasma cells (red). The medullar cords (1) are irregularly expanded and are stuffed with lymphocytes and reticular cells. The sinuses (2) are filled with much less lymphocytes and/or macrophages. Note the phagocytised brown pigment deposits (-->).