Unit I

Spleen:The spleen is a blood filtering lymphoid organ. It is covered by a capsule consisting of a connectivetissue and smooth muscle fibres. The capsule sends extensions known as trabeculae into the substance ofspleen. The parenchyma of the spleen is arranged as red and white pulp The white pulp consists of lymphoidnodules with an eccentrically placed arteriole. These are called splenic corpuscles or Malphigian corpuscles.Red pulp consists of splenic cords of Bilroth which have linearly arranged groups of lymphocytes andmacrophages. Between the splenic cords are the sinusoids of spleen lined by discontinuous endothelium.The arteries branch in the trabeculae and then they enter the white pulp. Smaller trabeculae showonly the veins. In the white pulp the arteries are covered by reticular sheaths (not seen in the slide) whichare heavily the infiltrated with the lymphocytes. These are the lymphatic nodules, which may show thegerminal centre. White pulp is distributed along arteries that leave trabeculae. When a sheath is expandedto form the nodule, the artery gives off a branch to supply the nodule called follicular artery. A follicularartery leaves white pulp and enters the red pulp. Here it divides into 2 to 6 branches, which radiate indifferent directions. These are straight or penicillar arteries. Each of these again divides in to 2 to 3 arterioles.They enter curious little structures called as ellipsoids. Perivascular lymphatic sheaths of the arteries andarterioles are the thymus dependent zone of the spleen. The border between the red and white pulp is notsharp. This is the transitional or marginal zone.In the red pulp there are the sinusoids lined by long narrow endothelial cells showing slits. The redpulp between two sinusoids is like a cord so called Bilroth cords. The red pulp contains RBCs, WBCs,plasma cells, macrophages and reticular cells. The basic cells of the germinal centre are activated B cells.The rim that surrounds the centre is composed of the T cells. From this position the plasma cells movereadily into the red pulp.Notes:49