Thymus and Spleen

The Spleen: What is it good for?Filters bloodIron RetrievalRBC reserveImmune Response*Fetal Hematopoiesis

The White PulpAppears basophilic on H&E and red on silver stainSite where immune response is mounted; formation of germinal centersGerminal centers with B cells and B cell derivatives push the central artery off to the side

White Pulp Vasculature The central artery is found in the white pulpThe central artery is surrounded by the PALS, which is T cellsPenicilli branch from the central artery into the red pulp

White Pulp Vasculature The central artery is found in the white pulpThe central artery is surrounded by the PALS, which is T cellsPenicilli branch from the central artery into the red pulp

White Pulp Vasculature The central artery is found in the white pulpThe central artery is surrounded by the PALS, which is T cellsPenicilli branch from the central artery into the red pulp

Red Pulp Vasculature: Penicilli give rise to ellipsoidsEllipsoids are capillaries ensheathed by reticular cells and macrophages; their lumens are often occluded in histo sectionsBlood is filtered by macrophages through fenestrations in the sinusoids

Sinusoids See how the basal lamina is interrupted; evident with both stains

Sinusoids Lining of endothelial cells: apposed to one another, but remain separatedMacrophages extend their processes into the lumen of the sinusoid (you can see the remains of RBCs in macrophages)

The Red PulpAppears Red on H&EComposed of sinusoids and Cords of BillrothThe cords are the parenchyma of the red pulp; they are composed of reticular tissue w/ macrophages, red blood cells, and lymphocytes

Silver Stain*Notice how reticular fibers are evident with silver stain and not H&E*Notice the difference in appearance with the two different stains