1. Thymus and Spleen

8. The Spleen: What is it good for? 1.Filters blood 2.Iron Retrieval 3.RBC reserve 4.Immune Response* 5.Fetal Hematopoiesis

9. The ‘White’ Pulp Appears basophilic on H&E and red on silver stain Site where immune response is mounted; formation of germinal centers Germinal centers with B cells and B cell derivatives push the ‘central artery’ off to the side

10. White Pulp Vasculature The central artery is found in the white pulp The central artery is surrounded by the PALS, which is T cells Penicilli branch from the central artery into the red pulp

11. White Pulp Vasculature The central artery is found in the white pulp The central artery is surrounded by the PALS, which is T cells Penicilli branch from the central artery into the red pulp

12. White Pulp Vasculature The central artery is found in the white pulp The central artery is surrounded by the PALS, which is T cells Penicilli branch from the central artery into the red pulp

13. Red Pulp Vasculature: Penicilli give rise to ellipsoids Ellipsoids are capillaries ensheathed by reticular cells and macrophages; their lumens are often occluded in histo sections Blood is filtered by macrophages through fenestrations in the sinusoids

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

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

16. The Red Pulp Appears Red on H&E Composed of sinusoids and Cords of Billroth The cords are the parenchyma of the red pulp; they are composed of reticular tissue w/ macrophages, red blood cells, and lymphocytes

17. 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