Thymus & Spleen

1. Thymus&Spleen Sarah Murray sgm2106@columbia.edu November 30, 2005 Are you getting immune to exam blocks yet??

2. The Thymus:Gross Specimen

3. Thymic Structure • Connective tissue capsule, trabeculae. • Both contain blood vessels, efferent lymphatic vessels, and nerves. • Trabeculae demarcate thymic lobules. • Parenchyma is made up of both a cortex and medulla.

4. Thymus: Cortex and Medulla • The cortex stains darkly basophilic because there lots of small lymphocytes with intensely stained nuclei. • The medulla stains light because it has fewer lymphocytes with more cytoplasm. Medulla Cortex

5. Thymus: 3 cell types • Epithelioreticular cells: large, pale, and stellate. (They are not reticular fibers!) • Thymocytes: immature T cells. • Macrophages: phagocytose T cells that react too strongly with self. Thymocytes Epithelioreticular cells

6. Hassall’s Corpuscles • Concentrically arranged keratinizing and degenerating epitelioreticular cells and macrophages. • Function is poorly understood (thymic hormones?) • Found in the medulla. • Instantly signify the thymus!

7. Thymus: The Education of T cells • The thymus is the location where thymocytes mature and proliferate. • Thymocytes undergo positive and negative selection. • Schematic: multipotential stem cells enter thymus via postcapillary venule  positive selection in cortex  negative selection in medulla  naïve T cells exit thymus from medulla and enter blood circulation.

8. Blood-Thymic Barrier • Separates developing T cells from blood (prevents T cells from recognizing foreign proteins as “self”). • Components (from outside  inside) • Capillary endothelium • Endothelial basal lamina • Perivascular connective tissue sheath (and macrophages!) • Basal lamina of epithelioreticular cell • Epithelioreticular sheath

9. The Adult Thymus • Adult thymus shrinks (involutes). • Adipose tissue replaces thymic tissue. • The medulla and cortex are harder to differentiate because there are fewer lymphocytes.

10. The Spleen

11. The Spleen: What is it good for? • Filters blood • Iron Retrieval • RBC reserve • Immune Response • Fetal Hematopoiesis

12. The Spleen: Structure • Dense connective tissue capsule from which trabeculae extend; both contain myofibroblasts. • White pulp • Red pulp • Hilum (not pictured).

13. Spleen – Capsule and Trabeculae *Notice how reticular fibers are evident with silver stain and not H&E.

14. The Spleen – Vascular Schematic

15. White Pulp Vasculature • The central artery (branch of splenic artery) is found in the white pulp. • It is surrounded by the PALS, which is T cells. • Lymphatic nodules look like localized expansions of PALS; displace central artery. • Penicilli branch from the central artery into the red pulp.

16. Red Pulp Vasculature: • Leaving the white pulp and entering the red, 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.

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

18. The White Pulp • Mostly lymphocytes. • 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

19. The White Pulp

20. 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.

21. Question One • The function of this organ is to: • Secrete antibodies from B cells into the blood. • Present antigen to B cells and filter the blood. • Guide the maturation of T cells by positive and negative selection. • Present antigen on MHC molecules of mature T cells to epithelioreticular cells.

22. Question Two This structure at the pointer contains: • Type I collagen, which resists tension • Type II collagen, which resists tension • Type I collagen, which forms a filtration barrier • Type II collagen, which resists pressure.

23. Question Three The organ shown: • Contains blood vessel with a perivascular sheath. • Receives lymphocyte precursors via afferent lymphatics. • Both A and B. • None of the above.

24. Question Four The structure at the pointer contains: • Macrophages. • Lymphocytes. • Collagen type III. • Collagen type II. • I only • I and II only • I, II, and III. • I, II, III, and IV.

25. Question Five Which is the correct order that blood flows through the spleen? • Capsular artery, cortex, medulla, pulp veins. • Central artery, penicilar arterioles, sinusoids, ellipsoids, pulp veins. • Afferent lymphatics, subcapsular sinuses, trabecular sinuses, medullary sinuses, efferent lymphatics. • Central artery, penicillar arterioles, ellipsoids, sinusoids, pulp veins.

26. Question Six This organ: • Is encapsulated, capsule contains reticular fibers. • Is encapsulated, capsule contains smooth muscle. • Is encapsulated, capsule contains both reticular fibers and smooth muscle. • Is encapsulated, capsule contains neither reticular fibers nor smooth muscle. • Is not encapsulated.

27. That’s all, folks… We hope you got a good education(THYMUS) and that you filtered out what was important (SPLEEN).

28. Shameless Plug ** WILDERNESS MEDICINE ** A presentation by Dr. Jay Lemery, Director of Wilderness Medicine Education at Weill Cornell Medical College.Thursday, December 16:00 p.m.HSC 305Dinner will be provided.