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

The Thymus: Gross Specimen

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.

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

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

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!

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.

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

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.

The Spleen

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

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

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

The Spleen – Vascular Schematic

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.

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.

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

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

The White Pulp

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.

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.

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.

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.

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

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.

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.

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

** WILDERNESS MEDICINE ** Shameless Plug ** WILDERNESS MEDICINE ** A presentation by Dr. Jay Lemery, Director of Wilderness Medicine Education at Weill Cornell Medical College. Thursday, December 1 6:00 p.m. HSC 305 Dinner will be provided.