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Thymus & Spleen. Sarah Murray [email protected] 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.

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Thymus spleen l.jpg


Sarah Murray

[email protected]

November 30, 2005

Are you getting immune to exam blocks yet??

The thymus gross specimen l.jpg
The Thymus:Gross Specimen

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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 l.jpg
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.



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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.


Epithelioreticular cells

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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!

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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.

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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

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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.

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The Spleen: What is it good for?

  • Filters blood

  • Iron Retrieval

  • RBC reserve

  • Immune Response

  • Fetal Hematopoiesis

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The Spleen: Structure

  • Dense connective tissue capsule from which trabeculae extend; both contain myofibroblasts.

  • White pulp

  • Red pulp

  • Hilum (not pictured).

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Spleen – Capsule and Trabeculae

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

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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.

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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 l.jpg

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

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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

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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That’s all, folks…

We hope you got a good education(THYMUS) and that you filtered out

what was important (SPLEEN).

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Shameless Plug


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.