Thymus & Spleen

Are you getting immune to exam blocks yet??

Sarah Murraysgm2106@columbia.edu

November 30, 2005

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?

1. Filters blood

2. Iron Retrieval

3. RBC reserve

4. Immune Response

5. 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 OneThe function of this organ is

to:A) Secrete antibodies from B

cells into the blood.B) Present antigen to B cells

and filter the blood.C) Guide the maturation of T

cells by positive and negative selection.

D) Present antigen on MHC molecules of mature T cells to epithelioreticular cells.

Question Two

This structure at the pointer contains:

A) Type I collagen, which resists tension

B) Type II collagen, which resists tension

C) Type I collagen, which forms a filtration barrier

D) Type II collagen, which resists pressure.

Question Three

The organ shown:

A) Contains blood vessel with a perivascular sheath.

B) Receives lymphocyte precursors via afferent lymphatics.

C) Both A and B.

D) None of the above.

Question Four

The structure at thepointer contains:I. Macrophages.II. Lymphocytes.III. Collagen type III.IV. Collagen type II.

A) I onlyB) I and II onlyC) I, II, and III.D) I, II, III, and IV.

Question Five

Which is the correct order that blood flows through the spleen?

A) Capsular artery, cortex, medulla, pulp veins.

B) Central artery, penicilar arterioles, sinusoids, ellipsoids, pulp veins.

C) Afferent lymphatics, subcapsular sinuses, trabecular sinuses, medullary sinuses, efferent lymphatics.

D) Central artery, penicillar arterioles, ellipsoids, sinusoids, pulp veins.

Question Six

This organ:A) Is encapsulated, capsule

contains reticular fibers.B) Is encapsulated, capsule

contains smooth muscle.C) Is encapsulated, capsule

contains both reticular fibers and smooth muscle.

D) Is encapsulated, capsule contains neither reticular fibers nor smooth muscle.

E) Is not encapsulated.

That’s all, folks…

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

what was important (SPLEEN).

Shameless PlugShameless 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 305

Dinner will be provided.