Thymus & Spleen

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Thymus Spleen
Sarah Murray sgm2106_at_columbia.edu November 30,
2005

• Are you getting immune to exam blocks yet??

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The ThymusGross 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.

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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
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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.

Thymocytes
Epithelioreticular cells
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Hassalls 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
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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 HE.
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The Spleen Vascular Schematic
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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.

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Sinusoids
See how the basal lamina is interrupted evident
with both stains.
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The White Pulp

• Mostly lymphocytes.
• Appears basophilic on HE 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 White Pulp
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The Red Pulp

• Appears Red on HE
• 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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Thats 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

• WILDERNESS MEDICINE
• A presentation by Dr. Jay Lemery, Director of
  Wilderness Medicine Education at Weill Cornell
  Medical College.Thursday, December 1600
  p.m.HSC 305Dinner will be provided.