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?
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 One
The 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 the
pointer contains:
I.
II.
III.
IV.
A)
B)
C)
D)
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?
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 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.