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Dr. Janet D. Smith
MICROSCOPIC ANATOMY
MICROSCOPIC ANATOMY SLIDE REVIEW
LYMPHOID ORGANS
A. THYMUS
Is a primary lymphoid organ (i.e., a site of lymphocyte differentiation rather
than site of lymphocyte function in immune responses)
Has 2 lobes derived from the fusion of right & left thymic primordia
Thin connective tissue (CT) capsule
Sheets of CT called septa arise from the capsule
Septa divide the thymus into lobules
Each lobule has a medulla (the pale central region) & cortex (the darker outer
region of the lobule)
Medulla is continuous throughout the organ
Thymus normally contains no lymphoid nodules. Do not confuse the medulla
with the germinal center of a secondary nodule.
Blood vessels run in the capsule & septa, & then at the corticomedullary boundary
Prothymocytes from the bone marrow enter the thymus via these vessels
Stroma is composed of thymic epithelial cells (TECs), also called epithelial
reticular cells (ERCs)
Pale, euchromatic cells
Recognizable by EM because they are joined to one another by desmosomes or
tight junctions, & because they contain tonofibrils (bundles of tonofilaments)
Poorly phagocytic (distinguishes them from macrophages)
Functions of TECs:
Provide physical support (stroma) throughout cortex and medulla
Act as nurse cells that promote thymocyte differentiation (Types II & V)
Create a protected environment in the cortex
Contribute to blood-thymus barrier (Type I), which is tight in cortex; leakier in
medulla
Form a layer deep to the capsule (Type I) that shields the cortex from the
environment outside the thymus
Separates cortex from medulla (Types III & IV)
Form Hassall’s corpuscles in medulla (Type VI)
- center of corpuscle may become necrotic, keratinized, or calcified
- play an indirect role in the production of regulatory T cells (T REG)
Thymic cell types include:
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TECs
Thymocytes (= any lymphocyte within the thymus)
Macrophages (recognizable by their phagocytic activity)
Dendritic cells (DCs), also called interdigitating dendritic cells (IDCs)
- a type of bone marrow-derived antigen-presenting cell (APC)
- difficult to distinguish from TECs by light microscopy
- by EM, DCs lack tonofilaments
- found in the medulla; the other cell types are in both cortex & medulla
- play a role in negative selection
Thymus receives committed T cell precursors (prothymocytes) from bone
marrow throughout life
Prothymocytes enter thymus from blood vessels at cortico-medullary boundary,
migrate to outer cortex of a lobule, & then migrate toward medulla
Undergo positive selection in cortex (thymocytes that bind with sufficient
strength to TECs will survive)
Undergo negative selection mainly in medulla (thymocytes that bind with
sufficient strength to DCs, TECs, or macrophages will die)
> 95% of thymocytes fail these selection processes and die (by apoptosis) without
ever leaving the thymus
Naïve CD4+ cells and CD8+ cells that survive leave the thymic medulla via
blood vessels or lymphatics
Thymus begins to involute around puberty, but some functional tissue
remains, & thymocyte maturation continues throughout life
Involution affects cortex before medulla:
- cortex becomes a discontinuous layer
- white fat accumulates in the areas previously occupied by cortex
B. LYMPH NODES:
Are secondary lymphoid organs
Filter antigens from afferent lymph & initiate immune responses to them
Have a capsule & trabeculae (finger-like projections) of dense irregular CT
Have a cortex (including the paracortex), and a medulla
Cortex
Lies immediately deep to subcapsular sinus
Dark staining due to numerous lymphocytes & few trabecular sinuses
Outer cortex includes lymphoid nodules (follicles) & internodular cortex
- nodules are B cell territory (thymus-independent)
- internodular cortex is T cell territory (thymus-dependent)
Paracortex lies deep to the outer cortex
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-
lacks lymphoid nodules
dark staining due to densely packed lymphocytes
is predominantly T cell territory
contains high endothelium venules (HEVs)
rich in interdigitating dendritic cells (IDCs), which act as APCs to
activate T cells
- is the site where immune responses are most often initiated in a node
Medulla
The central region of the node
Has 2 components:
- medullary sinuses: numerous pale-staining lymph sinuses
- medullary cords: dark-staining regions of closely packed cells that
surround the medullary sinuses
- cords have a mixed cell population including T & B lymphocytes,
plasma cells & macrophages
Can be distinguished from the light-staining medulla of the thymus because
thymic medulla is uniformly pale; medulla of lymph node contains pale
medullary sinuses plus dark-staining medullary cords
HEVs
Are postcapillary venules
Characterized by cuboidal to columnar endothelial cells
Most lymphocytes enter a node by crossing the wall of an HEV
Other leukocyte types do not usually cross the HEV wall
Stroma of a node consists of reticular CT cells and reticular fibers
Attached to capsule and trabeculae
Present throughout cortex & medulla, including within the lumen of lymphatic
sinuses (subcapsular, trabecular, & medullary sinuses)
PATHWAY OF LYMPH FLOW
Afferent lymphaticsà subcapsular sinusà trabecular (= intermediate or
cortical) sinusesà medullary sinusesà efferent lymphatics
Afferent lymphatics
Carry lymph to the node
Nodes are the only organs with afferent lymphatics
They pierce capsule anywhere except at the hilum
Have no stroma within their lumen
A valve is often present where an afferent enters the capsule
Antigens (ags) most often enter a node via afferent lymphatics
Subcapsular, trabecular, & medullary sinuses
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Are all located interior to the capsule (i.e., within the node)
Have stroma across their lumen, with macrophages adhering to it
- are important sites of antigen trapping, especially subcapsular sinus
Endothelial lining is discontinuous in many places (highly permeable)
No valves
Efferent lymphatics
Leave from hilum
Have no stroma within their lumen
Have numerous valves
Lymphocytes most often leave a node in efferent lymphatics
Lymphatic vessels (afferent & efferent) have a continuous endothelium
A lymphatic that is efferent to one node is often afferent to another node
LYMPHOID NODULES (= LYMPHOID FOLLICLES)
Are the areas to which B cells preferentially migrate after entering a node
Contain:
B lymphocytes (& small number of T cells)
Follicular dendritic cells (FDCs) that bind ag-ab complexes via the Fc
portion of the antibody, but are not antigen-presenting cells according to
the strict definition of that term because they do not process the antigen
Macrophages whose functions include phagocytizing apoptotic lymphocytes
PRIMARY VS. SECONDARY NODULES
Primary nodule:
Uniformly dark-staining due to many small (inactive) B lymphocytes
Secondary nodule:
Develops from a primary nodule during an immune response
Consists of:
- germinal center: pale area where activated B cells enlarge & divide (blast
transformation)
- mantle (cap, corona): dark outer region surrounding ~ 3/4 of the germinal
center (contains small B cells including memory B cells)
Presence of secondary nodules indicates that an antibody-producing immune
response is in progress
Well-developed germinal centers have a dark zone & a light zone
Dark zone (usually closest to paracortex)
- contains the most recently activated B cells
- these B cells are rapidly dividing and are called centroblasts
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- they undergo somatic hypermutation, which helps generate antibody
diversity
Light zone
- centroblasts move from dark zone into light zone
- in the light zone they divide less actively & are called centrocytes
- centrocytes undergo processes including:
- affinity maturation, which results in individual B cells producing antibodies
with increasing affinity for the antigen during the course of an immune
reaction.
- involves somatic hypermutation to generate ab diversity, followed by a
selection process that involves interaction with ag-ab complexes on
FDCs
- class switching of antibody , e.g., IgM to IgG
- high rates of apoptosis
Some of the surviving centrocytes become plasmablasts
Begin producing abs, but are not morphologically recognizable as plasma cells
Migrate to medullary cords and develop into identifiable plasma cells
Some plasma cells leave the node via efferent lymphatics
Some centrocytes become memory B cells
C. SPLEEN
Is a secondary lymphoid organ that filters the blood to remove:
Antigens and initiate immune responses to them
Old or damaged red blood cells
Has lymphoid regions collectively called the white pulp
Has non-lymphoid regions called the red pulp that surround the white pulp
White pulp and red pulp are so-named because of their appearance in a fresh
unfixed specimen
In H&E-stained sections, white pulp is darkly basophilic due the heterochromatic
nuclei of its many lymphocytes; red pulp is eosinophilic due to large numbers
of erythrocytes
Spleen also has:
A hilum where blood vessels enter and leave, and where efferent lymphatics
leave
A capsule & trabeculae composed of dense irregular CT
No subcapsular or trabecular sinuses
Trabeculae contain blood vessels (trabecular arteries & veins)
A mesothelium that covers the outer surface of the capsule since the spleen is
intraperitoneal
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A stroma composed of reticular cells and reticular fibers
Most abundant in the splenic cords of the red pulp and the basement
membrane of splenic sinusoids (but absent from the lumen of the
sinusoids)
Less abundant in germinal centers of the white pulp
NOTE: Spleen has no cortex or medulla
WHITE PULP:
Consists of discontinuous masses of tissue surrounded by red pulp
Each area of white pulp is organized around a central artery
White pulp has 2 components:
Periarteriolar lymphatic sheath (PALS) (a cylindrical collar of T lymphocytes
surrounding the central artery)
Lymphoid nodules (primary or secondary) embedded in the PALS
- where nodules are present, they often push the central artery aside, i.e.,
toward the edge of the white pulp
There is often no obvious boundary between a nodule and the PALS
MARGINAL ZONE:
Is the area where immune responses are often initiated in the spleen because
it is a region into which antigens, T cells, & B cells are delivered by side
branches of a central artery
Sometimes identifiable in H&E as several layers of flat reticular cells oriented
circumferentially around white pulp
Can be visualized by specialized techniques (immunocytochemistry,
immunofluorescence) to reveal:
Reticular fibers encircling the white pulp
Cell types that are specific to the marginal zone (e.g. marginal zone B cells,
several types of macrophages)
Marginal zone B cells differ from other B cells in that:
They don’t recirculate
Many are specific for bacterial or viral antigens, and can be activated without T
cell help
They shuttle antigens from marginal zone to nodules of white pulp
RED PULP:
Looks red due to many RBCs, fewer lymphocytes
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2 components:
Splenic (or venous) sinuses (sinusoids) characterized by:
- cigar-shaped endothelial cells oriented longitudinally in the vessel
- discontinuous basement membrane (organized into interconnected hoops
that encircle the vessel)
Splenic cords (Bilroth’s cords)
- cords are the areas that surround the sinuses
- contain stroma, macrophages and all blood cell types
Blood from open circulation is dumped into splenic cords
Macrophages in the cords trap particles and antigens; act as ACPs
Red and white cells in the cords must cross the wall of a splenic sinus to reenter circulation
NOTE: Splenic sinuses are blood vessels, not lymph sinuses
BLOOD FLOW THROUGH THE SPLEEN:
Human spleen has a vasculature that is probably entirely an open circulation
In an open circulation the blood is not contained within a vessel at all times
Pathway of splenic blood flow in open circulation:
Splenic artery
Capsular arteries
Trabecular arteries
Central arteries (leave trabeculae & are surrounded by white pulp)
- have side branches, some of which open into the marginal zone
Penicillar arterioles
Sheathed capillaries
- open into splenic cords, i.e., blood leaves the vascular system
- sheath is composed of macrophages
Splenic cords (where ags are trapped & old or defective RBCs are destroyed)
Splenic sinuses
Pulp veins (squamous endothelium & continuous basement membrane)
Trabecular veins
Capsular veins
Splenic vein
Hepatic portal vein
NOTE: In a closed circulation, all the capillaries would be directly continuous
with the splenic sinuses
D. KEY FEATURES OF LYMPHOID ORGANS
MICROSCOPIC ANATOMY
FEATURE
LYMPH NODE
SPLEEN
THYMUS
Capsule
Yes
Yes
Yes (thin)
Mesothelium
No
Yes
No
Cortex & medulla
Yes
No
Yes
Cords & sinuses
Yes (medullary)
Yes (splenic)
No
Lymphoid nodules
Yes
Yes
No
Special features
Lymph sinuses
Red/white pulp
Blood sinuses
Hassall's
corpuscles