Chapter 20: The Lymphatic System
and Lymphoid Organs and Tissues
M.C. Shamier BSc
Shenzhou University
Subjects
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Lymphatic Vessels
Lymphoid Cells and Tissues
Lymph Nodes
Other Lymphoid Organs
Developmental Aspects of the Lymphatic System
and Lymphoid Organs and Tissues
The Lymphatic System
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Lymph
Lymphatic Vessels
Lymph Nodes
Lymphatic Vessels
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Function
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Return excess tissue fluid to the bloodstream
Return leaked proteins to the blood
Carry absorbed fat from the intestine to the blood
Lymphatic Vessels
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Nutrients, wastes and gases are exchanged
between blood and interstitial fluid
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Fluid that remains in tissue spaces (3L/day)
becomes part of the interstitial fluid
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To prevent a loss of volume, this fluid must find a
way back to the heart and the blood circulation
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Once interstitial fluid enters lymphatic vessels it is
called lymph
Lymphatic Vessels
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One way system towards the heart
System begins in blind ended lymphatic capillaries
Lymphatic Vessels
Lymphatic Vessels
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Capillaries are very permeable (allowing passage of
substances)
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Endothelial cells are not joined, but overlap loosely
forming minivalves
Collagen filaments anchor endothelial cells to surrounding
structures so that any increase in interstitial fluid volume
opens the minivalves
So valves open when interstitial pressure >
lymphatic pressure and vice versa
Lymphatic Vessels
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Take up proteins, but even larger particles when
tissue is inflamed
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Cell debris, bacteria, viruses, cancer cells
In lymph nodes the immune system ‘examines’ and
cleanses the lymph
Lymphatic Vessels
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Duct (right lymphatic duct and thoracic duct)
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Trunk (lumbar, bronchomediastinal, subclavian, jugular, intestinal)
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Collecting vessel
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Capillaries
Successively larger and thicker walled channels
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Right lymphatic duct: right upper limb, right side of head
and thorax
Thoracic duct: left upper limb, left side of head and thorax
and rest of the body
Thoracic Duct
Lymph transport
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Slow and sporadic
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Skeletal muscle activity
Pressure changes in thorax during breathing
Valves to prevent backflow
Smooth muscle contractions in lymphatic ducts and
trunks
Immobilization of an inflamed body part can prevent
spread of infection
Lymphedema results from blockage of the
lymphatics
Lymphedema
Lymphoid Cells
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Lymphocytes (B and T), macrophages, dendritic
cells, reticular (fibroblast-like) cells
Lymphoid Tissue
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Houses and provides a proliferation site for
lymphocytes
Ideal surveillance point for lymphocytes and
macrophages
Loose connective tissue: reticular connective tissue
Diffuse lymphatic tissue: scattered reticular tissue
elements, found in virtually every organ
Lymphoid follicles: spherical bodies of tightly packed
reticular elements and cells
Lymphoid Follicle
Lymphoid Organs
Lymph Nodes
Lymph Nodes
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Cluster along lymphatic vessels of the body
Functions
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Lymph filter: macrophages destroy microorganisms and
debris
Activation of the immune system
Lymph Nodes
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Dense fibrous capsule
Trabeculae extend inward dividing it in
compartments
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Two distinct regions:
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Cortex
Medulla
Lymph Nodes
Lymph Nodes
Lymph Nodes
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Lymph enters at the convex side
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Through the subcapsular sinus into smaller sinuses
through the cortex into the medulla
Exits the node at the hilum on the concave side
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Afferent vessel (to the organ)
Efferent vessel (away from the organ)
More afferent than efferent vessels lowering the
speed of flow, allowing time for cleansing.
Lymph always passes through several nodes.
Homeostatic Imbalance
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Swollen lymph nodes
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An overwhelming amount of microorganisms can lead to
inflammation and swelling of lymph nodes (99%)
Lymphoma is lymph cell cancer
Metastasizing of cancer
Spleen
Spleen
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Largest lymphoid organ
Splenic vein and artery enter/exit the hilum
Function
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Site for lymphocyte proliferation, immune surveillance and
response
Blood-cleansing (defective blood cells and platelets,
debris, foreign matter
Storing breakdown products for reuse
Storing platelets
Site of erythrocyte production in the fetus
Spleen
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Structure
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Surrounded by a capsule
Trabeculae extending inward
Huge number of erythrocytes
White pulp: lymphocytes around a central artery, immune
functions
Red pulp: all remaining splenic tissue, disposing red
blood cells and bloodborne pathogens
Spleen
Spleen
Thymus
Thymus
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Macroscopic Anatomy
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Inferior in the neck, extending into thorax, partially
overlies the heart
Atrophy after puberty, prominent in newborns and children
Microscopic Anatomy
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Thymic lobules each containing a medulla and a cortex
Consists mostly of T-cells, no B-cells.
Hassal’s corposcules: keratinized epithelial cells, site of
regulatory T-cell production?
Thymus
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Function
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Thymus is where T-cells mature
Only lymphoid organ not directly involved in immune
response
Blood-thymus barrier keeps bloodborne antigens out to
prevent activation of immature lymphocytes
Stroma consists of epithelial cells rather than reticular
cells
Destruction of autoreactive T-cells and T-cells with a weak
or too strong antigen binding affinity.
Tonsils
Tonsils
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A ring of lymphatic tissue around the entrance of the
throat (pharynx)
Gather and remove pathogens from food or inhaled
air
Follicles with germinal centers and scattered
lymphocytes
Not fully encapsulated
Tonsillar crypts are the entrance for bacteria
They are ‘invited in’ and information is stored in the
immunologic memory: known intrudors can more
easily be fought than unknown intrudors.
Aggregates of Lymphoid Follicles
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Digestive Tract
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Peyer’s patches: structurally similar to tonsils, located in
the wall of the intestine
Lymphoid follicles in the appendix
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Destroy bacteria before they can enter the internal environment
Generate memory lymphocytes for longterm immunity
Respiratory Tract and Urogenital Tract
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Mucosa Associated Lymphoid Tissue: MALT
Protect open passages
End of Chapter 20