HIS11 Structure And Function Of Lymphatic Organs Flashcards
Immunity
Innate (non-specific) immunity:
- NOT involve specific recognition of a microbe
- Act against all microbes the same way
—> 1st line: Skin + Mucous membrane (Physical + Chemical barrier)
—> 2nd line: Phagocytes, NK cells, Humoral factors (Antimicrobial substances) —> Inflammation / Fever
Adaptive (specific) immunity: - Specific recognition of a microbe - acts when microbe breached innate immunity defences —> Humoral immunity + Cellular immunity —> B cells + T cells
Lymphatic system: consists of both Innate + Adaptive immunity
Organisation of lymphatic system
- Lymph
- Lymphatic vessel
- Lymphatic capillaries —> Lymphatic vessels —> Lymphatic trunks - Lymphatic organs (containing lymphatic tissue)
***Function of lymphatic system
- Immune responses (***filtration by LN)
- microbes
- abnormal cells - Drainage of ***excess interstitial fluid (15% of total interstitial fluid, 2-4 L) —> return to blood
- Dietary **lipid transportation
- lipids, lipoprotein particles and lipid-soluble vitamins (A, D, E, K) absorbed by small intestine
- Lacteals: lymphatic capillaries at small intestine (Centre of villus)
—> carry dietary lipid droplets into lymphatic vessels
—> **Chyle (creamy white juice)
Lymphatic circulation
Lymphatic capillaries:
- thin-walled microvessels located throughout body
- except in **Non-vascular tissues, **Splenic pulp, ***Bone marrow
***Morphological characteristics of Lymphatic capillaries
- Slightly larger in diameter than blood capillaries
- ***Blind ended
- Loosely overlapped endothelial cells —> ***lack of tight junction —> when interstitial fluid pressure is greater —> allow entry
- Anchoring filaments from endothelial cells (containing elastic fibres) —> project into interstitial space —> **1. Prevent lumen from collapsing + **2. Pull open endothelial cell passively when interstitial pressure is large to allow entry
- ***Incomplete / Absent basal laminae
Overall, greater permeability —> allow absorption of large molecules e.g. Proteins, Lipids
***Organisation of lymphatic vessels
Lymphatic capillaries —> Lymphatic vessels —> Lymphatic trunks —> Major Lymphatic ducts
Lymphatic vessels (~ to venules but thinner walls, more ***valves):
- Lymphatic capillaries unite
- **Afferent lymphatic vessels —> **LN —> ***Efferent lymphatic vessels
Lymphatic trunks:
- Lymphatic vessels unite
1. Lumbar: **lower limb, wall + viscera of pelvis, kidneys, adrenal glands, abdominal wall
2. Intestinal: organs in **abdominal cavity (e.g stomach, pancreas, intestine, spleen, part of liver)
3. Bronchomediastinal: **thoracic wall, lungs, heart
4. Subclavian: **upper limbs
5. Jugular: ***head and neck
Major Lymphatic ducts
1. ***Thoracic duct (3/4)
—> much longer / larger than right lymphatic duct
—> left upper 1/4 + lower 1/2
***Lymph flow:
Lumbar (L/R) trunks + Intestinal trunk
—> Cisterna chyli
—> Thoracic duct (joined by Left Bronchomediastinal + Left Subclavian + Left Jugular trunks)
—> empty into junction of Left internal jugular and Left subclavian vein (left venous angle)
—> Left Brachiocephalic vein
- ***Right lymphatic duct (1/4)
- right upper 1/4
—> joined by Right Bronchomediastinal + Right Subclavian + Right Jugular trunks
—> empty into junction of Right internal jugular and Right subclavian vein (right venous angle)
—> Right Brachiocephalic vein
Cisterna chyli:
- prominent ***sac collecting large amount of Chyle after meal
- proximal end: ***just below diaphragm —> Thoracic duct begins
Lymphatic flow sequence
Blood capillaries
—> Interstitial space
—> Interstitial fluid collected by Lymph capillaries
—> Afferent lymphatic vessels
—> LN (filtration, defence against foreign substances and toxins, release lymphocytes)
—> Efferent lymphatic vessels
—> Lymphatic trunks
—> Major lymphatic ducts
—> Venous angles (return to blood stream)
***Maintenance of lymph flow
- Pressure differences
- **Interstitial fluid pressure > Lymph pressure
- **Skeletal muscle pump (milking action of skeletal muscle contraction ~ venous drainage)
- **Smooth muscle pump (wall of large lymphatic vessels, Tunica media (elastic fibres + smooth muscles), Tunica adventitia —> contract when distended)
- **Respiratory pump (pressure changes during inhalation: Abdominal pressure > Thoracic pressure) - Bicuspid valves
- ensure ***unidirectional flow (even when pressure reverse during breathing / relaxation of skeletal muscles)
- beaded appearance when vessel is distended
Lymphoedema
Lymphatic drainage not sufficient —> Accumulation of tissue fluid —> Swelling
- Primary lymphoedema
- rare inherited condition
- female > male
- abnormal development of LN / valves - Secondary lymphoedema
- Tumour compression / invasion
- Post-treatment
—> failure of lymphatic vessel regeneration (normally can regenerate)
—> scar tissue formation after surgery / radiation therapy -
**Filariasis (most common)
—> invasion of lymphatic vessels by **Filarioidea (parasite) —> obstructive lymphodema
—> severe swelling in legs and genitals (elephantiasis)
—> chemotherapy
Treatment:
- Compression garment: force tissue fluid out of interstitial space —> return to lymph vessels
- Surgery: build bypass of lymph flow / remove excess fat to reduce volume of whole affected area
Lymphatic organs and tissues
Primary lymphatic organs
- lymphoid stem cells **divide and **mature into immunocompetent B and T cells (able to recognise and respond to antigens)
1. Red bone marrow
2. Thymus
Secondary lymphatic organs and tissues
- immunocompetent cells **populate and **initiate immune responses
1. LN
2. Spleen
3. Mucosa-associated lymphatic tissue (MALT) (lack a complete capsule —> considered tissue not organ) - Peyer’s patch in ileum
- Tonsils
Thymus
Location:
- Mediastinum between sternum and aorta
- extend from top of sternum down to 4th costal cartilage
Size:
- functional portion largest in early part of life, particularly around puberty (Rate of T cell production greatest before puberty)
- functional portion atrophied with age (T cell differentiation and proliferation reduced) —> ***replaced by adipose tissue (red to yellow colour)
- actual size does not change
Lobe:
- 2 lobes being encapsulated separately
- connected by connective tissue
- adhere to fibrous pericardium
- Connective tissue capsule extend into thymus —> form **Trabeculae —> subdividing thymus into **incomplete lobules
***Lobules: Functional part of Thymus
- Cortex
- dark-staining outer part of lobule
- **Immature T-cells (densely packed, large number)
- **Epithelial reticular cells / Thymic cortical epithelial cells (scattered)
—> several long processes serve as a framework for T cells
—> “educate” pre-T cells (develop ability of self-recognition (Positive selection))
—> produce thymic hormones (aid maturation of T cells)
- ***Dendritic cells, Macrophages (clean up debris of dead T cells) - Medulla
- light-staining central part of lobule
- may be continuous with medulla of adjacent lobules due to incomplete lobule
- **Mature T cells (fewer) (paler staining of nuclei, more cytoplasm)
- **Epithelial reticular cells
—> cluster to form thymic (Hassall’s) corpuscles (numerous)
—> uncertain role, may involve in T cell “education“
- ***Dendritic cells, Macrophages
During aging, cellular portion gradually replaced by CT and Adipose cells
Hassall’s corpuscles:
- concentric layers of flat reticular cells
—> degenerate and filled with keratin
- unique in Thymus
Thymoma
Thymic epithelial tumour
- Asymptomatic: 1/3 to 1/2 patients, mass usually found by CT / X-ray for unrelated problem
- Symptomatic: compression of the surrounding organs / associated autoimmune disorder (Myasthenia gravis most common)
- Medulla / Cortex structure cannot be seen
Lymph nodes
- Small, oval bodies lying along course of lymphatic vessels
- ~600 throughout body
Structure:
- Capsule
- CT with ***trabeculae extensions
- Afferent lymphatic vessels
- Subcapsular sinuses (divide cortex from capsule)
- Trabecular sinuses - Cortex
- Primary + Secondary ***lymphoid follicles - Paracortex
- **NO lymphoid follicles
- **HEV - Medulla
- **Medullary sinuses (continuation of trabecular sinuses)
- **Medullary cords (cord of lymphatic tissues separated by medullary sinuses) - Hilum
- Blood vessels
- ***Efferent lymphatic vessels
**ALL Sinuses:
- lined by endothelium
- contain
—> branching **Reticular fibres (filtration)
—> **Lymphocytes (immune response)
—> **Macrophages (phagocytosis)
Lymph flow: Afferent lymphatic vessels (Capsule) —> Subcapsular sinus (Capsule) —> Trabecular sinus (Capsule) —> Medullary sinus (Medulla) —> Efferent lymphatic vessel (Hilum)
Lymph flow within LN
Afferent lymphatic vessels (several vessels per node, penetrating capsule)
—> Subcapsular sinus
—> Trabecular sinus
—> Medullary sinus
—> Efferent lymphatic vessel (1-2 vessels per node)
Several afferent vessels —> 1/2 efferent vessels —> ***Slow flow of lymph —> allow additional time for filtration of lymph
ALL Lymph flows through multiple LN along its path —> multiple filtering chances
***Functions of Lymph Node
- Lymph filtration
- foreign substances trapped by ***Reticular fibres within sinuses - ***Phagocytosis of bacteria / foreign substances by Macrophage
- **Lymphocyte **production (effector + memory cells), **storage, **activation, recirculation by parenchyma of LN (Secondary lymphoid organs)
***Parenchyma of LN: Cortex + Paracortex + Medulla
- Cortex (**B cells)
- Lymphoid follicles: mainly **B cells (some Dendritic cells for APC)
- Primary lymphoid follicles (一層)
—> NO immune stimulation
—> Inactive B cells (evenly distributed, dark-staining) -
**Secondary lymphoid follicles (兩層)
—> Under antigenic stimulation
—> **Plasma cell + Memory B cell formation
—> **Peripheral zone / Mantle zone (Inactive B cell with condensed nuclei, little cytoplasm)
—> **Germinal centre (Dendritic cell + Macrophage + Activated B cells in various stages of maturation) - Dendritic cell
—(present antigen)—> B cells
—(those with high affinity to antigen survive, proliferate, differentiate)—> Plasma cell + Memory B cells - Undifferentiated B cells —(apoptosis)—> Macrophage
- Paracortex (**T cells)
- NO lymphoid follicles
- **T cells + Dendritic cells
- expands in T cell-dominated immune responses
- postcapillary high endothelial venules (HEVs) —> specialised venules providing entry way for lymphocytes into LN - Medulla
- More densely packed lymphocytes in medullary cords than in medullary sinuses
- **B cells + Plasma cells (migrated from Cortex)
—> embedded in network of **Reticular cells + ***Reticular fibres
High endothelial venules (HEVs)
Postcapillary venules lined by:
- **Cuboidal endothelium (contains specialised lymphocyte **receptor recognised by circulating lymphocytes —> allow movement of B cell and T cell ***into LN) (比lymphocyte由血管走入去LN Paracortex)
- Basal lamina
- Tunica adventitia (sparse-spread)
—> also present in other ***lymphoid tissue e.g. Peyer’s patches, Thymus, Tonsils
—> but absent in Spleen
Function: Passage of lymphocytes between bloodstream and surrounding tissue
Metastasis through lymphatic vessels
All malignant tumours
- Secondary tumour site can be predicted based on direction of lymph flow from primary tumour site
- Cancerous LN: enlarged, **firm, **non-tender, ***fixed to underlying structures
- e.g. Breast cancer —> enlarged LN at axilla region
LN in infection: enlarged, softer, tender, movable
Sentinel lymph node
First lymph node encountered in the path of lymph drainage from a particular organ
—> guard rest of body from dissemination of tumour cells / microbes
Sentinel lymph node biopsy
—> determine whether tumour has metastasised
—> Radioactive substance / Dye injected into tissue around tumour
—> Sentinel LN identified by probe
—> examined for evidence of cancer cells
—> complete removal of tumour + sentinel + regional LN
Spleen
- Secondary lymphoid organ
- Largest lymphoid organ
- extensive blood supply
- left upper quadrant of abdominal cavity (between stomach and diaphragm)
Function:
- ***Blood-forming organ during fetal life
- ***Blood-destroying organ in postnatal life
- ***Filter blood from blood-borne antigens / microorganisms
Structure of Spleen
NO cortex / medulla
- Fibrous capsule with trabeculae extension
- Red pulp (main region, peripheral)
—> Splenic sinuses
—> Splenic cords (Billroth cords) - White pulp (immune component, central)
—> Follicles (Lymphocyte + Macrophage) around central arterioles
—> Periarteriolar lymphatic sheath (PALS)
—> Marginal zone
***Red pulp - Parenchyma of Spleen
- up to 90% total splenic volume
- ***Mechanical filtration of blood via macrophages
- ~ LN Medulla
- Splenic cords
- **Reticular fibres (dense network) form Matrix which surround sinuses
- Matrix contain many **Macrophages + Lymphocytes + Mast cells
- remove antigens, microorganisms, old/abnormal RBC - Splenic sinuses (separated by Splenic cords)
- **discontinuous endothelium (allow blood vessels in Splenic cord to squeeze into sinus lumen)
- drains into **Splenic vein tributaries
***White pulp - Parenchyma of Spleen
- 5-20% of total splenic volume
- Splenic artery —> Central artery
- Immune function (carried out by B + T cell, Macrophage)
- ~ LN Cortex
- Periarteriolar lymphatic sheath (PALS)
- end of Central artery: adventitia replaced by ***sheath of T cells wrapping around central artery - Lymphoid follicles
- located near terminal branches of central arterioles
- white semi-opaque dots (visible by naked eye) near PALS
- intensive **B cell proliferation after **antigenic stimulation (APC by Dendritic cells)
—> activated B cell migrate to ***Germinal centres
—> some Plasma cells migrate to Red pulp
- atrophy during aging - Marginal zone (~ LN Paracortex)
- junction between Red and White pulp
- location of **blood delivery to Red pulp
- Lymphocyte leave circulation and **enter White pulp
- ***trap antigens by Macrophages + Lymphocytes
