L11 – Structure and Function of the Lymphatic System

Question 1

3 components of the lymphatic system?

Answer 1

Lymph, lymphatic organs, lymphatic vessels

Question 2

3 function of lymphatic system?

Answer 2

1. Fluid balance (maintain homeostasis): recover fluid lost from blood capillaries
2. Defense (immunity):
   i) Carries away larger particles (e.g. bacteria, viruses) to be filtered, destroyed in lymph nodes
   ii) Guards against pathogens
3. Fat absorption via intestinal lacteals:
   - Chyle = lymph + lipids
   - Transport the fats into venous circulation

Question 3

5 Histological features of lymph vessels that differ from normal blood vessels/ capillaries? (think endothelium, basal lamina, lumen…)

Answer 3

• Blind ended
• Single layer of Loosely overlapped endothelial cells
• No tight junction, incomplete/ absent basal laminae
• Larger lumen than blood capillaries
• Large lymph vessels have Bicuspid valves (unidirectional flow)

Question 4

Distribution of lymphatic capillaries?

Answer 4

throughout the body except in nonvascular tissues, splenic pulp and bone marrow.

Particularly abundant within dermis of skin, mucous membranes

Question 5

List the sequence of drainage of lymph from interstitial fluid* to the venous circulation? (2 pathways)

Answer 5

Interstitial fluid
> afferent lymphatic vessel (multiple)
> lymph node 
> efferent lymphatic vessel (1 or 2)
>  various lymphatic trunks: drain into 2 lymphatic DUCTS

1) 75%, Left upper body + lower body (intestinal + lumbar into Cisterna chyli) into Thoracic duct > Left venous angle > Left brachiocephalic vein
2) 25%, right upper body into right lymphatic duct > Right venous angle> Right brachiocephalic vein

Question 6

List all the lymphatic trunks and the areas they each drain? (5) (their names literally say where they drain!)

Answer 6

§ L/R Lumbar – lower limbs, pelvis, kidneys, adrenal glands and abdominal wall

§ Intestinal – organs in the abdominal cavity

§ L/R Bronchomediastinal – thoracic wall, lungs and heart

§ L/R Subclavian – upper limbs

§ L/R Jugular – head and neck

Question 7

List all the lymphatic trunks and their accompanying blood vessels?

Answer 7

§ L/R Lumbar – Aorta and its branches

§ Intestinal – Aorta and its branches

§ L/R Bronchomediastinal – independent of blood vessels

§ L/R Subclavian – Subclavian vein

§ L/R Jugular – Internal jugular vein

Question 8

3 mechanisms that provide pressure differences to drive lymph flow? (very similar to venous blood flow!)

Answer 8

• Skeletal muscle pump
• Respiratory pump
• Smooth muscle wall in tunica media and adventitia of large lymphatic vessels

Question 9

What vessels form the venous angles?

Answer 9

junction of internal jugular and subclavian veins

Question 10

2 types of lymphedema and causes?

Answer 10

Primary = developmental abnormality of lymph nodes, rare and inherited condition

Secondary = surgery/ radiation therapy/ tumour compression…etc cause scar tissue formation and failure of lymph vessels to regenrate

Question 11

Cause and symptoms of Filariasis?

Answer 11

• invasion of the lymphatic vessels by filarioidea

- severe swelling in legs and genitals (elephantiasis)

Question 12

Define the primary lymphatic organs and function?

Answer 12

The organs where lymphoid stem cells are divided and matured into immunocompetent B and T cells

§ Red bone Marrow (B cells)

§ Thymus (T cells)

Question 13

Define secondary lymphatic organs and function?

Answer 13

where immunocompetent cells populate and initiate immune responses

§ Lymph nodes

§ Spleen

§ Mucosa-associated lymphatic tissue (MALT)
- Peyer’s patch - Tonsils

Question 14

Location and gross structure/shape of thymus?

Answer 14

Location - in the mediastinum between sternum and the aorta

• two lobes being encapsulated separately and connected by connective tissue
• adhere to the fibrous pericardium

Question 15

Change in the size of thymus over lifetime?

Answer 15

Size

• largest in the early part of life, particularly around puberty
• the functional portion atrophies with age and is replaced by fatty infiltrations

Question 16

Describe the gross internal structure of the thymus?

Answer 16

Connective tissue capsule extends into thymus
» form trabeculae** subdividing the thymus into incomplete lobules**:

Each lobule has:
Cortex
Medulla

Question 17

Describe the difference in staining between cortex and medulla of thymus lobules. (staining: think about the cells that reside there/ lifecycle of lymphocytes!)

Answer 17

1) Cortex
- dark-staining outer part of the lobule with densely packed immature lymphocytes

2) Medulla
- light-staining central part of lobule with mature lymphocytes

Mature lymphocytes have less dense nucleus + more cytoplasm = less staining

Question 18

Describe the change in thymus parenchyma with aging?

Answer 18

cellular portion of thymus is gradually replaced by connective tissue and adipose cells.

Question 19

List the cells in the cortex of lobules in the thymus?

Answer 19

§ large number of T cells

§ scattered epithelial reticular cells

§ dendritic cells and macrophages

Question 20

Function of epithelial reticular cells in the lobules of the thymus?

Answer 20

Depends on location:

1) Cortex of lobule:
i) serve as a framework to “educate” the pre-T cells to develop self- antigen recognition in Positive Selection
ii) Produce thymic hormones***

2) Medulla of lobule:
- cluster to form Hassall’s corpuscles***
- assist in T cell “education” too

Question 21

List the cells in the medulla of lobules in the thymus?

Answer 21

§ mature T cells

§ reticular epithelial cells

§ dendritic cells and macrophages

Question 22

Define the neoplasm of thymic epithelium and its presentation?

Answer 22

Thymoma
- Non-symptom: 1/3 to 1/2 patients have no symptoms

• Symptoms: compression of the surrounding organs or associated autoimmune disorder (myasthenia gravis)

Question 23

List the 4 layers of lymph nodes?

Answer 23

Capsule
Cortex
Paracortex
Medulla

Question 24

Describe the structure of the lymph node capsule?

Answer 24

• connective tissue with trabeculae extensions into the node
  » form subcapsular and trabecular sinuses
• afferent lymphatic vessels penetrate the capsule

Question 25

Function of the lymph node cortex?

Answer 25

Contain primary and secondary lymphoid follicles: 1) Primary lymphoid follicles - absence of immune stimulation - evenly distributed dark-staining inactive B cells 2) Secondary lymphoid follicles - under antigenic stimulation - sites of plasma cell and memory B cell formation

Question 26

Function of the medulla of lymph nodes?

Answer 26

Contain medullary sinuses and medullary cords > Connect trabecular sinuses with efferent lymph vessels at the Hilum

Question 27

The cortex and paracortex both have primary and secondary lymphoid follicles. T or F?

Answer 27

False Paracortex has no lymphoid follicles

Question 28

3 functions of lymph nodes?

Answer 28

§ lymph filtration § phagocytosis of bacteria or foreign substances § lymphocytes production, storage, activation and recirculation

Question 29

Describe the flow of lymph through the lymph nodes? Which layers are for immune defense?

Answer 29

``` Several afferent vessel Subcapsular sinus Trabecular sinus Medullary sinus 1-2 Efferent lymph vessels ``` All 3 sinuses have reticular fibers, macrophages and lymphocytes for immune defence

Question 30

Why are there more afferent lymph vessels into lymph nodes than efferent?

Answer 30

Lower number of efferent vessels to slow down lymph flow >> more time for filtration and immune defense

Question 31

Compare the cells that populate the cortex vs paracortex of lymph nodes?

Answer 31

Cortex = B cells mainly, some dendritic cells and macrophages as APCs ,has primary and secondary lymphoid follicles Paracortex = T cells and dendritic cells, no lymphoid follicles

Question 32

Distinguish the cells that populate the secondary lymphoid follicles in LN?

Answer 32

- germinal center: dendritic cells and macrophages activated B cells in various stages of maturation - peripheral zone (mantle zone): inactive B cells with condensed nuclei and little cytoplasm

Question 33

Function of the lymph node paracortex?

Answer 33

- absence of lymphoid follicles = no B cell production - T cells and dendritic cells >> expands in T cell dominated immune responses - postcapillary high endothelial venules (HEVs) ***

Question 34

Describe the ultrastructure of the medulla of lymph nodes?

Answer 34

- Medullar cords separated by sinuses - More lymphocytes in cords - B cells and plasma cells migrated from cortex embed between reticular fibers in medulla

Question 35

Function of HEVs in Lymph nodes? What are the histological layers of HEV?

Answer 35

High endothelial venules @ paracortex of LN - Postcapillary venules lined by cuboidal** endothelium,THICK basal lamina and sparse tunica adventitia (endothelium has specialized lymphocyte homeoreceptors) - Shortcut Passage of lymphocytes between bloodstream and LN and surrounding tissue

Question 36

Describe difference between metastatic LN and Infected LN?

Answer 36

Metastatic = enlarged, firm, nontender lymph nodes fixed to underlying structures Infected = Soft, tender, moveable

Question 37

Define sentinel lymph nodes?

Answer 37

first lymph node encountered in the path of lymph drainage from a particular organ. guard rest of body from dissemination of tumour cells or microbes

Question 38

Differentiate the change in function of the spleen over lifetime?

Answer 38

- blood-forming organ in foetal life - blood-destroying organ in postnatal life - filters blood from blood-borne antigens and microorganisms

Question 39

Describe the gross structure of the spleen with anatomical divisions?

Answer 39

fibrous capsule with trabeculae extension White pulp: - extends from central to marginal zones - contains follicles,periarterial lymphatic sheath (PALS) Red pulp: (major, 90% volume) - at the periphery - Contains splenic sinuses and splenic cords (Billroth cords)

Question 40

Describe the anatomical structures and functions of the red pulp of the spleen.

Answer 40

90% splenic volume Mechanical filtration of blood via macrophages Splenic cords: - Reticular fibers form matrix of WBC surrounding sinuses - Macrophages destroy RBC Splenic siuses: - Drain into splenic vein branches - Lined by discontinuous endothelium

Question 41

Describe the anatomical structures and functions of the white pulp of the spleen.

Answer 41

- periarterial lymphatic sheath (PALS): T cells wrapping terminal end of central artery - Lymphoid follicles: i) Germinal center for B-cell proliferation under Ag stimulation ii) Activated B cells and plasma cells move to germinal center - Marginal zone: Junction between red and white pulp - Trap antigens by macrophages and lymphocytes

Question 42

List the locations in the body where MALT is found?

Answer 42

Unencapsulated lymphoid tissue >> in walls of GI, Resp, Reproductive, Urinary tracts + skin e. g. - gut-associated lymphoid tissue (GALT) - tonsils (palatine,nasopharyngeal,lingual and tubal)

Question 43

Describe the ultrastructure of MALT?

Answer 43

- Close to epithelial surface, unencapsulated - Follicles in lamina propria and submucosa >> lymphocyte activation and proliferation - Has APCs (dendritic, macrophages)

Question 44

What are the 2 types of GALT in the gut?

Answer 44

Peyer's patches @ ileum Isolated lymphoid follicles @ throughout intestines

Question 45

Describe the special cell types and functions of Peyer's Patches?

Answer 45

1) specialized epithelial cells (M cells) - phagocytose luminal antigens - transport antigens to act as APC to lymphocytes & macrophages at follicles 2) Lymphoid follicles - B and T cells, macrophages and plasma cells

Question 46

Which layer of the intestine is GALT resided?

Answer 46

in the lamina propria & submucosa of ileum

Question 47

Name of the ring of MALT at the oro- and nasopharynx?

Answer 47

Waldeyer’s ring surrounds the openings into the digestive and respiratory tracts

Question 48

List the tonsils that make up Waldeyer's ring? Structure of these tonsils?

Answer 48

§ anteroinferiorly by the lingual tonsil § laterally by the tubal and palatine tonsils § posterosuperiorly by the pharyngeal tonsil - partially encapsulated + lymphatic follicles with germinal centers

Question 49

Where is the lingual tonsil located?

Answer 49

lamina propria of the root of the tongue

Question 50

Describe the epithelium, capsule and ultrastructure of the lingual tonsils?

Answer 50

stratified squamous nonkaratinized epithelium with invaginations >> deep tonsillar crypts incomplete capsule primary or secondary lymphoid follicles

Question 51

Location of palatine tonsils?

Answer 51

either side of pharynx between palatopharyngeal and palatoglossal arches

Question 52

Describe the epithelium, capsule and ultrastructure of the palatine tonsils?

Answer 52

stratified squamous nonkaratinized epithelium with invaginations >> deep tonsillar crypts incomplete capsule mainly secondary lymphoid follicles, some primary

Question 53

Compare the Aetiology and appearance of tonsils in acute vs chronic tonsillitis?

Answer 53

Acute: Viral or bacterial cause Enlarged, red and inflammed tonsils Chronic: repeated acute attacks Small and fibrosed

Question 54

Treatment options for tonsilitis?

Answer 54

Treat pathogen with antibiotics (viral usually self resolve) Indicate tonsillectomy* for airway obstruction + dysphagia and dysphonia in chronic tosilitis (>3 months)

Question 55

List the structural features that distinguish pharyngeal tonsil from lingual or palatine tonsils?

Answer 55

- pseudostratifed ciliated columnar goblet cell / respiratory epithelium (NOT NKSS) - NO DEEP CRYPTS - Mostly secondary lymphoid follicles

Question 56

Sequence of arterial blood flow into spleen?

Answer 56

Splenic artery (hilum) >> trabecular arteries >> central arteries >> Periarterial lymphatic sheath (PALS) to penetrate lymphatic nodule >> pencillar artery/arterioles and terminal macrophage-sheathed capillaries >> splenic sinuses

Question 57

Sequence of venous blood flow out of spleen?

Answer 57

splenic sinusoids/ sinuses>> pulp veins >> trabecular veins >> splenic veins

Question 58

If diffuse lymphoid tissues are filled with eosinophils, what does that indicate?

Answer 58

 Chronic inflammation |  Hypersensitivity reactions