Lecture 9: The Lymphatic System

Question 1

What is the lymphatic system?

Answer 1

A collection of organs and tissues used for defence from pathogens.
Such components include:
- Lymphocytes
- Spleen
- Bone Marrow
- Thymus Gland
- Lymph Nodes
- Lymph Vessels

Question 2

What are the lymph vessels?

Answer 2

The main transport vessel for cells and large molecules to migrate from tissue back into blood (they drain into the vena cava).

Question 3

What is the structure of lymph vessels?

Answer 3

The structure is similar to capillaries,
Single layer of squamous epithelium, and has layers of smooth muscle to aid in slow movement of lymphatic fluid. Some vessels rely of skeletal muscle to move fluid. Larger vessels have valves that aid in one way movement. They are also more porous than capillaries.

Question 4

Where are lymph vessels most common?

Answer 4

In the skin and mucous membranes.

Question 5

Why are lymph vessels more porous than capillaries?

Answer 5

It allows antigens and immune cells to be transported to and from the lymph vessels into tissues.

Question 6

What is diffuse lymphatic tissue?

Answer 6

The least complex form of lymphoid tissue. It is an unencapsulated aggregation of lymphocytes and macrophages + other leukocytes (eosinophils) associated with a reticular fiber network.

These tissues are strategically placed in entry points of infection such as the colon, in submucosal layers in the case of mucosa-associated lymphoid tissue (MALT), which is also located in genitals, respiratory and urinary tissues.

Lymphocytes, post-exposure can migrate from diffuse tissues into lymph vessels.

Question 7

What are lymphatic nodules?

Answer 7

Different from lymph nodes.
Lymphatic nodules have no encapsulation.

Made up of complex of lymphocytes and macrophages

Question 8

What is the difference between lymphatic nodules and diffuse lymphatic tissue?

Question 9

Where are lymphatic nodules located?

Answer 9

They are associated with the alimentary canal. Some examples are:
- Adenoids (Pharyngeal tonsils)
- Palatine and lingual tonsils (palate
and tongue)
- Peyer’s Patches (in ileum)
- Appendix

Question 10

What

Question 11

What are lymph nodes?

Answer 11

These are fully encapsulated organs which ‘filter’ lymph fluid. The size of these organs can vary from 1mm to 20mm.

Widely distributed throughout the body, but found in high concentration in certain areas:
- Axilla
- Groin
- Mesentery

Each lymph nodes supplied by 2 vessels, the afferent and efferent lymph vessels.

They are supported by the reticular network.

Question 12

What is the axilla?

Answer 12

The anatomical region under the shoulder where the arm connects to the shoulder.

Question 13

Where is the mesentery?

Answer 13

The fold of membrane located in the abdomen that connects the intestine to the abdominal wall.

Question 14

What role does the lymphatic system play in the metastasis of cancer?

Question 15

What is the reticular network?

Answer 15

Made up of cells that support the function of lymphocytes within the node. These cells include:
- Reticular cells
- Dendritic cells
- Macrophages
- Follicular dendritic cells.

Question 16

What are reticular cells?

Question 17

What is reticulin?

Question 18

What are follicular dendritic cells?

Question 19

What stain can be used to highlight reticulin?

Answer 19

Silver stains can be used, two examples would be:
- Jones Methenamine Silver
- Gordon and Sweet’s

In the presence of reticulin, these silver stains leave a black deposit.

Question 20

What is the histology of a lymph node?

Answer 20

The lymph node is divided into a cortex and a medulla.
The cortex contains significant amount of lymphatic tissue.
The medulla consists of medullary cords and sinuses.

Lymphocytes within the cortex are organised into nodules.

Question 21

What roles do the lymph nodes play?

Answer 21

Important in immunological response.

They are responsible for phagocytosis of particulate matter.

Play a key role in antibody response. With antigens being presented to naive B-cells in the germinal centre, then migrating to the medullary cord where they secrete antibodies.

Question 22

What is lymphadenitis?
What illnesses would it appear in?

Answer 22

The swelling of the lymph nodes as a complication of infection, occurs in mononucleosis, tonsillitis, pharyngitis, etc.

Question 23

What causes lymphadenitis?

Answer 23

An increase in the proliferation of lymphocytes within germinal centres, and an increase in the proportion of plasma cells in the node, as well as an increase in exudative fluid draining into the lymph node.

Treatment is antimicrobial therapy for infection. Excision if episodes of lymphadenitis are reoccurring.

Question 24

What is the thymus gland?

Answer 24

A gland that is part of the lymphatic system, known as a lymphoepithelial organ.

Located in the superior mediastinum region, anterior to the heart.

It begins as fully functional at birth but gets infiltrated by adipose tissue and shrinks as you age, until almost gone around 40, this process is known as involution.

It plays a vital role in T-cells maturation.

Question 25

What is the structure of the thymus?

Question 26

How many types of epithelioreticular cells are there?
Where do these cells reside?

Answer 26

6 types:
- Type I to Type VI

Type I - III reside in the cortex.

Type IV - VI reside in the medulla.

Question 27

What is myasthenia gravis?

Answer 27

An autoimmune disease in which auto-antibodies attack nicotinic acetylcholine receptors (important in neuromuscular junction), and thus leads to neuromuscular damage and lack of control.

Strong link between an enlarged thymus gland and MG, with around 20% of MG patients presenting thymoma.