Lymphatic System Flashcards
Define primary and secondary lymphatic tissue
- *Primary:** origin points of lymphocytes
- *Secondary:** site where cells undergo antigen dependent differentiation
What are the primary lymphatic tissues?
- Bone marrow
- Thymus
What are examples of secondary lymphatic tissue?
- Lymph Nodes
- Spleen
- Uncapsulated lyphatic tissues
- MALT
- GALT
- Tonsils
- Peyer’s Patches
Cellular composition and location of the adult thymus
- epithelioreticular cells
- packed with stem cells
- superior mediastinum; bilobed
Describe epithelioeticular cells
- Attached to each other by desmosomes
- provides supportive network for developing lymphocytes (similar to the reticular fiber network)
Function of epithelioreticular cells
- Barrier between developing T cells & antigens
- Provides self antigens to test T cells (negative selection)
What are the three gross anatomical areas of the thymus?
- Capsule
- Cortex
- Medulla
Describe the thymus capsule
- Surround thymus with blood vessels, efferent lymphatic, and nerves
- Trabeculae (capsule extensions) divides the cortex into lobules
- hilum: where arteries, veins and lymphatic vessels exit
Describe the thymus cortex
- Peripheral portion of thymus
- High density of lymphocytes
- basophilic
- Presence of macrophages & epithelial cells
- lymph follicles or nodules
Describe the thymus medulla
- Central portion of thymus
- Large number of epithelioreticular cells
- Thymic (Hassall’s) corpuscles (unique to medulla)
Thymic (Hassall’s) corpuscles
- unique structures in medulla
- Small groups of epithelial cells that are keratinized/calcified
- May be involved in auto-reactive T-cell destruction
- Eosiniphilic
Function of thymus
- Development of immunocompetent T-cells
- Elimination of incompetent T cells
T Cell development in the thymus
Arteries enter the capsule through the trabeculae of the corticomedullary junction forming a capillary bed which enters the cortex
Three major elements of the blood-thymus barrier
- Capillary endothelium and is basal lamina
- Periascular connective tissue space occupied by macrophages
- Type 1 epithelioreticular cells with their basal lamina
Function of blood-thymus barrier
- Creates a continuous capillary bed ensheathed by a layer of epithelioreticular cells
- Used to reduce new T-cells’ exposure to blood-borne antigens
- Creates a pathway for stem cells to develop into thymocytes in the cortex and migrate towards the medulla to mature
How do immunocompetence cells enter circulation?
The venous system
Describe thymus involution
Fully developed at birth & continuously makes T-cells until puberty, then becomes fat
Where are the lymph nodes located?
Throughout the vascular system
1ry follicle of thymus cortex
unstimulated and homogenous appearance
2ry follicle of thymus cortex
- lighter staining central region (germinal center) during immune response
- Immunoblasts – Large and stain light, responsible for the germinal center’s appearance
- Mantle – Layer of mature lymphocytes around the germinal center
Two parts of the thymus medulla
- Medullary cord: dense group of cells mostly plasma cells and amino blast
- Medullary sinuses: spaces between cord lined by macrophages
Explain flow of lymph through the lymph node
- Afferent lymph vessels–>Subcapsular sinus –> Trabecular sinuses –> collect at the hilum –> leave the node through the efferent lymphatic vessels
(this exposes the lymph and antigens to a large number of lymphocytes within)
Define subcapsular sinus of thymus
Space immediately adjacent to the capsule that provides a path for lymph flow
Define trabecular sinus of thymus
Continual opening with subcapsular sinus that allows the lymph to flow through the cortex
Explain blood flow and lymphocyte recirculation in a lymph node
Arterial supply enters through the hilum and branches into the medulla and cortex (i.e. corticalmedullary bed) –> Loop back into the medulla and drain into the postcapillary venules –> exits from the hilum
What are high endothelial venules?
Specialized postcapillary venules in the lymph nodes lined with unusually thick endothelial cells
What is the purpose of high endothelial venules?
Endothelial cells have special adhesion molecules on their luminal surface. B-cells and T-cells are able to shuttle back and forth from the lymph node to the blood.
(maximizing the odds of contact with antigen)
What are the main histological components of the spleen?
- Trabeculae extend into spleen
- Hilum
- Parenchyma: white & red pulp
define white pulp of the spleen
Round elongated accumulations of densely packed cells (lymphocytes)
Define red pulp of spleen
- Surrounds white pulp
- Accumulation of blood cells
Functions of the spleen
- Eliminate blood-borne antigens
- Destruction of old or defective blood cells
(macrophages monitor blood cells by cytoplasmic extensions in the inter endothelial slits. Any difficulty of blood cell passing through is destroyed)
Explain blood flow through the spleen and resulting filtration of defective blood cells.
-
Splenic artery
- Arterial supply that enters in hilum and branches into trabecular arteries Central artery is an arteriole branch of the trabecular arteries that area surrounded by white pulp (called the periarterial lymphatic sheath)
- Branches from the central artery are penicillar arterioles at right angles.
* At the end of the penicillar arterioles are sheath capillaries that are blind-ended and at the end of the marginal zone
- Branches from the central artery are penicillar arterioles at right angles.
- Arterial supply that enters in hilum and branches into trabecular arteries Central artery is an arteriole branch of the trabecular arteries that area surrounded by white pulp (called the periarterial lymphatic sheath)
Marginal zone
The border of the red-white pulp border where blood passes form the sheathed capillaries and enters the splenic cords (i.e. it has exited the vasculature)
Explain the mechanism of blood-borne antigen detection in the spleen
- Blood is released from the sheathed capillaries at the marginal zone –> Ags detected by the blood are processed and presented by macrophages lining the area of the splenic sinus –> T-cells in the spleen that recognize the Ag will be activated and leave the spleen to attack
- B-cells that are activated will accumulate at the marginal zone and become plasma cells to release Abs
Describe locations, histology, & function of major diffuse lymphoid tissues.
- MALT (mucosal-associated lymphoid tissues)
- BALT – bronchi-associated
- GALT – gut-associated
- Aggregates of lymphocytes and LNs that are not completely surrounded by capsules providing a first line of defense against exogenous antigens.
