Histology of the Thymus and Peripheral Lymphoid Organs Flashcards
Name three encapsulated lymphoid organs.
- Lymph Nodes
- Thymus
- Spleen
What structures are considered primary lymphoid organs?
Bone marrow
Thymus
How does lymph enter and exit a lymph node?
Lymph enters in the afferent lymphatic vessels into the subcapsular space
Then they exit through the efferent lymphatic vessels
The lymph node is made up of three regions, what are they?
Cortex
Paracortex
Medulla
What separates the cortex?
Trabeculae
What is contained in the cortex?
Germinal centers full of active cell proliferation and apoptosis
Mostly B cells
Some macrophages
What is the paracortex mostly comprised of?
T cells and some B cells
What is contained in the medulla?
B cess, a few T cells and plasma cells
Where do the blood vessels enter and exit?
At the hilum
Why is the high endothelial venule important?
It is the site for recognition and diapedesis of lymphocytes from the blood into the lymphatic space of the node.
Contain selectins
What occurs in the thymus?
Proliferation, differentiation and selection of thymocytes
When does the thymus involution occur?
Around puberty
What composes the cortex of the thymus?
More densely packed set of developing thymocytes having dark nuclei. It also stains darker
Where can you find more mature thymocytes in the thymus?
In the less densely packed medulla.
What kind of cells are found just beneath the capsule?
Blast cells that divide and give rise to other immature thymocytes
Where can you find returning T cells in the thymus?
In the medulla.
What other cell types can be found in the thymus?
Epithelial-like cells
dendritic cells
Macrophages
Stromal cells are important in what type of thymocyte maturation process?
Positive and negative selection
They secrete various cytokines and unique thymus hormones important in thymocyte maturation
Does the thymus contain reticular fibers?
No
What are Hassl’s corpuscles and why are they important?
They are comprised of concentric layers of reticular cells
These cells produce thymus stromal lymphoprotein that is important in suppressing autoimmune events
Where do arteries enter the thymus?
They enter through the outer capsule and penetrate into the thymus and bifucate within the connective tissue septa
Describe the blood-thymus barrier.
The endothelial cells of the vessels have tight junctions and are surrounded by a region of connective tissue that is then ensheathed in endothelioreticular cells
What are the main vessels going into and out of the spleen?
Splenic artery
Splenic vein
What is a special feature of the central arterioles of the spleen?
The endothelium is discontinuous allowing RBCs, platelets, and WBC to leave circulation.
Where is the bulk of lymphoid tissue found in the spleen?
They are found wrapped around the central arteriole in perarteriolar lymphoid sheath (PALS). Contain germinal centers
Blood flows into _________ and more organized lymphoid tissue is found in the __________
Red Pulp
White pulp
Loosely-arranged reticular fibers are primarily found in _________ pulp
Red
Are macrophages found more in the red or white pulp?
Both
Where are mucosal-associated lymphoid tissues found
Mostly in the digestive tract and respiratory system.
Ex: Tonsils, adenoids, appendix, Peyer’s patches
Is MALT encapsulated?
No
What is the role of M cells?
They deliver antigens to underlying lymphoid tissue
