Histology of the thymus and peripheral lymphoid organs Flashcards
What do secondary lymphoid organs are?
Secondary organs include areas where aggregates of lymphocytes are found in close proximity to antigen presenting cells and can also furnish an adaptive immune response.
The lymph node
Lymph Node
1- What are reticular fibers (present in entire lymph node) used for?
2- High endothelial venules are specific for?
1- Used to keep some cells in in a specific locations, while lymph flows through them.
2- Lymphocytes
Spleen
Thymus
What divides the thymus into a series of pseudolobules?
Why is the cortex darker staining?
Medulla is pale with mature T cells because of what?
What is what defines the difference between cortex and medulla?
If circulating T cells return to the thymus, where are they found at?
The bilobed thymus is ensheathed by a connective tissue capsule from which connective tissue septa (trabeculae) partially divide the organ into series of pseudolobules. it has no afferent, but only efferent lymphatics.
Examination of the lobules shows a darker staining cortex that shows a more densely packed set of developing thymocytes having dark nuclei.
-The more mature thymocyte precursors are, the less densely packed in the medulla.
Thus, it is the relative densities of thymocytes per unit area that define the cortex versus the medulla.
-The cells just beneath the capsule are blast cells that divide (many mitotic figures are seen here) and give rise to other immature thymocytes.
A small percentage of circulating T- cells may return to the thymus, and are found in the medulla.
Thymus
A population of other cells are also found in the thymus, what are they?
What is their function?
In which part of the thymus do these cells form a greater percentage?
These are referred as epithelioreticular cells, or stromal cells, which include epithelial - like cells, dendritic cells and macrophages.
These cells provide a matrix and generally envelop the developing thymocytes in large infolds as the thymocytes mature from the cortex of the thymus toward the medulla.
There is extensive interaction of these cells with developing thymocytes, both in the cortex and medulla.
In the medulla, these cells form a greater percentage of the cross - sectional area and can be more readily identified.
-Stromal cells are important in the negative and positive selection events as thymocytes mature —they also secrete various cytokines and unique thymic hormones that are important in thymocyte maturation. The majority of thymocytes are actually eliminated by the selection process, the molecular interactions of which remains an on- going area of active research and will be covered in immunology lectures.
Thymus
Blood flow through the thymus:
The endothelian cells of the vessels that enter the thymus have what type of junctions?
What is the function of the blood-Thymus Barrier?
Small arteries (from internal thoracic and inferior thymic artery) enter the thymus through the outer capsule and penetrate into the thymus and bifurcate within the connective tissue septa between the lobules.
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. These combined layers (but functionally primarily the endothelioreticular cells) form what is called the blood-
thymus barrier–> Hence maturing thymocytes are not exposed to just any molecules that might be circulating in the blood.
The stromal cells provide a microenvironment that protects them from exposure to foreign antigens as well as providing the conditions for their maturation and selection.
Thymus
What are Hassal’s corpuscles? Where are they located? What is their function?
The stromal cells provide support.
In the medullary regions of the thymus are seen varying numbers of circular bodies called Hassal’s corpuscles that are comprised of concentric layers of reticular cells. Recent evidence indicates that they produce thymic stromal lymphoprotein that is important ultimately in suppressing autoimmune events. At the end of their selection, mature thymocytes in the medullary region leave the thymus via lymphatics and blood vessels and become distributed through the body.
Mucosal Associaed Lymphoid Tissue
Hemisection of a lymph node:
H–> Hilar region
M–> Medulla
C–> Cortex
The outer region of a lymph node.
Cap–> outer capsule
T–> small trabelulae of the cortical region
LN–> lymphatic nodule or follicle (with germinal centers)
MC–> medullary cords,
MS–> medullary sinuses
The outer capsular region in more detail.
Cap, outer capsule
Tr, trabebulum
CS, subcapsular space.
RF, reticular fibers.
The cortical region showing the subcapsular space under the capsule. Various leukocytes enter in this region
A hilar region of the lymph node
Another region of the cortex, showing germinal centers in follicles
A section through a high endothelial venule in a lymph node. Note the endothelial cells that “bulge” into the blood vessel. This is a location for cellular interactions between lymphocytes and the endothelial cells for diapedesis—important for lymphocyte entry via the blood.
The thymus
Higher magnification of thymic trabeculae, showing how blood vessels stay within trabeculae. Reticuloendothelial cells that surround the trabeculae form the blood-thymus barrier.
A section of the thymus in the medullary region at the left, showing Hassall’s corpuscles and more relative numbers of the stromal cells involved in selection of thymocytes (the pinkish-stained cells in the medullary region).
A low magnification section through the spleen, showing the punctate regions of white pulp and the red pulp (blood sinuses). Note that this is unstained, just a section through the spleen where the red regions are sinuses containing red blood cells and white pulp is accumulations of (mostly) T-cells and some B cells.
White pulp (WP) in the spleen is mainly comprised of T-cells around central arterioles (CA), although groupings of B-cells and B-cells with germinal centers are spread throughout.
RP, red pulp, regions where blood percolates among macrophages primarily. TV, trabecular vein, which leads toward the splenic vein.
Higher magnification of the region of the spleen showing central arteries with periarteriolar sheaths of, primarily T cells, but also containing some B-cells
A scanning E.M. showing the arrangement of the discontinuous endothelium (stave cells) that line the ends of the central arteries. The spaces between the cells get so big that red blood cells, platelets, neutrophils (N) and macrophages (M) can leave the vessels into the sinusoids that make up the red pulp.
In this slide, the spleen was perfused with India ink that contains carbon particles that splenic macrophages engulf. Macrophages in the sinusoids engulf particulates, bacteria, and senescent platelets and red blood cells. Degraded hemoglobin (bilirubin) enters the splenic vein, thence to the liver which transports bilirubin to the bile. Macrophages hang on to reticular fibers in the spleen, as shown in the next slide.
Reticular fibers of the spleen.The round object is connective tissue around a central arteriole.
MALT
Are lymphoid cells encapsulated in MALT?
What is the name of the cell that takes antigents from the intestinal lumen and presents them to immune cell in the Peyer’ patches?
What is the main Ig secreted by B cells here?
This tissue is the spleen, which is able to be perfused via blood vessels.
It gets its cells fromm the thymus.
Differentiate white and red pulp, what different cells are present here?
Spleen
Lymph node
Circles area is a germinal center, where B cell proliferation/differentiation occurs.
A= medullary sinus
B= HEV
C= Afferent lymphatic
