4. Lymphoid Tissue Flashcards
What is the definition of secondary lymphoid tissue?
Tissue where lymphocytes are activated in response to antigens.
(Tonsils, lymph nodes, spleen, MALT in small intestine etc.)
Where would you find diffuse lymphoid tissue?
Lamina propria of the GI
Genitourinary tract (especially in females)
Respiratory passages
How could one identify a lymphatic nodule?
You would have a clear accumulation of lymphocytes in a dark staining nodule without a connective tissue border.
What is a primary lymphatic nodule?
A nodule full of inactive or nieve lymphocytes
Does not have a germinal center
What do we call the darker staining region around the germinal center of a lympatic nodule?
Mantle zone or corona
Where would you find aggregated lymphoid tissue?
Tonsils
Peyer’s patches
Veriform appendix
Mucosa-associated lymphatic tissue (MALT)
What area of aggregated lymphatic nodules can be partially encapsulated?
Tonsils
What lines the palantine and lingual tonsils?
Nonkeratinized stratified squamous epithelium
What are the invaginations of the palantine tonsil called?
Tonsillar crypts
What lines the pharyngeal tonsil?
Pseudostratified columnar ciliated epithelium
Which pharyngeal tonsil has a complete, but thin, capsule?
Pharyngeal tonsil
Where are Peyer’s patches found?
What covers them?
In the ileum associated with intestinal villi
Simple columnar epithelium with goblet cells
What covers the lymphatic nodules of the veriform appendix?
What differentiates it from peyer’s patches?
Simple columnar epithelium
The covering epithelium has crypts, but no villi
What is MALT?
How is it categorized?
Mucosa-Associated Lymphoid Tissue
It is categorized by the associated tissues, eg: GALT for GI-Associated Lymphoid Tissue
What sort of connective tissue composes lymph nodes?
Reticular tissue
What is the parenchyma of a lymph node?
The cortex and medulla - everything that isn’t the supporting connective tissue on the outside
What is the stroma of a lymph node?
The supporting connective tissue on the outside.
What is the hilum of a lymph node?
Exit for efferent lymph
Entry / exit for neurovasculature
What part of the lymph node receives lymph from afferent lymphatic vessels?
The superficial cortex
What happens at the medulla of a lymph node?
Sinuses come together at the efferent lymphatic vessel
What kind of lymphocytes do we see in the superficial cortex of a lymph node?
What kind of lymphocytes do we see in the deep cortex of a lymph node?
B Cells
T Cells
Where do we find nodules in the cortex of a lymph node?
The superficial cortex
(The deep cortex is free of nodules)
What kind of cells line postcapillary venules?
Cuboidal or columnar endothelial cells
What is the function of a high endothelial venule?
Facilitate movement of lymphocytes into the lymph node
How do lymphocytes leave lymph nodes?
Always through the efferent lymphatic vessel
What is a medullary cord?
A “cord” of lymphatic tissue in between medullary sinuses
How do you differentiate the thymus and a lymph node histologically?
The thymus lacks nodules, and instead has lobules with a cortex and a medulla - separated by trabeculae
What is the primary cell type in the thymus of a young person?
Thymocytes (developing T Cells)
What cells provide the framework for T Cell development?
Epithelioreticular cells I - VI
What is the function of type 1 cortical epithelioreticular cells in the thymus?
Separate boundaries (between cortex and capsule, cortical parenchyma and trabeculae, developing T cells and stroma)
Surrounds the walls of capillaries to participate in the blood-thymus barrier
Which type of cortical epithelioreticular cells of the thymus separate isolated areas for developing T Cells?
Type II
What is the function of type III cortical epithelioreticular cells?
Establish a boundary between the cortex and the medulla
Express MHC I and MHC II to help educate T Cells
Why does the medulla stain more lightly than the cortex of the thymus?
The cortex is more packed with lymphocytes, so you see less cytoplasm
What is the major identifying hallmark of the medulla of the thymus histologically?
Thymic (Hassal’s) Corpuscles
What is the generic function of Type IV and V epithelioreticular cells?
Protecting lymphocytes from unintentional interactions as they differentiate
What is the function of type VI epithelioreticular cells?
Forming thymic (Hassal’s) corpuscles
Contain keratohyalin granules
Secrete IL-4 and IL-7 that functions in thymic differentiation and T-cell education
What is the Blood-Thymus barrier?
A barrier which prevents developing thymocytes from interacting with antigens in the blood.
What is the function of macrophages in the blood-thymus barrier?
They phagocytose antigens before they reach the vulnerable thymocytes
What organ serves as a reservoir for platelets?
The spleen
What is the function of splenic myofibroblasts?
They are muscle like fibers that squeeze the spleen to help expel RBC’s when necessary
Also produce extracellular fibers to contribute to the framework of the spleen
What sort of connective tissue covers the spleen?
Dense CT capsule with trabeculae
What can be found in splenic white pulp?
T and B cells
Macrophages
What can be found in splenic red pulp?
Venous supply
Red blood cells
Platelets
Macrophages
Plasma Cells
How could we identify the spleen histologically in a stained section?
The hallmark is that there is no cortex and medulla
White pulp forms splenic nodules, with red pulp and trabeculae between.
What is unique about the lymphoid nodules (white pulp) of the spleen?
They contain a central arteriole with PeriArteriolar Lymphoid Sheaths (PALS)
Not a true nodule
Contain a marginal zone
What are the three distinct regions of a lymphoid nodule (white pulp) of the spleen?
Germinal center (light staining, innermost)
Corona / Mantle zone (dark staining, small lymphocytes surrounding germinal center)
Marginal zone (lymphocytes fading into surrounding red pulp)
What is the primary function of red pulp of the spleen?
Blood filtration and erythrocyte turnover
What are the three structures that compose the red pulp of the spleen?
Splenic sinusoids
Splenic cords
Penicillar arteries
What are the splenic cords made of?
Reticular tissue
Macrophages (covering the capillaries)
Lymphocytes
What structure in the splenic sinusoids helps to differentiate between healthy and unhealthy red blood cells?
Stave cells
Describe the vasculature of the spleen, starting with the splenic artery and ending with the splenic cords.
(Practice recall of the hierarchy of the splenic vasculature, rather than just memorizing the back side of the card)
Splenic Artery
Trabecular Arteries (enter into the parenchyma)
Central Arteries (run through white pulp; PALS)
Penicillar arterioles (red pulp - macrophage sheaths)
Splenic sinusoids
Splenic cords
What is the benefit of open circulation in the spleen?
Increases interaction with the stave cells of the splenic sinusoid
