3.01 Histology of Lymphoid Tissues

Explain the vascular supply and drainage of the spleen. Splenic artery STCPOSVL Are there any afferent lymphatic vessels?

-Splenic artery enters at hilium, branches into smaller vessels surrounded by collagenous tissue (trabecula) -Trebecular arteries then branch into central arteries, where collagenous collar is lost and now surrounded by T lymphocytes -Central arteries then branch into pencillar arteries (arterioles) -Pencillar arteries branch into capillaries and sinusoids, surrounded by macrophages. endothelium ends here -The open/non endothelial region eventually connects with sinusoids. -Sinusoids collect blood that has percolated through open area between ends of capillaries and sinusoids. Sinusoids drain into venules, then to veins Veins accompany trabecular arteries, drain into splenic vein at hilium Lymphatics begin in the white pulp (PALS) areas, coalesce to drain with splenic artery and vein at hilium. no afferent lymphatic vessels.

Architecture of a lymph node: What is the blood supply? What happens in the Deep Cortex (Paracortex)

At the hilium, arteries enter and veins leave the lymph node. Lymphatic vessels enter at the capsule and exit at the hilium. In the Deep Cortex, T lymphocytes predominate.

Organization of splenic parenchyma. White pulp, Red pulp, and capsule. What is white pulp? T cell regions? B cell regions?

Comes from fresh state appearance. HE staining is actually dark blue due to large numbers of lymphocytes. T cell regions - Periarteriolar lymphoid sheaths (PALS) these are irregular, discontinuous lymphoid aggregates surrounding central arteries B cell regions - scattered lymphoid aggregates present at intervals along course of central arteries, often with germinal centers, usually not central to the central artery.

There are two regions of the thymus. Explain the first. Explain what lymphocytes are doing, what thymic epithelial cells (sub capsular) are, nurse cells, and what macrophages are doing.

Cortex - has densely packed lymphocytes undergoing selection, differentiation, maturation. -sub capsular epithelial cells that produce a "blood thymus barrier" -nurse cells that produce a microenvironment for T cells -Also macrophages, cleaning out apoptotic lymphocytes

Tonsils now. Tonsils consists of lymph nodes with only what kind of lymph vessels? What is waldeyer's ring?

Efferent only. MALT organs, part of a collection of malt tissues collectively called Waldever's ring, because they are in a vaguely ring like arrangement around superior portion of pharynx.

What is the thymus? What is the major function, and what are two other main functions? What does it form embryologically from?

Ensures that mature T cells that leave thymus and enter circulation are restricted to MHC class I and II isoforms expressed by that person, therefore tolerant of SELF antigen, and responsive to foreign antigens prsesented by self MHC molecules. T cell production - t cells produced by bone marrow aren't competent. Hormone production - thymosin and cytokines that affect maturation of lymphocytes in other lymphatic organs. From third and part of fourth pharyngeal pouch endoderm,

We're gonna talk about thymic epithelial cells and their specific functions now. What are these generally? What do most thymic epithelial cells express (whats the exception) and produce?

Epithelial cells that have desmosomes and tonofibrils (kerain IFs). Connect to each other via desmosomes. Express both MHC I and MHC II, presumed to be involved in antigen presentation (exception Hassal's corpuscles only MHC I) All thymic epithelial cells produce hormones and cytokines for T cell development.

Capsule?

Fibrous capsule covers spleen. Spleen is intraperitoneal and has a mesothelial surface.

What is the function of the lymph node?

Filters the lymph, site of lymphocyte proliferation and differentiation. Found in series along lymph channels.

What makes up the blood-thymus barrier? How does it change with age and what is involution?

In the cortex, made up of the endothelium of capillary w/tight jxns basal lamina of capillary, thin connective tissue compartment with macrophages basal lamina next to a continous layer of epithelial cells thymic epithelial cells with tight jxns Thymus most fully developed in children, after puberty, "involution" occurs, replacing most of thymic tissue with fat. by young adulthood, thymus is almost all fat with scattered hassal's corpuscles spleen

Picture of lingual

Lingual - posterior tongue. Tongue muscle surrounds it. Shallow crypts

Explain lymphocytes (where are B and T lymphocytes found), and then differentiate them from the non lymphoid cells of lymph nodes (reticular cells, (interdigitating) dendritic cells, follicular dendritic cells, macrophages). Talk about location and function.

Lymphocytes are either B-lymphocytes mainly in cortex, or T lymphocytes in the paracortex. Reticular cells - present as part of sinus wall, present in stroma. Specialized fibroblasts that produce type III collagen of lymph node stroma. Have surface markers that attract lymphocytes and dendritic cells. IDCs - present in paracortex. bone marrow derived APCs, involved in presenting antigen to T lymphocytes FDCs - present in follicles. Have long cytoplasmic processes to which antigen-antibody complexes adhere. NOT APCs, but help B cells proliferate. Macrophages - found in most of node but prominent in germinal centers, contain phagocytosed material. Function as APCs

Peyer's Patches

MALT tissue, consisting of aggregates of lymph nodules in wall of ileum. Epithelium vcovering this is made up of flattened M cells (next) which endocytose macromolecules, transport to underlying lymphocytes

What is red pulp? What are splenic cords? What else can be found here? What are venous sinusoids and what collagen surrounds them?

Makes up 75% of splenic volume. Consists of splenic cords (billroth cords) and venous sinusoids. These are tissue surrounding and between sinusoids, contains many macrophages and erythrocytes (causes red color). This is where spleen moves senescent or damaged red cells from circulation. (also platelets, granulocytes, and 1/3 platelet population) Venus sinusoids lined by elongated endothelial cells, enclosed by circular reticular fibers (type III collagen) produced by reticular cells, forming loose stroma throughout spleen

Second? Explain what cells are here, how the thymic epithelial cells compare, and what the T lymphocytes are doing.

Medulla - has: -APCs (Dendritic) -even more thymic epithelial cells than cortex (more microenvironment) -Hassal's corpuscles (a population of thymic epithelial) -Mature T cells undergoing more selection.

What is MALT?

Mucosal associated lymphoid tissue. This is lymphoid tissue associated with wet epithilia, found in the tonsils, digestive tract (GALT, gut associated), and lower respiratory tract (BALT, bronchial associated)

What is the difference between primary, secondary, and tertiary lymphoid tissue?

PRimary: Organs where maturation and education of lymphocytes occur. B cells - bone marrow, T cells - thymus. Secondary: Everywhere else - peripheral, sites of antigen driven proliferation and differentiation. Lymph nodes, spleen, MALT structures (tonsils) Also scattered lymphocytes, plasma cells, common beneath wet epithelia. Tertiary - develops in response to injury/inflammation. Not a well defined organ, can develop/dissapear based on antigen stimulation. Persists long enough can actually develop characteristics of secondary.

Picture of palatine

Palatine - largest tonsil, covered in stratified squamous non keratinized epithelium. Deep crypts

What are the three portions of the tonsil?

Palatine - largest tonsil, covered in stratified squamous non keratinized epithelium. Deep crypts Lingual - posterior tongue. Tongue muscle surrounds it. Shallow crypts Pharyngeal - AKA adenoids. Covered by pseudostratified ciliated columnar epithelium with some stratified squamous mucosa. Posterior nasopharynx.

What is the vermiform appendix?

Part of the lower GI tract

Picture of pharyngeal

Pharyngeal - AKA adenoids. Covered by pseudostratified ciliated columnar epithelium with some stratified squamous mucosa. Posterior nasopharynx.

What are lymph nodules? Primary? Secondary?

Rounded aggregation of lymphocytes and plasma cells, found in any secondary lymphoid tissue (lymph nodes, spleen, MALT under wet epithelia). Can also be found in tertiary wherever there is an inflammatory of antigenic stimulus. Primary nodule lacks a germinal center (lacks central pale zone) Secondary nodule has a germinal center (central pale zone). Pale because large lymphocytes are present called lymphoblast. Follicular denriditic cells are present in germinal center too. Small lymphocytes crowded to periphery and make area darker.

What are M cells?

Speccialized epithelial cells found overlying lymphoid aggregates in GI tract. These have a pocket that contain immune cells (lymphocytes, dendritic cells, macrophages) Pocket causes a thinned cytoplasm, making transcytosis of engulfed antigens easier Apical glycocalyx has receptors for pathogens that can be endocytose and transported to waiting immune cells at basal aspect of M cell.

Before we get into regions and cells of the thymus, we gotta know these. There are 6 populations of thymic epithelial cells, but we're gonna focus on 4 subpopulations. Explain the location and function of these: Sub capsular epithelial cells Thymic epithelial cells (of the interface of cortex and medulla) Nurse cells Hassal's corpuscles One distinction to make here is that these are all thymic epithelial cells, but the second one is talking about general thymic epithelial and interface of cortex and medulla.

Sub capsular epithelial cells: Have tight junctions and thus, participate in blood-thymus barrier, separating vascular compartment from rest of thymus Thymic epithelial cells - at interface of cortex and medulla, participate in controlling transition of developing thymocytes between medulla and cortex Nurse cells - in thymic cortex, work with thymic epithelial cells to form an interconnected network of cytoplasmic processes and partially surround developing thymocytes, providing specialized microenvironment and antigen presentation, educating T lymphocytes.

What are lymphatic sinuses? Explain: Subcapular sinuses, Trabecular sinuses Medullary sinuses What do these allow for?

These are actually named by location in the lymph node. Subcapular sinuses are where afferent lymph vessels empty right beneath node capsule From that, sinuses extend through cortex (trabecular) to medulla (medullary) and exit node at helium. These allow for lymph fluid, with contents of lymphocytes and macrophages (Sometimes cancer cells!) to percolate through node , giving free access to node parenchyma.

What are Post Capillary Venules? Where are they? how do T lymphocytes circulate through these?

These are present in the deep cortex, and have HEV (high endethelial venules) These are the site of lymphocyte recirculation where T lymphocytes leave blood circulation and enter lymphatic circulation. Selective surface adhesion molecules on endothelial cells of HEVs allow lymphocytes to gain access to node parenchyma, and then lymphocytes exit via efferent lymph vessels. They can rent blood when lymphatics join venous blood, and circulate again until they reach lymph node; this keeps repeating.

What is the function of the spleen?

This is the largest single lymph organ in the body. Filters blood - destroys worn out/damaged cells, and surveys antigens (secondary lymphoid organ, contains proliferating T and B lymphocytes as part of immune system Blood storage - only a minor fxn in healthy humans Hematopoiesis in embryo