Lymphatic System Flashcards
Lymphoreticular System
To protect the body against pathogenic organisms and their products and to help in the removal and disposal of cells undergoing natural or induced degenerations
Phagocytosis: Macrophages
Production of immunologically competent cells: Antigen presenting cells (dendritic cells, Macrophage, B cell); B and T lymphocytes
Mononuclear Phagocytic System
Fixed Macrophages (within tissues): sinusoids of liver, sinusoids of spleen, sinusoids of lymph node, reticulum of bone marrow Free macrophages (monocytes): Blood, lung, serous cavities
Antigen Presenting Cells (APCs)
Cells that get exposed to an antigen (cancer, virus, toxin, etc)
Cells that capture, process, and present protein fragments from extracellular antigens (on MHC II)
Binds to antigen and then through cytokin processes, presents to T-cell– then mounts an immune response
Dendritic cells, Macrophages, B-cells
Lymphatic System: Primary Organs
Generate lymphocytes form progenitor cells
Bone Marrow (B lymphocytes)
Cloacal Bursa
Thymus (T lymphocytes): cortex and medulla
Lymphatic System: Secondary Organs
Sites of lymphocyte activation/differentiation in context of immune response
Lymph nodes
Spleen
Mucosal Associated Lymphoid Tissue
B and T Lymphocytes
In Bone marrow, Lymphoid stem cell leaves to Thymus where T cell differentiation takes place. (then populate secondary organs and can further differentiate in immune response) Helper/suppressor/effector, Natural Killer etc
In bone marrow, lymphoid stem cell stays and B cell differentiation takes place. (populate secondary organs and can further differentiate in immune response) Memory cell, plasma cell
Primary Lymphatic Organs
Where lymphoid stem cells originate from
Where T and B cells originate and their unique features are established
Access by antigen is strictly controlled (barriers): if primary lymphoid organs become exposed to antigens, process of positive selection would not occur properly
Apoptotic elimination of self-reactive cells
Released to circulation to sites where antigen encountered (secondary lymphoid)
Bone Marrow
Source of pluripotent stem cells (lymphoid line): B and T cells
B cells mature in bone marrow
T cells migrate to thymus
A background of stromal cells exist also: release factors aiding cell differentiation and macrophages for ion storage, antigen presentation etc.
Cloacal Bursa
In Birds
Dorsal wall of the cloaca
Functionally equivalent to mammalian bone marrow for B cell differentiation
Surface mucosa of bursal fold covered by pseudostratified columnar epithelium
3 lymphoid nodules, dark corices, light medulla
Lymphocytes emigrating through surface epithelium
Thymus
T cell differentiation
Located in mediastinum just cranial to heart
Composed of epithelial reticulum and lumphocytes
Lymphocytic stem cells migrate from bone marrow
Fill spaces between reticular epithelial cells of the developing organ
Develop into T cells
Thin connective tissue capsule, sends in septae. These divide the organ into partially separated lobules. Dark cortex, light medulla-which is continuous between adjacent lobules, despite island appearance
Thymic Cortex
Stains much darker than thymic medulla due to greater number of lymphocytes present
Area of positive selection (Good lymphocytes)
Tingible body macrophages- frequent near the medulla: Phagocytose and eliminate dead T cells
Positive Selection
During T lymphocyte development
Need to demonstrate that it has certain receptor (MHC II complex). This determines if down the line T cell binds to antigen presenting cell
If it can bind, it passes and moves to medulla of thymus
If not it dies by apoptosis
Lymphocytes pass positive selection by encountering Thymic epithelial cells. They traverse through the cortex and bump into many of these cells and possible and if they bind to any they pass the selection test
Thymic reticular cells present in medulla and cortex
Thymic Medulla
Also contains epithelial reticular cells
Area of negative selection (bad lymphocytes-autoreactive): tingible body macrophages
Some medullary reticular epithelial cells form thymic corpuscles or Hassall’s copuscles
Thymic epithelial reticular cells
Present as much of own DNA as possible on cell surface. So T cells bump into a lot and is exposed to a lot of self DNA
Negative Selection
Dont want T lymphocytes to be too sensitive to binding own antigens
If it binds too strongly to self antigen, it will fail and have to die
Hassall’s Corpuscles
Large central calcified or degenerated cells surrounded by concentric circles of keratinized cells
Blood Vessels in Thymus: Blood Supply
Arteries enter via the corticomedullary junction within the connective tissue septa
Divide into arterioles within the septa
Branch into a capillary network in the cortes
Blood Vessels in Thymus: Cortical Capillaries
Blood thymus barrier:
continuous endothelium
perivascular connective tissue
sheath of epithelial reticular cell processes: decrease antigen access to thymus, limits interference with positive T cell selection
Educated T cells
After passing positive and negative selection
Leave the thymus through the postcapillary venules at the coricomedullary junction
Enter blood, settle in T cell areas of secondary lymphatic tissue
Thymic Involution
Thymus is active in young animals, involutes after sexual maturity
Gradual depletion of lymphocytes
Replacement by adipocytes
Secondary lymphatic organs
Where lymphocytes are stores, mature and become activated
Lymph nodes, spleen, mucosal-associated lymphoid organs
Lymph nodes
Filter antigens from lymph before returning it to bloodstream Capsule Cortex Medulla Hilus
Lymph Node: Cortex
Primary and secondary follicles (B cell rich)
Paracortical tissue (T cell rich)
High endothelial blood vessels
Subcapsular Sinus
Lymph Node: Medulla
Medullary cords
Medullary Sinus
Lymph Node: Hilus
Slight indentation- arteries enter, and efferent lymphatics and veins leave
Lymph Node: Structure
Capsule-dense, irregular connective tissue (ruminants have smooth muscle cells too)
Trabecula extend from capsule into cortex and medulla: structural support, contain blood vessels and nerves. Surrounded by sinuses
Lymph node stroma- reticular cells and fibers that support lymphocytes, macrophages, and plasma cells
Lymph vessels and sinuses
Afferent lymph vessels enter the lymph node at several places along the capsule-enter through cortex
Open into the subscapsular sinus
Afferent and efferent lymph vessels have valves- one way flow of lymph
cortical sinuses from subcapsular sinus, follow trabeculae to form medullary sinuses: open network of channels, converge toward the hilus, open into efferent lymph vessels- all lymph leaves node this way
Blood Vessels
Major arteries enter at hilus: branch and supply medullary cords and cortex (Deep cortex: postcapullary venules-lines by cuboidal endothelium- high endothelial venules: only lymphcytes can extravasate via HEV) or supply traveculae- supply connective tissue and capsule
Smaller vessels enter capsule at various sites
Veins leave at hilus
Primary and secondary lymphoid follicles
When stimulated by antigen, they change histologic morphology from primary to secondary lymphoid follicles. (this is reversible)
Follicle: site of antigen presentation and B cell differentiation
Microanatomy of the secondary follicle
Germinal center is subdivided into basilar dark zone and apical light zone
Marginal zone
Cellular movement in lymph node
Lymphocytes and DCs enter lymph nodes by different routes
Most lymphocytes migrating to lymph nodes enter from the peripheral blood
Most DCs enter lymph nodes through the afferent lymphatics
Hemal Nodes
Run alongside (not connected) the bloodstream
Seen predominately in ruminants- dark color
Rich content of erythrocytes within sinuses
No lymphatic supply
Near the spleen, the kidney and large blood vessels along the ventral side of the vertebrae
Functions are probably like those of the spleen
Hemolymph nodes
In pigs
Probably only a lymph node that contains RBCs in its sinuses as a result of hemorrhage in its tributary field
Spleen: Filters Blood
Filters blood (phagocytize them)
Senescent (old) RBCs and WBCs
Recovers and stores iron
Macrophages of red pulp commonly contain portions of RBCs and hemosiderin
Samples and removes antigens from blood
Mounts immune responses against blood-borne antigens- B and T cells
Hematopoiesis in fetus, sometimes in adult
Spleen: Stores RBCs and PLTs
Stores RBCs and PLTs
mainly in horses, dogs, and cattle
Smooth muscle is a prominent feature of capsule and trabeculae in these species
Splenic contraction increases systemic hematocrit
Cats do not have storage type spleen
Splenic Structure
Outer capsule is dense connective tissue with underlying smooth muscle
Capsule gives rise to: trabeculae-collagen, elastic fibers, smooth muscle cells
Each trabecula contains a central artery or vein
White and Red pulp
Splenic Blood flow
In at central artery Branches to form penicillary arteries End in sheathed capillaries (CT and macrophages)- no endothelial cells, end blindly (do not connect to anything) Blood enters space in parenchyma Then to sinus system Sinuses become larger and larger Splenic vein Hepatic portal vein Called an open circulation system since the parenchyma is not a true blood vessel
White Pulp
Central arterioles
Periarterial lymphoid sheaths -PALS (t)
Follicles (b)
Parenchyma:
B cells- lymphoid follicles, will have germinal centers in young animals
T cells- located around central arterioles ~sheath, periarteriolar lymphoid sheath (PALS)
Red pulp
Sinusoids 1. Parenchyma (functional tissue): macrophages of sheathed capillaries Other macrophages and blood cells that will eventually enter the venous sinuses 2. Venus sinuses (dog, rat, human) Compose the rest of the red pulp Long endothelial cells-contractile Basement membrane- discontinuous (blood cells easily enter from parenchyma to venous sinus)
Long endothelial cells delimit venous sinus (leaky and will ooze out to red pulp). Parenchyma composed of macrophages and blood cells
Mucosal Associated Lymphoid Tissue
Gut associated lymphoid tissue (GALT) Bronchial associated lymphoid tissue (BALT) Tonsil Ocular Urogenital Mammary
Secondary lymphatic tissue
Strategic location at sites of antigen entry
Antigen-presenting cells induce immune response
Single or aggregations of lymphoid nodules
Augment mechanical and chemical barriers of surface mucosal epithelium
Gut associated lymphoid tissue (GALT)
Includes: Solitary and aggregated lymphatic nodules Intraepithelial lymphocytes Subepithelial lymphocytes Plasma cells Macrophages
Aggregated lymphatic nodules
In GALT
Peyer’s patches
Small intestin-ileum
Ileal peyer’s patch may involute with age, may function differently to smaller patches
Smaller aggregated nodules in small-intestine, colon and rectum persist into adulthood
Lacteals
Blind-ended lymphatic capillaries within intestinal villi of the small intestine
Micelles from the chyme are absorbed, converted by the cell to chylomicrons- these enter the lacteals to become part of the chyle
Smooth muscles in the villi contract the villous up and down pumping the lacteal to force the chyle into the one-way lymphatic vessels of the submucosa
Lymphatic nodules and diffuse lymphatic tissues
lots of lymphatic tissue because it comes across stuff a lot
The mucosa and submucosa contain diffuse lymphatic tissue and large lymphatic nodules with germinal centers
Peyer’s Patches
Microscopically: aggregation of lymphatic nodules in the lamina propria and submucosa of the distal jejunum/ileum
B cells migrate there from bone marrow (as some level of B cell maturation occurs here, often considered as a primary lymphatic organ)
Same central light and peripheral dark zones as typical lymphoid nodules
Overlain by simple columnar (M-cells; microfold cells)
Bursa of fabricius in chicken considered its equivalent hence the suggestion of its role in B cell development
Anti-mesenteric side of ileum contain GALT
Peyer’s Patch domes are covered by a simple columnar layer of microfold cells
M cells
Close proximity to luminar surface
Specialized epithelial cells
Pinocytose GI contents and secrete content on the lymphocytes and macrophages of the peyer’s patch
Tonsils
Aggregated lymphatic nodules in pharynx -Adjacent to lumen of host organ- sample luminal content -No afferent lymphatic vessels Local production of antibodies occurs -important in immune response
squamous epithelium
collection of lymphatic nodules present in tunica mucosa of pharynx
May or may not have crypts
Efferent lymphatics drain tonsils (into lymphoids)
Lymphatics drain tissue fluid
Frank-starling
Inbalances between hydrostatic and osmotic pressure will result in 10% of fluid being unabsorbed back to blood circulation
the 10% ends up in interstitium and then lymph
lymph willhave antigens etc
Lymphatic vessels
Blind-ending tubes lined by endothelial cells
Absorb fluid from interstitium, pass it back into blood stream
Inflammatory cell and antigen movement from peripheral tissue to nearest local lymph node for recognition and response
Anchoring fibrils pull endothelial cells apart, fluid enters vessel
Lymphatics deliver lymph to lymph nodes- regional monitoring centers for immune response
Lymph flows in one direction from periphery to heart
surrounding tissues compress or expand vessel
one way valve prevents backflow
