Lymphatics Flashcards
Peripheral lymphatic tissue
Where lymphocytes respond to antigens
Lymphoid tissue characteristics
Populated with lymphocytes
Small lymphocyte types
B and T lymphocytes, null cells
Null cells
Lack surface characteristics of T or B cells. 5-10% of the peripheral blood cells
Large lymphocytes
About 3x the volume of small lymphocytes
Lymphoid tissue function
- Lymphocyte production
2. Immune resonse
Role of small lymphocytes in immune response
- Recirculate continuously between blood and lymph.
2. Serve as immunocompotent cells capable of responding to antigen
B-cells
Differentiate into plasma cells or are retained in tissue as memory cells
Plasma cells
Produce humoral antibodies
T-lymphocytes
Involved in cell-mediated immune response or are retained in tissue as memory cells.
When do T-cells usually respond to an antigen
When it presented on surface of an accessory cell
What is characteristic of secondary immune response?
Immediate and extensive
Components of lymphoid tissue
Stroma (includes cells and reticular fibers) and lymphoid cells.
Cells in the stroma
Reticular cells, macrophages, dendritic cells, follicular dendritic cells
Reticular cells
Fibroblast-like, produce reticular fibers (Type III collagen)
Macrophages (APCs)
- Phagocytosis of foreign particle matter.
2. Play a role in processing and presenting antigens
Dendritic cells
Very efficient APCs
Follicular dendritic cells
- Bind antibody-antigen complexes on surface of processes.
2. Don’t endocytose and process antigen, thus these cells aren’t APCs
Reticular fibers
- Produced by reticular (fibroblast) cells
2. Form a lot of supporting framework of lymphoid tissue
Lymphoid cells
- small, med, large lymphocytes and plasma cells.
2. Enlargement and proliferation on exposure to antigen
How is non-encapsulated lymphatic tissue classified?
Based on gross histological arrangement
Diffuse lymphatic tissue
- In GI, respiratory tracts and other places where lymphatic tissue accumulates.
- Loose
- Dense
Nodular lymphatic tissue
- Scattered throughout digestive, respiratory, urinary systems in loose connective tissue beneath wet epithelial membranes
- Represent local immune responses to antigens
- Characterized by solitary lymphatic nodules (follicles)
Primary lymphatic nodule
Dark staining, spherically organized ball of small lymphocytes
Secondary lymphatic nodule sites have what?
Reaction center
General features of secondary lymphatic nodules?
- Don’t appear until after birth.
- Disappear in absence of antigen and reappear with re-exposure to antigen
- Need thymus for development
- Decline in # w/age
What are the cells of the germinal center or a secondary nodule?
- Large and med sized lymphocytes
- Small lymphocytes.
- Macrophages
- Follicular dendritic cells
- Developing plasma cells
Large and med lymphocytes (of germinal center)
- Mitotic
2. Also known as lymphoblasts (Activated lymphocytes)
Small lymphocytes (of germinal center)
- More numerous
- Don’t see dividing very often
- Process antigens for presentation to lymphocytes
Macrophages (of germinal center)
- Morphological features variable.
- Often contain remains of digested lymphocytes
- Process antigens for presentation to lymphocytes
Where are developing plasma cells found in the germinal center?
Near the periphery
Follicular dendritic cells (of germinal center)
- Lightly eosinophilic cytoplasm and large, euchromatic nucleus.
- Bind antigen-Ab complexes to surface via Fc receptors
Where are lymphatic nodules found?
Lymph nodes and spleen. If in thymus, suggests pathological condition!
Examples of nodular non-encapsulated lymphatic tissue
Tonsil, Peyer’s patches, appendix
Functions of non-encapsulated lymphatic tissue
- traps antigen
- produces lymphocytes in response to antigens (antigens stimulate lymphocyte proliferation, differentiation of B cells occurs)
- destroys antigen
Appendix
groups of lymphatic nodules in submucosa w/ many lymphocytes extending into the mucosa
General tonsil features
No afferent lymphatics and no lymphatic sinuses
Lingual tonsil
- In root or posterior 1/3 of tongue and epiglottis
- Consists primarily of dense lymphatic tissue and some nodules.
- Tonsillar tissue covered by stratified squamous mucosal epithelium
Pharyngeal and tubal tonsils
- Found in nasal pharynx
- Lymphatic tissue embedded in connective tissue and extending through epithelium.
- Tonsillar tissue covered by pseudostratified/columnar ciliary epithelium w/patches of stratifed squamous mucosal epithelium.
Peyer’s patches
- groups of lympatic nodules
2. mostly in wall of ileum of small intestine
Lymph node general points
- Only lymphatic organ in lymphatic vessels, contains lymphatic sinuses, that filters lymph
How is lymph filtered?
By macrophages before returning it to the blood stream
Lymph node components
- Connective tissue (capsule, trabeculae, stroma)
- Fibers
- Endothelial cells lining sinusoids
Cells in lymph nodes
Reticular cells, dendritic cells, macrophaqes, follicular dendritic cells
Areas of lymph node
- Cortex
2. medulla
Lymph node cortex
- Tissue supported by reticular fiber meshwork
- Germinal centers contain many lymphocytes and lymphatic nodules (transitory and have a tail that extends into medulla as the medullary cords)
- Stromal cells
Medulla
- Also has reticular fiber meshwork and stromal cells.
- Contains medullary cords
- Many small lymphocytes, macrophage, plasma cells (differentiating and mature)
Superficial cortex
Located between the capsule and outer limits of the germinal center. Contains majority of B-lymphocytes
Deep cortex
Area between germinal centers and medullary cords. Contain a majority of T-lymphocytes (that extend into area between nodules of superifcial and mid-cortex and also into the medulla)
Sinuses
- Lined by endothelial cells with large intercellular gaps.
- Macrophages, lymphocytes, reticular cells
What happens to lymphocytes after they enter lymphatic sinuses
Leave lymph nodes through efferent lymphatics and become part of the re-ciculating lymphocytes
What happens to macrophages after they enter lymphatic sinuses?
Enter lymph fluid and blood, but then become trapped in the lung where they are disposed of by entering lung parenchyma.
Superficial cortex
Located between the capsule and outer limits of the germinal center. Contains majority of B-lymphocytes. Lined by simple squamous epithelium
Mid-cortex
Area containing mainly germinal centers. Contain majority of B lymphocytes with other cells of the germinal center. Lined by simple squamous epithelium
Lymph flow through lymph node
Afferent lymphatic vessels–> subcapsular (marginal) sinus–> trabecular (peritrabecular) sinus–> paracortical (subcortical) sinus–> medullary sinus–> efferent lymphatic vessel
Blood flow through lymph node
Arterial vessels (hilum)–> trabecular vessels–> arterioles and capillaries–> post-capillary venules (outer cortex and deep cortex)–> venous vessels (hilum)
Thymectomy at birth leads to?
Depletion of lymphocytes from deep and mid-cortex. Change in venule endothelium from cuboidal to squamous
Source of lymphocytes in lymph nodes
- Mitotic division in lymphatic nodules
2. Circulating lymphocytes enter from post-capillary venules
Path of recirculating T lymphocytes
Afferent arterioles–>pre-capillary arterioles–> capillaries–> post-capillary venules–> lymphatic tissue–> lymphatic sinuses–> efferent lymphatic vessels
Lymph node function
- Lymph filter.
- Produce lymphocytes.
- Produce antibodies (produced by plasma cells)
Thymus general points
- No afferent lymphatic vessels
- Lymphatic vessels limited to capsule and connective tissue
- Site for differentiation of T lymphocytes
- Size largest in relation to body in utero and for first 2 yrs post-natal life. Continues to grow until puberty. Regresses in size into old age
Thymus development
- Epithelium of 3rd branchial pouch.
2. Reticular stroma composed almost entirely of reticular cells and very few reticular fibers
Thymus morphological features
2 lobes beneath sternum, each enclosed by connective tissue capsule. Lobes divides into incomplete lobules by connective tissue trabeculae or septae
Thymus cortex cells
Reticular cells, macrophages, lymphocytes
Reticular cells in the thymus
- Derived from endoderm.
- Loose cellular network that supports developing T-lymphocytes
- Large nucleus, prominent nucleolus, voluminous eosinophilic cytoplasm.
- Produce thymic hormones (promote differentiation of stem cells into T-cells, induce formation of T-cell surface markers
- Help form blood-thymus barrier, prevents antigens from entering the thymic cortex
3 regions of lymphocytes in the thymus
- Outer region
- Middle
- Inner
Outer region (lymphocyte)
- Stem cells enter from vascular system, make lymphoblasts, undergo mitosis
- largest lymphocytes in cortex
- maturing cell move toward medulla
Middle region (lymphocyte)
- Differentiating cells derived from outer region
2. Cell proliferation continues
Inner region (lymphocyte)
- Smallest lymphocytes of cortex.
- No cell division
- Mature T- cells enter blood vessels at corticomedullary junction where they go to thymic dependent zones
Medulla (of thymus)
- Primarily epithelial reticular cells with ome lymphocytes, other connective tissue, fibers
- Hassall’s (thymic) corpuscles are the characteristic feature!
Vascular supply (of thymus)
- Major branches from internal thoracic and inferior thyroid arteries.
- Cortex has small arteries and arterioles at the corticomedullary junction.
- Medulla has small arteries and arterioles that give rise to capillaries. Post-capillary venules drain into veins that drain major efferent vessels of the thymus
Blood-thymus barrier
Prevent antigens present in the blood stream from entering the corex
Components of the blood thymus barrier
- Endothelial cells with occluding junctions
- External lamina
- Perivascular space
- External lamina
- Epithelial reticular cells held together by desmosomes
How do lymphocytes enter and leave the circulatory system?
Post-capillary venules (at coticomedullary junction)
Congenital absence of the thymus
Patient lacks T-lymphocytes (and therefore cellular immunity)
Thymomas
Most common tumor found in anterior mediastinm
Neoplasms of thymus reticular cells
Spleen general points
Largest lymphatic organ, no afferent lymphatic vessels, efferent lymphatic vessels present, no lymph sinuses
2 types of splenic pulp
White pulp and red pulp
White pulp
Made of: periarterial lymphatic sheaths, splenic nodules
Periarterial lymphatic sheaths
- Surround white pulp artery or central artery
2. Contain primary T-cells
Splenic nodules
- Scattered throughout splenic pulp
2. Contain primarily B-cells except in area assocated with white pulp or central artery
Red pulp
Made of splenic sinuses, splenic cords
Splenic sinuses (sinusoids)
Vascular passageways, lined by specialized endothelial cells
Splenic cords (billroth cords)
Between sinuses, contain RBCs, granulocytes, lymphocytes, macrophages, platelets, plasma cells, reticular cells adn fibers
Marginal zone
Transitional zone between red and white pulp where small vessels empty blood.
Recirculating lymphocytes return to enter periarterial lymphoid sheath or splenic nodule
Splenic circulation
- Splenic artery
- Trabecular artery
- White pulp or central artery
- Red pulp artery
- Sheathed artery (capillaries)
6 Terminal arterial capillaries
Penicillus
Red pulp artery and sheathed artery
Whic is more prevalent ? Open or closed circulation?
Open
Venus sinusoids or red pulp
Closed or open circulation
Open circulation
Terminal capillaries open into red pulp
Closed circulation
Terminal capillaries open into sinusoids
Sinusoids
Made of elongated and narrow endothelial cells
Endothelial cells supported by anastomosing ring of basement membrane and reticular fibers that encircle the sinusoid
Spleen function
- Ab in response to blood-borne antigens (main source of circulating Ab in the body)
- Removal and destruction of defective cells and debris (through macrophages)
- Concentrates and stores blood cells and platelets
- In prenatal life, form RBCS, granulocytes, lymphocytes, platelets
