Lymphatic System Histology Flashcards
Central lymphatic tissues
Bone marrow: source of B-cells
Thymus: source of T-cells
Peripheral lymph tissue
Mucosa-associated lymphatic tissue or nodular non-encapsulated lymphatic tissue
Lymph nodes
Spleen
Components of Lymphoid tissues
Stroma (cells - fibroblasts, APCs, DCs, FDCs, reticular fibers), lymphoid cells
Non-encapsulated Lymphatic tissues
Diffuse (GI, respiratory, GU, etc.) - dense or loose
Nodular (GI, respiratory, GU, etc)
Lymphatic nodules role vs lymph nodes
localized production of lymphocytes
smaller, no capsule, not a filter (no lymph vessel)
Primary lymphatic nodule
dark staining spherical balls of lymphocytes
secondary lymphatic nodule
contain a reaction (germinal) center
Cells of germinal center (secondary lymphatic nodule)
Large & medium lymphocytes (mostly Bs - mitotic)
Small lymphocytes (B and T - not mitotic)
Macrophages
FDCs
Plasma cells
Follicular Dendritic Cell
Special cell used to select lymphocytes involved in the generation of antibody and memory.
These are not true DC because they do not present antigen – just morphologically look like DC.
Occurrence of Lymphatic Nodules
Solitary, lymph nodes, spleen
NOT THE THYMUS
Examples of non-encapsulated nodular lymphatic tissue
Tonsil (palatine, lingual, pharyngeal and tubal)
Peyer’s patches
Appendix
Functions of non-encapsulated lymphatic tissues
trapping of antigen
Lymphocyte production in response to antigen (B cell proliferation)
Destruction of antigen
Selection of memory lymphocytes
Encapsulated lymphatic tissues
lymph node, spleen, thymus
Only lymphatic organ located in course of lymphatic vessels
lymph node
Only lymphatic organ that has lymphatic sinuses
Lymph node
Only lymphatic organ that filters lymph
Done by macrophages
Lymph node
Morphological features
Capsule, trabeculae, stroma (cells, fibers), endothelial cells
Divisions of lymph node
cortex and medulla
cortex of lymph node
Lymphoid tissue supported by reticular fibers & stromal cells
Lymphocytes
Lymphatic nodules
Germinal centers
Transient
Have tails that extend into medulla as medullary cords
Stromal cells
Lymphatic sinus flow
Afferent Lymphatic vessel to Subcapsular sinus to Trabecular sinus to Paracortical sinus to Medullary sinus to Efferent Lymphatic vessel
Medulla of lymph node
Lymphoid tissue supported by reticular fibers & stromal cells
Consists of medullary cords & medullary sinuses
Cells: small lymphocytes; differentiating and mature plasma cells; macrophages
Cortical subdivisions of lymph node
Superficial cortex
Mid-cortex
Deep cortex (paracortex)
Superficial cortex lymph node
Between capsule & outer limits of germinal centers
Majority of cells are B-lymphocytes
Midcortex of lymph nodes
Area containing the germinal centers
Primarily B-cells
Deep cortex of lymph nodes
Between germinal centers & medullary cords
Majority of cells are T-lymphocytes
Medullary cords are
mainly b and plasma cells
Blood flow LN
Arterial vessels (at hilus)
Trabecular vessels
Arterioles & capillaries
Post-capillary venules
Outer (superficial & mid): simple squamous epithelium
Deep (paracortex): simple cuboidal endothelium (HEV’s)
Venous vessels (at hilus)
HEV
Site of passage of lymphocytes (T- & B-lymphocytes) from blood vessels into lymphatic tissue
These lymphocytes then re-circulate via the efferent lymphatics back into the blood vascular system
Sources of lymphocytes to lymph nodes
Mitotic division of cells in nodules
Circulating lymphocytes (Pathway: afferent arterioles; pre-capillary arterioles; capillaries; post-capillary venules (HEV’s); lymphatic tissue; lymphatic sinuses; efferent lymphatic vessels)
Afferent lymphatic vessels
Lymph node functions
Lymph filter (macrophages) Lymphocyte production Antibody production (plasma cells)
Thymus
Site of differentiation of T lymphocyts
Thymus size
Size (proportionally) largest during fetal & first 2 years of postnatal life
Increases in size up to puberty
Regresses in size as we subsequently age
Thymus development
endoderm of third branchial pouch
–> reticualr stroma of thymus and hassals corpuscles
Reticular stroma of thymus composed almost entirely of
epithelial reticular cells (no reticular fibers)
Morphological features of thymus
Two lobes
Enclosed by connective tissue capsule
Beneath sternum on upper anterior thoracic wall
Divided into incomplete lobules by c.t. trabeculae or septae
Cortex and medulla
Cells of cortex of thymus
epithelial reticular cells (endoderm, produce thymic hormones, blood-thymus barrier, support)
Macrophages Lymphocytes (outer = stem cells, middle, proliferation and differentiation, inner - smallest, no mitoses, mature at corticomedullary junciton)
Thymic hormones
Promote differentiation of stem cells into T-cells
Induce formation of T-cell surface markers
thymus medulla
Epithelial reticular cells
Some lymphocytes
Some connective tissue cells and fibers
Hassall’s (thymic) corpuscles
Thymus vascular supply
Major branches: internal thoracic and inferior thyroid arteries
Cortex Small arteries & arterioles at corticomedullary (c-m) junction Cortical anastomsing capillary loops Post-capillary venules at c-m junction Venous drainage away from thymus
Medulla Small arteries & arterioles Capillaries Post-capillary venules Venous drainage away from thymus
Blood-thymus barrier
Prevents antigens in blood stream from entering the thymic cortex
Endothelial cells, basal lamina, perivascular space, basal lamina, epithelial reticular cells with desmosomes
Spleen gross features
Hilus
Thick capsule with some smooth muscle
Thick connective tissue trabeculae
Splenic pulp (white & red)
NO CORTEX OR MEDULLA
Splenic pulp consists of
blood cells embedded in, and supported by, a reticular fiber stromal network
two types of splenic pulp
red, white
white splenic pulp
2 compartments:
Periarterial lymphatic sheaths (PALs)
- Surround white pulp artery (central artery)
- Contain primarily T-cells
Splenic (lymphatic) nodules
- Scattered throughout the splenic pulp
- Contain primarily B-cells (except in area associated with white pulp or central artery)
PALs
- Surround white pulp artery (central artery)
- Contain primarily T-cells
Splenic lymphatic nodues
Scattered throughout the splenic pulp
Contain primarily B-cells (except in area associated with white pulp or central artery)
Red splenic pulp
Splenic sinuses (sinusoids)
- Vascular passageways
- Lined by specialized endothelial cells
Splenic cords (Billroth cords)
- Located between sinuses
- Contain: red blood cells; granulocytes; lymphocytes; macrophages; platelets; plasma cells; reticular cells & fibers
Marginal zone (spleen)
The transitional zone between the red & white pulp
Small blood vessels dump their blood into this area
Recirculating lymphocytes return to enter either the periarterial lymphoid sheaths or the splenic nodules
what kinds of cells in PALS vs splenic nodules
T cells in PALs
B cells in splenic nodules
Splenic circulation
splenic artery – trabecular artery – white pulp or central artery – penicillus OR follicular artery – venous sinusoids or red pulp – pulp veins – trabecular veins – splenic veins
closed vs open circulation (spleen)
Closed circulation
Terminal capillaries open into sinusoids
Open circulation
Terminal capillaries open into red pulp
Humans use open
spleen sinusoids
Consist of elongated and narrow endothelial cells
Endothelial cells supported by an anastomosing ring of basement membrane and reticular fibers that encircle the sinusoid like the hoops of a barrel
Splenic immune functions
Antibody presentation by APCs
Mostly dendritic cells and macrophages
Activation and proliferation of B and T lymphocytes
Production of antibodies against antigen present in circulating blood
Removal of macromolecular antigens from the blood
Splenic hematopoietic functions
Removal and destruction of senescent, damaged, and abnormal erythrocytes and platelets
Retrieval of iron from erythrocyte hemoglobin
Formation of erythrocytes & granulocytes during early fetal life
Storage of blood, especially red blood cells and platelets
