10 - Lymphatic System Histology Flashcards
(126 cards)
1
Q
Major functions of the lymphatic system:
A
Drain Excess Interstitial Fluid
-Return excess interstitial fluid to blood stream to help maintain normal blood volume & prevent edema
Provide an Alternative Transportation Route for Hormones, Nutrients, & Waste Products
Immune Response
-Produce, maintain, and distribute lymphocytes
2
Q
Components of Lymphatic System:
A
Lymphatic vessels – connect parts of lymphatic system to blood circulatory system Lymphocytes & variety of supporting cells Diffuse lymphatic tissue Lymphatic nodules Lymph nodes Spleen Bone marrow Thymus
3
Q
Lymphatic vessels – connect parts of
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lymphatic system to blood circulatory system
4
Q
Lymphatic vessels begin as networks of
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blind-ended capillaries in microcapillary beds
5
Q
Lymphatic capillaries form network within
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loose connective tissues (especially numerous beneath epithelium of skin & mucous membranes)
6
Q
Lymphatic vessels serve as adjuncts to the
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blood vessels; unlike blood vessels, which carry blood to and from the body tissues, lymphatic vessels convey fluid only from tissues (to ultimately be returned to blood vascular system)
7
Q
Smallest lymphatic vessels are called
A
lymphatic capillaries
8
Q
Lymphatic capillaries: endothelial tubes lacking
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continuous basal lamina results in high permeability
Allows for entry of large molecules, antigens, cells, etc.
9
Q
Lymphatic vessels Collect
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excess protein-rich extracellular fluid from extracellular spaces
Extracellular fluid referred to as lymph once it enters lymphatic vessel
10
Q
Vessel wall thickness increases (connective tissue & bundles of smooth muscle) as
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vessels become larger
11
Q
There is no
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central pump in lymphatic system
12
Q
Lymph moves
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sluggishly; relies primarily on skeletal muscle compression of vessels
13
Q
Lymphatic vessels possess
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valves prevent backflow of lymph; aid in unidirectional flow
14
Q
Lymphatic capillaries converge to form
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lymphatic vessels
15
Q
Vessels unite to form two main channels:
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Thoracic duct – largest lymphatic vessel; drains majority of body
Right lymphatic duct – drains right upper limb, right thorax, right neck, & right head
16
Q
Right lymph valve Valve formed by
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very small layer of connective tissue covered on both sides with endothelium
17
Q
Left lymph vessel This layer of connective tissue present on the
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outside of the endothelium blends with dense connective tissue (DCT) beyond the wall of the vessel
18
Q
Lymphocytes –
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definitive cells of lymphatic system; effector cells
3 major types of lymphocytes: B lymphocytes, T lymphocytes, & Natural Killer (NK) cells
19
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Supporting cells –
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interact with lymphocytes; antigen presentation & regulation of immune responses
20
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B Lymphocytes: First recognized in
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Bursa of Fabricius of birds (mass of lymphatic tissue associated with cloaca)
21
Q
B Lymphocytes: Represent
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20%-30% of circulating lymphocytes
22
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B Lymphocytes: Membrane-bound
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B-cell receptor (antigen-specific binding site) = Immunoglobulin
23
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B Lymphocytes: Activated B cells (Plasma cells) secrete
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non-membrane bound form of immunoglobulins = Antibodies
24
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B Lymphocytes: Humoral immunity =
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secretion of antibodies into the blood upon B cell activation
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T Lymphocytes: Mature in the
thymus
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T Lymphocytes: Represent
60%-80% of circulating lymphocytes
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T Lymphocytes: Cell-mediated immunity – involves
cell-cell interactions and release of cytokines; directed against intracellular pathogens
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T Lymphocytes: Subclassified based on
presence/absence of cell surface markers: CD4+ and CD8+
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Natural Killer (NK) cells : Represent
5%-10% of circulating lymphocytes
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Natural Killer (NK) cells : Potent killers of
certain types of target cells, namely virally infected and malignant cells
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Natural Killer (NK) cells : Component of
innate immunity
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Diffuse lymphatic tissue – accumulations of
lymphatic tissue found within alimentary canal, respiratory passages, and genitourinary tract
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Accumulations NOT surrounded by a
connective tissue capsule
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Lymphocytes, plasma cells, fibroblasts & eosinophils found in the
lamina propria at these sites
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Lymphatic nodules (follicles) –
discrete concentrations of lymphocytes contained in meshwork of reticular cells
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Localized concentrations of lymphocytes commonly found in
walls of alimentary canal, respiratory passages, and genitourinary tract
Sharply defined, but NOT surrounded by connective tissue capsule
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Primary nodule – a nodule consisting primarily of
small lymphocytes
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Secondary nodule – contains
light staining germinal center in central region of nodule & an outer mantle zone (corona)
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Germinal center – morphologic indication of
response to antigen; develops when lymphocyte binds antigen, returns to primary nodule, & proliferates
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Germinal center
Light staining
(large immature lymphocytes with large amounts of dispersed euchromatin in nuclei)
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Follicular dendritic cells (FDCs) also present; found interspersed between
Germinal center
B lymphocytes
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Mantle zone (corona) –
darker outer ring of small lymphocytes that encircle the germinal center
43
However, aggregations of lymphatic nodules are found in specific locations:
Tonsils
Peyer’s patches
Vermiform appendix
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Tonsils – form ring of lymphatic tissue at entrance of oropharynx & nasopharynx
(“Waldeyer’s ring”)
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Lingual tonsils – at the base of the
tongue
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Pharyngeal tonsils (adenoids) – located in
roof of pharynx
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Palatine tonsils – between
palatoglossal & palatopharyngeal arches
Squamous epithelium of tonsillar surface dips into underlying connective tissue forms tonsillar crypts (walls of crypts contain numerous lymphatic nodules)
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Palatine tonsil: Stratified squamous epithelium forms the
surface of the tonsil and dips into the underlying connective tissue, forming tonsillar crypts
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Palatine tonsil:
| Stratified squamous epithelium (SSE) lines
tonsillar crypt. The stratified squamous epithelium is separated by a connective tissue layer (CT) from the lymphatic nodule (LN). In the upper portion of the of the image, the stratified squamous epithelium is just barely recognized because of the heavy infiltration of lymphocytes.
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Peyer’s patches – located within the
ileum of small intestine; consists of numerous aggregations of lymphatic nodules containing T & B lymphocytes
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Vermiform appendix – arises from
cecum of large intestine
| Lamina propria heavily infiltrated with lymphocytes and contains numerous lymphatic nodules
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Mucosal membranes are a
major site of entry for pathogens
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Mucosa-Associated Lymphatic Tissue (MALT) –
accumulations of lymphocytes (i.e., diffuse lymphatic tissue & lymphatic nodules) associated with different mucosal areas (i.e. wall of alimentary canal, respiratory, urinary, & reproductive tracts)
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MALT can be given specific names according to region/organ where they appear:
GALT (Gut-Associated Lymphatic Tissue) – associated with the alimentary canal
BALT (Bronchus-Associated Lymphatic Tissue) – associated with respiratory system
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Lymph nodes – small,
bean-shaped encapsulated lymphatic organs
Variable in size (1mm to 1–2 cm long)
Interposed along pathway of lymphatic vessels
Filter lymph prior to its return to blood circulation
Dispersed throughout body, but concentrated in certain regions such as the axilla, groin, & mesenteries
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Afferent Lymphatic Vessels – convey lymph
towards the node; enter at convex surface of capsule
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Efferent Lymphatic Vessels – convey lymph
away from the node; leave at hilum
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Lymph Node Stroma: Capsule –
dense connective tissue surrounding node
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Lymph Node Stroma: Trabeculae –
dense connective tissue extending from capsule
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Lymph Node Stroma:
| 3Reticular tissue –
reticular cells & reticular fibers that form fine supporting meshwork
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Reticular cells : Synthesize and secrete
reticular fibers (type III collagen) & ground substance
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Reticular cells - Elongated cytoplasmic processes wrap around bundles of
reticular fibers; isolates structural components from parenchyma of lymphatic tissues/organs
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Reticular cells : play
supporting role
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Reticular cells
| Express
surface molecules & produce substances that attract T cells, B cells, and dendritic cells
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Dendritic Cells: Bone marrow-derived
antigen presenting cells
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Dendritic Cells: Monitor environment for
antigens; process & present antigen to T cells
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Dendritic Cells: In lymph node, usually localized to
T cell-abundant areas
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Macrophages
| Act as both
phagocytes & antigen presenting cells
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Follicular dendritic cells (FDCs) : Have multiple,
thin, hairlike branching cytoplasmic processes that interdigitate between B cells in germinal centers
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Follicular dendritic cells (FDCs) : Antigen-antibody complexes adhere to
FDC cytoplasmic processes via Fc receptors
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Follicular dendritic cells (FDCs) : Not technically
antigen presenting cells
(act similarly to macrophages, but lack
MHC II molecules)
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Parenchyma of lymph node divided
into a cortex and medulla
Cortex – forms outer portion of lymph node (except at hilum)
Medulla – inner portion of lymph node
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Cortex - Superficial (nodular) cortex –
outer part of cortex; just deep to capsule
Primary nodules – consist chiefly of small lymphocytes
Secondary nodules – contain germinal centers
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Deep cortex (paracortex) – between
superficial cortex and medulla
No nodules
Contains the majority of T cells within lymph node
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Medulla – consists of
cords of lymphatic tissue (medullary cords) separated by lymphatic sinuses (medullary sinuses)
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medulla - Network of
reticular cells and fibers traverses medullary cords and sinuses; serve as framework for the parenchyma
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medulla Medullary cords contain:
lymphocytes (mostly B cells), macrophages, dendritic cells, plasma cells
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Medullary sinuses converge near
hilum & drain into efferent lymphatic vessels
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Thymus structure:
Capsule
Trabeculae
Cortex
Medulla
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Thymus structure 2 -
```
Capsule
Trabeculae
Thymic Lobules
Cortex
Medulla
```
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Flow of lymphocytes from
| blood through lymph node:
High Endothelial Venules (HEVs)
Deep Cortex
Superficial Cortex
Medulla
Efferent Lymph Vessel
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Flow of lymphocytes from
| lymph through lymph node:
Afferent Lymph Vessel
Subcapsular (Cortical) Sinus
Trabecular Sinus
Medullary Sinus
Efferent Lymph Vessel
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Flow of lymphocytes from
| lymph through lymph node:
Afferent Lymph Vessel
Subcapsular (Cortical) Sinus
Trabecular Sinus
Medullary Sinus
Efferent Lymph Vessel
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Splenic Sinuses:
Wall has lattice-like structure
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Splenic sinuses: 3Prominent intercellular spaces between endothelial cells
Allow blood cells easy access into/out of sinuses
| Macrophage processes extend between endothelial cells into lumen of sinuses to monitor blood for antigens
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Thymus – bilobed organ located in the
superior mediastinum, anterior to heart and great vessels
Fully formed & functional at birth
Persists as large organ until puberty, at which point:
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Thymus at puberty: T cell differentiation & proliferation
reduced; most lymphatic tissue replaced with adipose tissue
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Thymus at puberty: However, thymus can be
restimulated under conditions that demand rapid T cell proliferation
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Capsule – thin
connective tissue capsule
Connective tissue contains: collagen fibers, fibroblasts, variable numbers of plasma cells, granulocytes, lymphocytes, mast cells, adipocytes, & macrophages
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Trabeculae – extend from
capsule into parenchyma of thymus
Establish domains called thymic lobules cortical caps over portions of highly convoluted but continuous medullary tissue
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*Note: In some planes of section, the “lobular” arrangement of the cortical cap and medullary tissue resembles a
lymphatic nodule with a germinal center (often confusing)
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Cortex – outer portion of
thymic parenchyma
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Thymus cortex: Markedly
basophilic (closely packed developing T lymphocytes with intensely staining nuclei)
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Thymus cortex: Developing lymphocytes (thymocytes) occupy
spaces within extensive meshwork of epithelioreticular cells
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Thymus cortex: Macrophages also
present; phagocytose apoptotic T cells (those that do not fulfill thymic education requirements)
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Epithelioreticular cells: Provide framework for
developing T cells (therefore serve role played by reticular cells & reticular fibers found in other lymphatic organs. HOWEVER, reticular cells and their fibers are NOT present in thymic parenchyma)
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Epithelioreticular cells: Exhibit certain features characteristic of
epithelium (i.e., intercellular junctions & intermediate filaments)
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Epithelioreticular cells
| Serve as the supporting
stroma; thymocytes occupy spaces within extensive meshwork of epithelioreticular cells
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Epithelioreticular cells
| 6 types recognized on the basis of function
3 Types in Cortex:
Type I
Type II
Type III
3 Types in Medulla:
Type IV
Type V
Type VI
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Medulla –
inner portion of thymic parenchyma
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Medulla thymus: Contains large number of
epithelioreticular cells & loosely packed T cells
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medulla thymus; Stains less intensely than
cortex large lymphocytes with pale-staining nuclei & more cytoplasm than small lymphocytes
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medulla thymus: type VI epithelioreticular cells form the most
characteristic feature of the medulla:
| Thymic (Hassall’s) corpuscles
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Thymic (Hassall’s) corpuscles : Isolated masses of
closely-packed, concentrically arranged type VI epithelioreticular cells with flattened nuclei
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Thymic (Hassall’s) corpuscles : Possess
keratohyalin granules make corpuscles very eosinophilic
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Thymic (Hassall’s) corpuscles: Center of corpuscle contains
fully keratinized cells
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Thymic (Hassall’s) corpuscles
| Distinguishing feature of
thymic tissue!
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Thymic (hassall's) corpuscles: Function not fully
understood
| [Believed to produce interleukins (IL-4 & IL-7) involved in thymic differentiation & T cell education]
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Spleen – largest lymphatic organ; located in
upper left quadrant of abdominal cavity
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Spleen Has
rich blood supply filters blood (analogous to lymph node filtering lymph)
Abundant lymphocytes, macrophages, and dendritic cells
Specialized vascular spaces/channels
Meshwork of reticular cells and reticular fibers
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Spleen Enclosed in
dense connective tissue capsule from which trabeculae extend into the parenchyma
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Spleen Hilum – located on
medial surface of spleen; site for passage of splenic artery and vein, nerves, and lymphatic vessels
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Spleen contains:
| red pulp
Red Pulp - primary role of red pulp is blood filtration (i.e., removal of particulate materials, removal of antigens, and removal of worn-out or defective RBCs, WBCs, and platelets)
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Red pulp Consists of
splenic sinuses separated by splenic cords (cords of Billroth)
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Splenic sinuses –
blood-filled sinuses; few contact points between adjacent endothelial cells prominent intercellular spaces; allow blood cells to pass readily into/out of sinuses
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Splenic cords (cords of Billroth) –
loose meshwork of reticular cells and reticular fibers; contain large numbers of erythrocytes, macrophages, lymphocytes, dendritic cells, plasma cells, and granulocytes
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Red pulp has red appearance in fresh state, as well as in histologic sections because it contains
large numbers of red blood cells.
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Splenic macrophages phagocytose
damaged red blood cells; iron from these cells used for formation of new red blood cells.
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Spleen contains:
| white pulp
White Pulp – consists of lymphatic tissue, mostly lymphocytes
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White pulp: Appear basophilic in
H&E sections (dense heterochromatin of lymphocyte nuclei)
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White pulp: Branches of splenic artery course through
through capsule and trabeculae and enter white pulp
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White pulp: Central artery branch of
splenic artery initially in central position within white pulp
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White pulp: Periarterial lymphatic sheath (PALS) aggregates of
lymphocytes around central artery
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White pulp: In cross-section, PALS appear
circular; develop lymphatic/splenic nodule (with germinal centers) in some locations
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White pulp: Lymphatic nodules displace
central artery to an eccentric position
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capsulated structures are
organs
