10 - Lymphatic System Histology Flashcards
Major functions of the lymphatic system:
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
Components of Lymphatic System:
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
Lymphatic vessels – connect parts of
lymphatic system to blood circulatory system
Lymphatic vessels begin as networks of
blind-ended capillaries in microcapillary beds
Lymphatic capillaries form network within
loose connective tissues (especially numerous beneath epithelium of skin & mucous membranes)
Lymphatic vessels serve as adjuncts to the
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)
Smallest lymphatic vessels are called
lymphatic capillaries
Lymphatic capillaries: endothelial tubes lacking
continuous basal lamina results in high permeability
Allows for entry of large molecules, antigens, cells, etc.
Lymphatic vessels Collect
excess protein-rich extracellular fluid from extracellular spaces
Extracellular fluid referred to as lymph once it enters lymphatic vessel
Vessel wall thickness increases (connective tissue & bundles of smooth muscle) as
vessels become larger
There is no
central pump in lymphatic system
Lymph moves
sluggishly; relies primarily on skeletal muscle compression of vessels
Lymphatic vessels possess
valves prevent backflow of lymph; aid in unidirectional flow
Lymphatic capillaries converge to form
lymphatic vessels
Vessels unite to form two main channels:
Thoracic duct – largest lymphatic vessel; drains majority of body
Right lymphatic duct – drains right upper limb, right thorax, right neck, & right head
Right lymph valve Valve formed by
very small layer of connective tissue covered on both sides with endothelium
Left lymph vessel This layer of connective tissue present on the
outside of the endothelium blends with dense connective tissue (DCT) beyond the wall of the vessel
Lymphocytes –
definitive cells of lymphatic system; effector cells
3 major types of lymphocytes: B lymphocytes, T lymphocytes, & Natural Killer (NK) cells
Supporting cells –
interact with lymphocytes; antigen presentation & regulation of immune responses
B Lymphocytes: First recognized in
Bursa of Fabricius of birds (mass of lymphatic tissue associated with cloaca)
B Lymphocytes: Represent
20%-30% of circulating lymphocytes
B Lymphocytes: Membrane-bound
B-cell receptor (antigen-specific binding site) = Immunoglobulin
B Lymphocytes: Activated B cells (Plasma cells) secrete
non-membrane bound form of immunoglobulins = Antibodies
B Lymphocytes: Humoral immunity =
secretion of antibodies into the blood upon B cell activation
T Lymphocytes: Mature in the
thymus
T Lymphocytes: Represent
60%-80% of circulating lymphocytes
T Lymphocytes: Cell-mediated immunity – involves
cell-cell interactions and release of cytokines; directed against intracellular pathogens
T Lymphocytes: Subclassified based on
presence/absence of cell surface markers: CD4+ and CD8+
Natural Killer (NK) cells : Represent
5%-10% of circulating lymphocytes
Natural Killer (NK) cells : Potent killers of
certain types of target cells, namely virally infected and malignant cells
Natural Killer (NK) cells : Component of
innate immunity
Diffuse lymphatic tissue – accumulations of
lymphatic tissue found within alimentary canal, respiratory passages, and genitourinary tract
Accumulations NOT surrounded by a
connective tissue capsule
Lymphocytes, plasma cells, fibroblasts & eosinophils found in the
lamina propria at these sites
Lymphatic nodules (follicles) –
discrete concentrations of lymphocytes contained in meshwork of reticular cells
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
Primary nodule – a nodule consisting primarily of
small lymphocytes
Secondary nodule – contains
light staining germinal center in central region of nodule & an outer mantle zone (corona)
Germinal center – morphologic indication of
response to antigen; develops when lymphocyte binds antigen, returns to primary nodule, & proliferates
Germinal center
Light staining
(large immature lymphocytes with large amounts of dispersed euchromatin in nuclei)
Follicular dendritic cells (FDCs) also present; found interspersed between
Germinal center
B lymphocytes
Mantle zone (corona) –
darker outer ring of small lymphocytes that encircle the germinal center
However, aggregations of lymphatic nodules are found in specific locations:
Tonsils
Peyer’s patches
Vermiform appendix
Tonsils – form ring of lymphatic tissue at entrance of oropharynx & nasopharynx
(“Waldeyer’s ring”)
Lingual tonsils – at the base of the
tongue
Pharyngeal tonsils (adenoids) – located in
roof of pharynx
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)
Palatine tonsil: Stratified squamous epithelium forms the
surface of the tonsil and dips into the underlying connective tissue, forming tonsillar crypts
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.
Peyer’s patches – located within the
ileum of small intestine; consists of numerous aggregations of lymphatic nodules containing T & B lymphocytes
Vermiform appendix – arises from
cecum of large intestine
Lamina propria heavily infiltrated with lymphocytes and contains numerous lymphatic nodules
Mucosal membranes are a
major site of entry for pathogens
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)
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
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
Afferent Lymphatic Vessels – convey lymph
towards the node; enter at convex surface of capsule
Efferent Lymphatic Vessels – convey lymph
away from the node; leave at hilum
Lymph Node Stroma: Capsule –
dense connective tissue surrounding node
Lymph Node Stroma: Trabeculae –
dense connective tissue extending from capsule
Lymph Node Stroma:
3Reticular tissue –
reticular cells & reticular fibers that form fine supporting meshwork
Reticular cells : Synthesize and secrete
reticular fibers (type III collagen) & ground substance
Reticular cells - Elongated cytoplasmic processes wrap around bundles of
reticular fibers; isolates structural components from parenchyma of lymphatic tissues/organs
Reticular cells : play
supporting role
Reticular cells
Express
surface molecules & produce substances that attract T cells, B cells, and dendritic cells
Dendritic Cells: Bone marrow-derived
antigen presenting cells
Dendritic Cells: Monitor environment for
antigens; process & present antigen to T cells
Dendritic Cells: In lymph node, usually localized to
T cell-abundant areas
Macrophages
Act as both
phagocytes & antigen presenting cells
Follicular dendritic cells (FDCs) : Have multiple,
thin, hairlike branching cytoplasmic processes that interdigitate between B cells in germinal centers
Follicular dendritic cells (FDCs) : Antigen-antibody complexes adhere to
FDC cytoplasmic processes via Fc receptors
Follicular dendritic cells (FDCs) : Not technically
antigen presenting cells
(act similarly to macrophages, but lack
MHC II molecules)
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
Cortex - Superficial (nodular) cortex –
outer part of cortex; just deep to capsule
Primary nodules – consist chiefly of small lymphocytes
Secondary nodules – contain germinal centers
Deep cortex (paracortex) – between
superficial cortex and medulla
No nodules
Contains the majority of T cells within lymph node
Medulla – consists of
cords of lymphatic tissue (medullary cords) separated by lymphatic sinuses (medullary sinuses)
medulla - Network of
reticular cells and fibers traverses medullary cords and sinuses; serve as framework for the parenchyma
medulla Medullary cords contain:
lymphocytes (mostly B cells), macrophages, dendritic cells, plasma cells
Medullary sinuses converge near
hilum & drain into efferent lymphatic vessels
Thymus structure:
Capsule
Trabeculae
Cortex
Medulla
Thymus structure 2 -
Capsule Trabeculae Thymic Lobules Cortex Medulla
Flow of lymphocytes from
blood through lymph node:
High Endothelial Venules (HEVs)
Deep Cortex
Superficial Cortex
Medulla
Efferent Lymph Vessel
Flow of lymphocytes from
lymph through lymph node:
Afferent Lymph Vessel
Subcapsular (Cortical) Sinus
Trabecular Sinus
Medullary Sinus
Efferent Lymph Vessel
Flow of lymphocytes from
lymph through lymph node:
Afferent Lymph Vessel
Subcapsular (Cortical) Sinus
Trabecular Sinus
Medullary Sinus
Efferent Lymph Vessel
Splenic Sinuses:
Wall has lattice-like structure
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
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:
Thymus at puberty: T cell differentiation & proliferation
reduced; most lymphatic tissue replaced with adipose tissue
Thymus at puberty: However, thymus can be
restimulated under conditions that demand rapid T cell proliferation
Capsule – thin
connective tissue capsule
Connective tissue contains: collagen fibers, fibroblasts, variable numbers of plasma cells, granulocytes, lymphocytes, mast cells, adipocytes, & macrophages
Trabeculae – extend from
capsule into parenchyma of thymus
Establish domains called thymic lobules cortical caps over portions of highly convoluted but continuous medullary tissue
*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)
Cortex – outer portion of
thymic parenchyma
Thymus cortex: Markedly
basophilic (closely packed developing T lymphocytes with intensely staining nuclei)
Thymus cortex: Developing lymphocytes (thymocytes) occupy
spaces within extensive meshwork of epithelioreticular cells
Thymus cortex: Macrophages also
present; phagocytose apoptotic T cells (those that do not fulfill thymic education requirements)
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)
Epithelioreticular cells: Exhibit certain features characteristic of
epithelium (i.e., intercellular junctions & intermediate filaments)
Epithelioreticular cells
Serve as the supporting
stroma; thymocytes occupy spaces within extensive meshwork of epithelioreticular cells
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
Medulla –
inner portion of thymic parenchyma
Medulla thymus: Contains large number of
epithelioreticular cells & loosely packed T cells
medulla thymus; Stains less intensely than
cortex large lymphocytes with pale-staining nuclei & more cytoplasm than small lymphocytes
medulla thymus: type VI epithelioreticular cells form the most
characteristic feature of the medulla:
Thymic (Hassall’s) corpuscles
Thymic (Hassall’s) corpuscles : Isolated masses of
closely-packed, concentrically arranged type VI epithelioreticular cells with flattened nuclei
Thymic (Hassall’s) corpuscles : Possess
keratohyalin granules make corpuscles very eosinophilic
Thymic (Hassall’s) corpuscles: Center of corpuscle contains
fully keratinized cells
Thymic (Hassall’s) corpuscles
Distinguishing feature of
thymic tissue!
Thymic (hassall’s) corpuscles: Function not fully
understood
[Believed to produce interleukins (IL-4 & IL-7) involved in thymic differentiation & T cell education]
Spleen – largest lymphatic organ; located in
upper left quadrant of abdominal cavity
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
Spleen Enclosed in
dense connective tissue capsule from which trabeculae extend into the parenchyma
Spleen Hilum – located on
medial surface of spleen; site for passage of splenic artery and vein, nerves, and lymphatic vessels
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)
Red pulp Consists of
splenic sinuses separated by splenic cords (cords of Billroth)
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
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
Red pulp has red appearance in fresh state, as well as in histologic sections because it contains
large numbers of red blood cells.
Splenic macrophages phagocytose
damaged red blood cells; iron from these cells used for formation of new red blood cells.
Spleen contains:
white pulp
White Pulp – consists of lymphatic tissue, mostly lymphocytes
White pulp: Appear basophilic in
H&E sections (dense heterochromatin of lymphocyte nuclei)
White pulp: Branches of splenic artery course through
through capsule and trabeculae and enter white pulp
White pulp: Central artery branch of
splenic artery initially in central position within white pulp
White pulp: Periarterial lymphatic sheath (PALS) aggregates of
lymphocytes around central artery
White pulp: In cross-section, PALS appear
circular; develop lymphatic/splenic nodule (with germinal centers) in some locations
White pulp: Lymphatic nodules displace
central artery to an eccentric position
capsulated structures are
organs
