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Microscopic Anatomy > Unit III- Lymphoid Organs > Flashcards

Unit III- Lymphoid Organs Flashcards

1
Q

Immune system

A
  • protects against most pathogens
  • composed of two parts- innate and adaptive
  • lymphocytes and antigen presenting cells interact with antigens
  • macrophages and dendritic cells are antigen presenting cell
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2
Q

Organs involved with immune system

A

Central (primary) lymphoid organs:
-bone marrow, thymus, production of lymphocytes

Peripheral (secondary) lymphoid organs:
-lymph nodes, spleen, tonsils, Peyer’s patches

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3
Q

Extralymphoid tissue in non-lymphoid organs

A

aggregates of lymphoid tissue in non-lymphoid organs:

  • digestive system (GALT)
  • respiratory tract
  • urinary tract
  • reproductive tract
  • skin (SALT)

lymphocytes of blood and lymph

wandering lymphocytes

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4
Q

Recirculation of lymphocytes

A
  • from blood to lymphoid organs and back to blood
  • facilitates immune surveillance
  • ensures rapid response to antigens
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5
Q

B lymphocytes

A
  • differentiate into plasma cells
  • humeral immunity
  • production of antibodies(immunoglobulins) - soluble molecules, act in extracellular spaces of the body
  • account for 10-15% of circulating lymphocytes
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6
Q

T lymphocytes

A
  • account for about 70-80% of circulating lymphocytes
  • cytotoxic lymphocytes (effector cells)
  • cell-mediated immunological responses- against foreign cells, against bacterial or viral infected cells, against malignant cells
  • mechanism of killing- production of perforin and granzymes, induction of apoptosis (FAS- FAS ligand)
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7
Q

Reticular tissue

A
  • lymphoid tissue
  • also occurs in hematopoietic organs (lymphoid and myeloid)
  • framework of reticular tissue
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8
Q

Reticular fibers

A
  • can not be observed in H and E preps
  • PAS +
  • reduce silver salts
  • composed of Type III collagen
  • reticular cell
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9
Q

Retricular cells

A
  • most originate from mesenchyme
  • pale staining cytoplasm with numerous processes
  • diffuse nucleus (with prominent nucleolus)
  • synthesize and maintain reticular fibers and ground substance
  • macrophage
  • trophic role in blood cell maturation
  • storage place of ferritin
  • antigen presentation (dendritic cells)- roam body and spot and ingest bacteria and viruses, process antigens and travel to nearest lymph, present antigen to naive T lymphocytes, activated T cells travel to where needed
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10
Q

Types of reticular tissue in hematopoietic organs

A
  • lymphoid tissue- free cells are largely lymphocytes

- myeloid tissue- free cells are developing erythrocytes and granular leukocytes (bone marrow)

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11
Q

Loose and dense lymphoid tissue

A
  • Loose lymphoid tissue:
  • open meshwork of cells and fibers
  • fixed cells (reticular cells) are most numerous
  • Dense lymphoid tissue:
  • denser meshwork of cells and fibers
  • free cells (lymphocytes) are most numerous
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12
Q

Nodular lymphoid tissue

A
  • compact
  • spherical
  • lack at CT capsule
  • consist mostly of B lymphocyes
  • primary and secondary nodules
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13
Q

Primary nodules

A
  • tightly packed small lymphocytes

- not under antigenic challenge

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14
Q

Secondary nodules

A

-germinal centers:
dark zone containing dividing B lymphocytes
light zone containing non-dividing B lymphocytes and T helper cells
mantle zone containing young plasma cells and memory B cellls

  • follicular dendritic cells and reticular fibers form framework
  • functional correlations:
  • proliferation of activated B lymphocytes in dark zone, selection of B lymphocytes in light zone, apoptosis of B lymphocytes that cannot discriminate between self and non self, differentiation in light zone , temporary storage of young plasma cells and memory B cells take place in mantle zone

-appear during primary antigenic response, and involute after about 4 weeks

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15
Q

General characteristics of Lymph nodes

A
  • occur along course of lymphatic vessels
  • usually kidney shaped with a definite indentation or hilus
  • blood vessels enter and leave at hilus
  • efferent lymphatics leave at hilus
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16
Q

Histological organization of lymph node

A

1) capsule of dense irregular CT
2) CT trabeculae- provide structural support, subdivide node into incomplete compartments
3) cortex
4) medulla

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17
Q

Component of lymph node cortex

A

: outer, more densely stained region, loose lymphoid tissue (sinuses)- just beneath capsule (subcapsular sinus), along trabeculae (peritrabecular sinus)
-lymphatic nodules- may or not have germinal centers, contain mostly B lymphocytes

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18
Q

Components of lymph node medulla

A
  • composed of dense lymphoid tissue (medullary cords)- populated by B lymphocytes and plasma cells)
  • medullary sinuses (loose lymphoid tissue)
  • communicate with peritrabecular sinuses and efferent lymphatics
  • permeable walls- permit free passage of wandering cells
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19
Q

Flow of lymph through node

A
  • afferent lymphatics- enter node at multiple sites on convex surface, valves open toward node
  • subscapular sinus
  • peritrabecular (intermediate) sinuses
  • medullary sinuses
  • efferent lymphatics- leave node at the hilus, valves point away from hilus
20
Q

Filitration of lymph

A
  • occurs in sinuses
  • lymph moves slowly
  • reticular cells are phagocytes- reticular cell failure facilitates spread of infection and/or metastases
21
Q

Lymphocyte circulation

A
  • produced in germinal centers
  • forced to periphery of nodules
  • enter sinuses
  • leave node by way of efferent lymphatics
  • enter blood circulatory system by way of thoracic duct
22
Q

HEV

A
  • High Endothelial Venules
  • lined by tall endothelial cells( vascular addressins on their surface)
  • permeable to small lymphocytes
  • cell adhesion molecules: selectins, integrins, carbohydrates, immunoglobulin superfamily members, clever-1
  • also found in tonsils and Peyer’s patches (but not the spleen)
23
Q

Medical Relevance of HEVs

A
  • play a role in lymphocytes recirculation (contribute to specificity)
  • decrease in number with age (immunologic function down with age)
  • implicated in metastasis of lymphoid malignancies (lymphomas)
24
Q

Recirculation

A
  • permits lymphocytes to encounter antigens
  • facilitates communication with other leukocyte types
  • allows targeting of lymphocytes
25
Q

Paracortical zone

A
  • located at boundary between cortex and medulla
  • site of most HEVs
  • under thymic influences (populated mostly by T lymphocytes)
  • composed of deep cortex units- semi-rounded structures, contiguous with cortex and bulging into medulla, centered on opening of afferent lymphatic, sometimes fused to form complexes, central portion (few reticular fibers, high concentration of small lymphocytes, site of cellular storage and proliferation)
  • peripheral portion- dense framework of reticular fibers, fewer lymphocytes, site of post-capillary venules with tall endothlium (HEV), site of rapid migration of lymphocytes
26
Q

Functions of lymph nodes

A
  • filtration of lymph
  • production and selection of B lymphocytes
  • play a role in the immune response to lymph borne antigens
27
Q

General characteristics of thymus

A
  • situated in the superior mediastinum
  • dual embroyological origin: epithelial lining of 3rd and 4th branchial (pharyngeal) pouches and mesenchyme
  • fully developed at birth
  • begins to involute at about two years of age- parenchyma is replaced by fat and fibrous tissue
28
Q

Histological organization of thymus

A
  • thin capsule of dense CT
  • CT septae subdivide organ into two main lobes and numerous lobules
  • lobules:
  • cortex- peripheral, more darkly staining, Dense lymphoid tissue, lymphocytes predominate, site of thymocyte proliferation, apoptosis and selection
  • medulla- central, more lightly staining zone, loose lymphoid tissue, rich in eptithelial reticular cells, site of thymocyte maturation
  • continuity between adjacent lobes
29
Q

Cell types in the thymus-thymocytes

A
  • predominate in cortical regions
  • proliferate and undergo selection in cortex
  • populate T dependent regions of other lymphoid organs
30
Q

Cell type in the thymus- epithelial reticular cells

A
  • originate in endodermal lining of 3rd and 4th branchial pouches
  • are not phagocytic
  • do not manufacture reticular fivers
  • have secretory function- contain dense granules, produce a family of polypeptides thymosins that promotes T cell differentiation, may produce thymopoietin
  • form supporting framework (cytoreiculum)- desmosomes and tonofilaments
  • form Hassall’s corpusles
  • contribute to the blood-thymus barrier
31
Q

Hassal’s corpulses

A
  • unique to thymic medulla
  • concentrically arranged epithelial reticular cells
  • frequently keratinize or calcify
  • decrease in number and increase in size with age
  • probably degenerated structures- may remove apoptotic thymocytes?? may place a role in generation of regulatory T cells???
32
Q

Mesenchymal Reticular Cells (Dendritic Cells)

A
  • small fraction of thymic stroma

- identical to reticular cells of lymph nodes- are phagocytoic, clean up debris resulting from T cell selection

33
Q

Functions of thymus

A
  • development of a diverse population of T lymphocytes that can respond to foreign antigens (thymosin)
  • trophic role in the development of other lymphoid organs (thymopoietin)
34
Q

Spleen general characteristics

A
  • largest accumulation of lymphatic tissue in body

- complex filter interposed in bloodstream

35
Q

Histological organization of spleen

A

1) dense irregular CT capsule
2) branching network of trabeculae arises from capsule- provide support, contain blood vessels
3) capsule and trabeculae contain small amount of smooth muscle
4) splenic pulp

36
Q

Red pulp

A

-80% of splenic parenchyma
-contains large quantity of blood
-made up of two structures
-splenic sinuses:
long, irregular vascular channels
elongate endothelial cells with spaces between them
discontinuous basement membrane
macrophages are on the outside surface of the basement membrane
allow free exchange between cords and sinuses

-splenic cords:
plates of cellular tissue lying between sinuses
site of destruction of RBCs
site of storage of blood

37
Q

While pulp

A
  • 20% of splenic parenchyma
  • composed of lymphoid tissue
  • periarterial lymphoid sheath (PALS)- surrounds major arterial branches (central arteries), T dependent area
  • lymphatic nodules- located within the PALS, contain mostly B lymphocytes, may or may not have germinal centers
38
Q

Marginal zone

A
  • found at the junction of red and white pulp
  • most arterial blood runs through marginal zone
  • contains lymphocytes, dendritic cells, and macrophages
  • site of initiation of immune responses to blood-borne antigens
  • from marginal zone blood enters the red pulp
39
Q

Blood flow through spleen

A

1) splenic artery- at hilus
2) trabecular arteries
3) central arteries
4) penicilli- straight branches into red pulp
5) capillaries
6) splenic sinuses
7) red pulp veins
8) trabecular veins
9) splenic vein leaves the spleen at the hilus

40
Q

Open and closed theories of splenic circulation

A
  • closed: blood flows directly from capillaries to sinuses (faster)
  • open: capillaries to cords to sinuses (slower)
41
Q

Splenic sinuses

A
  • wide vascular channels
  • spaces between endothelial cells
  • discontinuous basement membrane
  • macrophages are outside the basement membrane (within the cords)
  • free exchange between sinuses and cords
42
Q

Functions of the spleen

A
  • production of lymphocytes
  • immune response to blood-borne antigens
  • destruction of old erythrocytes (by macrophages)
  • storage of blood (especially the cells and platelets)- a reservoire of monocytes that can be rapidly released to regulate inflammation
43
Q

General characteristics of tonsils

A
  • composed of aggregates of lymphoid tissue that lie beneath an epithelium
  • located in the aperture through which the oral cavity communicates with the pharynx
  • consists of lymphatic nodules (many with germinal centers) embedded in dense lymphoid tissue
  • reach their maximal development in childhood
44
Q

Palatine tonsils (paired)

A
  • located in the lateral wall of oropharynx
  • covered by stratified squamous epithelium that is often infilitrated by lymphoctyes
  • epithelium invaginates to form numerous deep crypts
  • crypts often contain desquamated epithelial cells, lymphocytes and bacteria (detritus)
  • surrounded by a tick capsule of dense, irregular CT
45
Q

Pharyngeal tonsils (adenoids)

A
  • located in the roof of the nasopharynx
  • covered by pseudostratified ciliated columnar epithelium (respiratory epithelium)
  • characterized by the presence of numerous surface folds (pleats) rather than deep crypts
  • surrounded by a thin capsule of dense, irregular CT
46
Q

Lingual tonsils

A
  • located at the base of the tongue
  • smaller and more numerous than other tonsils
  • covered by stratified squamous epithelium
  • each lingual tonsil has a single large crypt
  • crypt is often filled with detritus
  • often associated with the mucous glands and skeletal muscle of the tongue
47
Q

Function of tonsils

A
  • detection and response to pathogens in the oral cavity

- production of lymphocytes

Microscopic Anatomy (7 decks)

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