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Microanatomy > Lymphoid Organs > Flashcards

Lymphoid Organs Flashcards

1
Q

Secondary Lymphoid Organs

A
  • Lymph nodes
  • Tonsils
  • Spleen
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2
Q

Lymphoid Tissue

A
  • Specialized CT made primarily of lymphocytes
  • Predominant tissue in lymphoid organs
  • Component of non-lymphoid organs
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3
Q

Blast Transformation

A
  • Cell enlarges to become a large lymphocyte

- Form new daughter cells which become effector/memory

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4
Q

Thymus development

A
  • 3rd pharyngeal pouch endodermal epithelium
  • Consists of mainly TECs until 8-9 weeks gestation
  • T cell precursors migrate into thymus and insert between TECs
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5
Q

Thymus Capsule

A
  • Dense irregular connective tissue
  • Collagen Type I
  • Reticular fibers
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6
Q

CT Septa

A
  • Trabeculae divide the thymus into lobules

- Extend inward from the capsule

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7
Q

Thymus Lobule

A

-Divided into cortex and medulla

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8
Q

Thymus Cortex

A
  • Outside

- Contains more lymphocytes

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9
Q

Thymus Medulla

A
  • Continuous throughout entire Thymus and each lobule
  • More macrophages
  • More TECs
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10
Q

Thymic Epithelial Cells

A
  • Large, Pale staining, euchromatic nuclei
  • Cytokeratin filmaents means that these are Epithelial cells
  • Desmosomes link TECs
  • Part of stroma (supportive function)
  • Chemokines/cytokines
  • Blood thymus barrier
  • Selection process
  • Hassal’s corpuscles
  • Non phagocytic
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11
Q

Type I TECs

A
  • Form sheets of cells connected by tight junctions
  • Line inner surface of capsule
  • Cover CT septa
  • Part of blood-thymus barrier in cortex
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12
Q

Type II TECs (Thymic Nurse Cells)

A
  • Long processes connected by desmosomes
  • Form network throughout cortex
  • Secrete factors that influence thymocyte development and migration
  • Positive selection
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13
Q

Type III TECs

A
  • Connected via tight junctions

- Form barrier between cortex/medulla

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14
Q

Type IV TECs

A
  • Joined by tight junctions

- Form a layer beneath Type III TECs at cortico-medullary boundary

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15
Q

Type V TECs

A
  • Stellate cells connected by desmosomes

- Form meshwork throughout medulla similar to type II TECs in cortex

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16
Q

Type VI TECs

A
  • Form Hassal’s Corpuscles
  • Center may be calcified, keratinized, necrotic
  • Produce cytokines
  • May help produce T-reg cells
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17
Q

Thymocyte migration path

A

1) Lead blood vessels at cortico-medullary boundary
2) Migrate to outermost part of cortex
3) Back to cortico-medullary boundary
4) Enter medulla
5) Leave via post capillary venules or via lymphatics
6) Leave as inactivated naive T lymphocytes and migrate to secondary lymphoid tissues/organs

18
Q

Positive Selection

A
  • Occurs mostly in the cortex
  • Double positive thymocytes bind to MHC molecules on Type II TECs
  • No binding means death by apoptosis
  • Eliminates defective thymocytes that cannot bind self-MHC and cannot be activated in immune response
19
Q

Negative selection

A
  • Occurs most often in medulla, but also in cortex
  • Cells that bind to Type V TECs will die b/c they adhere too strongly
  • Cells must be exposed to a wide variety of self-antigens
  • Medullary APCs synthesize “nonthymic” antigens such as PTH, insulin, AchR, etc.
20
Q

Blood Thymus Barrier (Medulla)

A

-Leaky in the medulla and allows self-antigens to be present

21
Q

Blood Thymus Barrier

Cortex

A
  • Not leaky in the cortex
  • Made of Type I TECs and capillary endothelial cells
  • Each set has their own basal lamina and are joined by tight junctions
22
Q

Thymic involution

A
  • Loses efficiency as size decreases
  • Affects the cortex most and forms cortical caps
  • Cortex becomes discontinuous
  • Old tissue becomes replaced by adipose tissue instead
  • Accidental involution stimulated by steroid hormones, infections, illness, etc.
23
Q

DiGeorge syndrome

A
  • Congenital defects of pharyngeal pouches
  • Very susceptible to infections
  • Underdeveloped/absent thymus
  • Stroma of thymus most affected
24
Q

Macrophages

A
  • Found in both cortex/medulla
  • More in the medullar (relatively)
  • Dark-staining apoptotic bodies (tingible bodies) from dead thymocytes
  • Large lysosomes are PAS+
25
Q

Thymic Dendritic Cells

A
  • Found in medulla
  • Large, pale staining with elongated cytoplasmic processes
  • Difficult to distinguish from medullary TECs
  • APCs
26
Q

Lymph nodes

A
  • Encapsulated organs (Dense irregular CT)
  • Trabeculae are finger-like projections into organ
  • Stroma made of reticular CT/fibers
  • Afferent lymphatic vessels penetrate capsule at multiple sites
27
Q

Lymph cortex

A
  • Outermost region
  • Stains darkly basophilic due to lymphocytes

1) Lymphoid nodules: B cells (thymus independent region)

2) Inter-nodular cortex: T cells
(thymus dependent region)

28
Q

Lymph paracortex

A
  • Lies deep to cortex
  • No nodules
  • Dark staining
  • Contains mainly T cells (thymus dependent region)
  • Contains HEV which deliver lymphocytes to a node
29
Q

Lymph medulla

A
  • Central portion of node
  • Medullary sinuses (light staining)
  • Medullary cords (dark staining)
  • Lighter staining than cortex/paracortex due to lymphatic sinuses
30
Q

High Endothelial Venules (HEV)

A
  • Cuboidal/columnar epithelial cells
  • Deliver lymphocytes to paracortex
  • Diapedesis
31
Q

Nodules

A
  • Found at outer cortex separated by internodular cortex

- Macrophages, Follicular dendritic cells (FDCs), some Th Cells, B cells

32
Q

Internodular Cortex

A
  • Mostly T cells

- Between nodules in the outer cortex

33
Q

Internodular Cortex

A
  • Mostly T cells

- Between nodules in the outer cortex

34
Q

Primary nodules

A
  • Stain uniformly dark
  • Small inactive B cells
  • Develop into secondary nodules when B cells activate and proliferate during immune response
35
Q

Secondary nodules

A
  • activated B cells are dividing and maturing as part of humoral response
  • Germinal center
  • Cap (mantle, corona)
  • Activated B cells proliferate/divide
36
Q

Germinal center

A
  • Light staining central region because activated B cells are large lymphocytes
  • Activated B cells proliferate/mature
37
Q

Cap/mantle/corona

A
  • Inactive lymphocytes that get pushed to edge of nodule during rapid division of active B cells
  • Dark staining peripheral region
38
Q

Germinal center dark zone

A
  • Site of intense proliferation of activated B cells
  • Dividing cells are called centroblasts
  • Somatic hypermutation of immunoglobulins
39
Q

Germinal center light zone

A
  • Centroblasts migrate into light zone and are then known as centrocytes
  • Mitosis decreases
  • Centrocytes interact with follicular dendritic cells
40
Q

Lymphocyte entry into lymph node

A
  • Enter via high endothelium venule
  • Found in paracortex of the node
  • Some enter via afferent lymphatics
41
Q

Immune response in lymph node

A

1) Antigen arrives as free antigen, APC, or bound to Ab
- Langerhans cells; become IDCs in lymph node
2) Lymphocytes arrive via HEV in paracortex
3) Migrate to T/B cell territory
- T cells go to paracortex/internodular cortex
- B cells go to lymphoid nodules in the cortex
4) APCs activate naive T cells which then activate B cells
5) Activated B cells differentiate/proliferate and form secondary nodule and germinal center (pale staining)
6) Plasma cells migrate to the medulla to secrete Ab into medullary sinuses

42
Q

Follicular Dendritic Cell

A
  • Aid in selection process
  • Interact in the light region with centrocytes
  • Not antigen presenting
  • Beaded processes due to inclusion bodies

Microanatomy (4 decks)

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