Term
| What is the location of the nodules and aggregates of the lymphoid system? |
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Definition
Subepithelial connective tissue (lamina propria) of hollow organs:
Digestive System (e.g., Peyer’s Patches)
Respiratory System
Reproductive System
Urinary System
The nodules occur especially at branch points (ex - in the respiratory tree), where two or more organs join (gastroesophageal junction), and where transitions in the type of epithelial lining occur
These barriers in the lymphoid system are crucial in preventing bacteria from entering the blood (bacterial sepsis) |
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Term
| What is the distinction between stroma and parenchyma in the lymphoid system? |
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Definition
Stroma = Reticular Connective Tissue
Parenchyma = Lymphocytes & Lymphoblasts |
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Term
| Explain the difference between primary and secondary lymphoid nodules |
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Definition
Primary nodules lack germinal centers and contain only small lymphocytes. They are present prenatally and in the absence of antigens
Secondary nodules appear after birth and they are activated by antigen exposure. They have a narrow, dark-staining halo of small lymphocytes surrounding a larger, lighter-staining germinal center that contains mainly lymphoblasts ( a lot of cell division occurs here) |
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Term
| What are the lymphoid organs? |
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Definition
Lymphoid Organs (Encapsulated):
Primary (Central) - where antigen independent proliferation occurs and naive lymphocytes are produced
-Bone Marrow
-Thymus
Secondary (Peripheral)
-Lymph Nodes
-Spleen
-Tonsils |
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Term
| What are the basic characteristics of the thymus? |
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Definition
- Primary (Central) Lymphoid Organ
- Located in Mediastinum (middle part of thoracic cavity)
- T-Lymphocyte Programming (antigen independent)
- relative to the size of the rest of the body and esp. the heart, the thymus is a lot bigger in children than in adults |
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Term
| What is the basic structure of the thymus? |
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Definition
It has two lobes joined and covered by a thin loose CT capsule that penetrates the lobes as septa, dividing the lobes into incomplete lobules (adjacent lobules can share a common medulla)
Each lobule has a peripheral dark-staining cortex, adjoining the capsule and the septa.
Each lobule contains a central light-staining medulla |
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Term
| Explain the cortex component of the thymus |
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Definition
Cortex
-Dense cell packing(mainly thymic lymphocytes - thymocytes)
-Stroma=Epithelial reticular cells - contain cytokeratin and not vimentin!, do not make reticular fibers
-Blood-thymus barrier (capillaries only) - prevents antigens from pre-programming naive cells
-Antigen-independent thymocyte proliferation
-T-cell programming |
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Term
| Explain the medulla component of the thymus |
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Definition
Medulla
-Loose cell packing
-Fewer thymocytes (programmed T-cells)
-Programmed T-cells enter general circulation
-Stroma=Epithelial reticular cells, dendritic (aka Langerhans) cells, macrophages, fibroblasts
Hassal’s corpuscles - spherical structures with concentric layers of epithelial reticular cells. ONLY found in thymus -> identifying feature |
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Term
| What are the basics of epithelial reticular cells? |
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Definition
- Derive from endoderm
- Produce thymic hormones & cytokines
- Contribute to blood-thymus barrier
- joined at their tips by desmosomes |
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Term
| How does the thymus change over time? |
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Definition
It involutes with age
- Gradual depopulation of cortex
- Replacement by adipose tissue
- Hassal’s corpuscles enlarge & may calcify
In involution the fat levels increase while the medulla and cortex decrease |
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Term
| How do programmed T-cells enter the circulation? |
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Definition
1)Bone-marrow-derived precursors enter blood, home to thymus, & populate thymic cortex
2)Multi-step commitment process promoted by stromal cells
3)Programmed T-cells migrate to thymic medulla,
re-enter circulation, & home to secondary organs |
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