Flashcards in Lecture 24: Immune And Lymphatic Tissue II Deck (13):
Describe the basic histology of the thymus
* See Slide 5
* Most developed at puberty:
- 10-15 grams at birth to 30-40 grams at puberty
* Involutes during adolescence
* No lymph follicles (nodules)
* No afferent lymph vessels
* No lymph sinuses
- Blood vessels
- Efferent lymphatics are present.
- Afferent lymphatics are not present: Therefore, lymph does not circulate through thymus.
- Extends trabeculae (septa) into the parenchyma
* Trabeculae (Septa):
- Delicate CT
- Divide the thymus into incomplete lobules
Describe the lobules of the thymus
* Each lobule is composed of an outer, darker staining cortex and an inner, lighter staining medulla.
* Cortex: (dark staining):
- Stained densely with basic dyes such as H&E
- Cell population:
-- Epithelial reticular cells: Secrete thymosin
-- T cells in various stages of differentiation
- Thymocytes migrate from cortical areas to medullary areas
- Blood vessels surrounded by continuous epithelial barrier.
-- Allows thymus to maintain lymphopoiesis while segregated from antigens.
Describe the medulla (usually light stained)
* Specialized to allow entry channel into blood stream of mature lymphocytes
* Capillary beds are not sheathed by epithelial cells.
* Hassall’s corpuscles:
- Whorls of highly keratinized medullary epithelial cells:
-- Produce cytokine thymic stromal lymphopoietin: Stimulates thymic dendritic cells needed for the maturation of single positive T cells
Describe differentiation in T Cells
*Double negative T cells:
- Lack cell surface molecules typical of mature T cells
- Enter cortex from blood vessels
- Proliferate in subcapsular area
* Double positive T cells move to outer cortex.
- Confronted with epithelial cells with cell surface MHC classes I and II for clonal selection
- Express both CD4 and CD8 coreceptors and TCR receptors.
* Single positive T cells move to inner cortex:
- Express TCR receptors and either CD4 or CD8 coreceptors
* Clonal deletion is completed in medulla.
- See Slide 10-13 and study the expression of Foxn1 and Aire genes and the expression of various keratins in the thymic epithelial cells that regulate differentiation of T-cells.
Describe what I think is the blood thymic barrier in the thymus
* Blood-thymic barrier.
* Located in thymic cortex
* Prevents antigens in the blood from reaching developing T cells in thymic cortex
* Leaky during fetal life to allow for development of immunologic tolerance to self-antigen.
- Endothelial basal lamina
- Perivascular space
- Basal lamina of reticular cells
- Reticular cells
- Thymicparenchymal cells
- See Slide 14
Describe the morphology of the spleen
* 5.6 x 4 inches
* No lymph sinuses
* No afferent lymph vessels
* Covered by peritoneum except at hilus
* Mesothelium-lined CT capsule contains some smooth muscle fibers and sends trabeculae into parenchyma
* Blood vessels enter and leave hilus
* Divided into red pulp and white pulp
Describe the blood filtering functions of the spleen
* Only lymphatic organ specialized to filter blood:
- Stores and removes worn-out RBCs
- Recycles iron
- Converts hemoglobin to bilirubin
- Blood formation in the fetus
Describe the immunologic functions of the spleen
* Screens foreign material in the blood
* Produces lymphocytes and plasma cells
* Removal leads to overwhelming bacterial infections in infants, children, and young adults.
Describe the histology of white pulp
* Elongated, branched strands always associated with arteries
* Zones of diffuse lymphoid tissue and germinal centers
* Site of clonal expansion of antigen-stimulated lymphocytes
* B cell area contains secondary follicles in which central arteriole is off center.
* T cells are found in the areas surrounding the central artery near the center of the white pulp.
- Forms the periarterial lymphatic sheath (PALS)
* Reticular fibers are associated with fixed macrophages and support splenic pulp
Describe the histology of the marginal zone
* Forms sinusoidal interface between red pulp and white pulp
* Has an abundance of antigen-presenting cells
* Lymphocytes first encounter antigens here.
* Activated T-helper cells activate B cells here.
Describe the histology of red pulp
* Surrounds white pulp and makes up about 80% of the spleen. • Functions to filter blood
* Contains large numbers of RBCs and other blood elements
* Billroth cords form red pulp parenchyma:
- Contain various blood cells, plasma cells, and antigen presenting cells
- Terminal capillaries open directly into substance of cords (open circulation).
- Macrophages destroy worn-out or defective red blood cells.
* Venous sinusoids:
- Endothelial-lined sinusoids with a discontinuous basement membrane
- Storage sites for healthy red blood cells
Describe the histology of the arteries within the spleen
* Splenic artery enters hilus
* Trabecular arteries branch off
* Central arteries:
- Review Figure 10-23 for vascular pattern
- Adventitia loosens and becomes mesh-like reticulum infiltrated with lymphocytes.
- Enlarged areas are splenic nodules
* After capillaries form, supplying white pulp, central arteries lose their white pulp investment and enter red pulp to form a penicillus.
- Composed of pulp arteriole, sheathed arteriole, and terminal capillary.
* Terminal capillary:
- Drains into: Intercellular spaces (open system) or
- Venous sinuses lined with reticuloendothelial cells (closed system)