Lecture 24: Immune And Lymphatic Tissue II

Question 1

Describe the basic histology of the thymus

Answer 1

• 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
• Capsule:
• 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

Question 2

Describe the lobules of the thymus

Answer 2

• 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.

Question 3

Describe the medulla (usually light stained)

Answer 3

• 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

Question 4

Describe differentiation in T Cells

Answer 4

• 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.

Question 5

Describe what I think is the blood thymic barrier in the thymus

Answer 5

• 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.
• Components:
• Endothelium
• Endothelial basal lamina
• Perivascular space
• Basal lamina of reticular cells
• Reticular cells
• Thymicparenchymal cells
• See Slide 14

Question 6

Describe the morphology of the spleen

Answer 6

• 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

Question 7

Describe the blood filtering functions of the spleen

Answer 7

• Only lymphatic organ specialized to filter blood:
• Stores and removes worn-out RBCs
• Recycles iron
• Converts hemoglobin to bilirubin
• Blood formation in the fetus

Question 8

Describe the immunologic functions of the spleen

Answer 8

• Screens foreign material in the blood
• Produces lymphocytes and plasma cells
• Removal leads to overwhelming bacterial infections in infants, children, and young adults.

Question 9

Describe the histology of white pulp

Answer 9

• 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

Question 10

Describe the histology of the marginal zone

Answer 10

• 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.

Question 11

Describe the histology of red pulp

Answer 11

• 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

Question 12

Describe the histology of the arteries within the spleen

Answer 12

Arteries:

• 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.
• 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)

Question 13

Describe the histology of veins in the spleen

Answer 13

• Venous sinuses are lined with reticuloendothelial cells.
• Drain into:
• Pulp veins which unite with:
• Trabecular veins, forming
• Splenic vein which
• Exits at hilus
• See Slide 28-32