L29: Lymphoid Tissues and Organs

Question 1

Classification of lymphatic tissues. Describe each and locations for each

Answer 1

1. ) Primary lymphatic tissue: site of development and maturation of lymphocytes into immunocompetent cells. Locations = bone marrow and thymus
2. ) Secondary lymphatic tissue: site of confrontation and formation of immunological defense against antigens or pathogens. Locations = 1.) Lymph nodes, 2.) MALT (GALT, BALT, GU tract, tonsils) and 3.) spleen

Question 2

How are lymphatic nodules/follicles classified?

Answer 2

Based on appearance in tissue

1. ) Primary: cells appear homogenous throughout, mostly small lymphocytes
2. ) Secondary: separation of cells into 2 distinct zones
   - Mantle zone (corona): outer dark-staining zone containing small mature lymphocytes
   - Germinal center: inner light-staining zone containing medium and large immature lymphocytes that encounter antigen causing GC to swell

Also based on them being:

1. ) Solitary: temporary structures that appear / disappear
2. ) Aggregates: permament structures

Question 3

Examples of aggregate lymphatic nodules/follicles?

Answer 3

• Peyer’s patches (B and T lymphocytes) in ileum
• Lymphatic tissue in appendix
• BALT in respiratory tract
• Lymphatic nodules: tonsils, lymph nodes and spleen

Question 4

What are tonsils? Types and what lymphocytes are contained in them?

Answer 4

• Aggregations of lymphatic nodules
• Specifically, there are pharyngeal, palatine and lingual tonsils that form a ring surrounding the entrance to the oropharynx
• B-lymphocytes

Question 5

Pharyngeal tonsil. Location, covering, description

Answer 5

• aka adenoid
• Located in posterior nasopharynx
• Covered by pseudostratified columnar epithelium with cilia
• No crypts, has pleats

Question 6

Palatine tonsils. Location, covering, description

Answer 6

• Paired along lateral wall of oropharynx between arches
• Covered by stratified squamous non-keratinized epithelium
• Has crypts/fissures
• Partial capsule (acts as cup/basket)

Question 7

Lingual tonsils. Location, covering, description

Answer 7

• Small bumps on dorsal surface of posterior 1/3rd tongue
• Covered by stratified squamous non-keratinized epithelium
• Has crypts

Question 8

Features of lymph nodes:

Answer 8

1. ) Capsule: dense CT that form trabeculae (septa) that extend into LN carrying BVs and nerves
2. ) Hilum: site for BVs and efferent lymph vessels to leave the node
3. ) Sinuses: subcapsular drain into trabecular sinuses drain into medullary sinuses. Sinuses lined by endothelial cells. They have meshwork of reticular cells with reticular fibers. Also lined by macrophages.
4. ) Reticular tissue: reticular cells and reticular fibers (type III collagen) form 3D meshwork to suspend cells and sinuses. Dendritic cells found here, macrophages found here, follicular dendritic cells found here
5. ) Parenchyma
   a. ) Cortex: contain B-lymphocytes, dendritic reticular cells
   b. ) Paracortex: contains T-lymphocytes, postcapillary venules known as high-endothelial venules (lined with simple cuboidal epithelium)
   c. ) Medulla: contain medullary cords, which contain reticular cells and fibers, B lymphocytes, plasma cells, DCs and macrophages

Question 9

What are high endothelial venules?

Answer 9

• These are postcapillary venules found in paracortex of lymph nodes. Circulating B and T lymphocytes escape from bloodstream via these to enter the paracortex of the LN (90% of them enter through this pathway). B lymphocytes then migrate to cortex, T lymphocytes stay in paracortex

Question 10

Discuss flow of lymph at lymph node

Answer 10

• Afferent lymphatic vessels
• Subcapsular sinus
• Trabecular sinuses
• Medullary sinuses
• Efferent lymphatic vessels

Question 11

Features of thymus

Answer 11

• 2 lobes – site of T-lymphocyte maturation
  1. ) Capsule:
• dense irregular collagenous CT with trabeculae that partion thymus into thymic lobules (each with cortex and medulla). Trabeculae contain BVs, nerves and efferent lymph vessels. Cells here = fibroblasts, plasma cells, granulocytes, lymphocytes, mast cells, adipocytes, macrophages
  2. ) Cortex: site of T-lympocyte maturation: contains Type I-III epithelioreticular cells and small T-lymphocytes known as thymocytes that undergo maturation
  3. ) Medulla (continous with medulla of other lobules): site of Type IV-VI epithelioreticular cells, contain mature T-lymphocytes that exit medulla via postcapillary venules

Question 12

Where do mature T-lymphocytes go after leaving the thymus?

Answer 12

• They are delivered to secondary lymphatic organs: paracortex of lymph nodes, PALS of the spleen, GALT (Peyer’s patches with mostly B lymphocytes) and BALT (mostly B lymphocytes)

Question 13

Function of type I-VI epithelioreticular cells?

Answer 13

1. ) Form seal around cortex, separating it from CT capsule and trabeculae. Form sleeve around tunica adventitia of vessels. These help form the blood-thymus barrier
2. ) Stellate-shaped cells form meshwork in midcortex. They are also teachers involved in thymic cell education
3. ) Reside in deep cortex and form seal between cortex and medulla, isolating the two
4. ) Associate with type III and participate in barrier at corticomedullary junction
5. ) Meshwork of medulla
6. ) Form thymic corpuscles known a Hassall’s corpuscle which are eosinophilic whorls unique to thymic medulla

Question 14

Which epithelioreticular cells help establish the blood-thymus barrier?

Answer 14

• Type-I

Question 15

Composition of the blood-thymus barrier?

Answer 15

• Cortical capillary endothelium (continuous type)
• Thick basal lamina
• Macrophages
• Type I epithelioreticular cells

Question 16

What is DiGeorge’s Syndrome?

Answer 16

• Developmental disorder that leads to thymic hypoplasia or aplasia (deficit in third pharyngeal arch development), resulting in a decreased or inability to produce T lymphocytes. These individuals die from infection.

Question 17

Features of spleen

Answer 17

1. ) Capsule: covered by mesothelium – dense irregular collagenous CT, also contains SM cells and elastic fibers. Trabeculae carry branches of splenic artery deep into spleen
2. ) Hilum: BVs and nerves enter and leave the organ
3. ) Stroma: CT with reticular fibers forming a 3D meshwork
4. ) Parenchyma:
   a. ) White pulp (stain blue with hematoxylin)
   - PALS: thymus-dependent zone of splenic pulp contain T-lymphocytes, central artery located centrally within the PALS
   - Splenic nodes (aka Malpighian Corpuscles): lymphatic nodules containing B- lymphocytes, may contain GC around central artery (eccentrically located)
   b. ) Marginal zone
   - between red and white pulp
   c. ) Red pulp
   - Splenic cords (cords of Billroth): irregular and branching cords of splenic tissue, contain framwork of reticular cells and fibers, spaces between are filled with blood that is filtered, also with macrophages, plasma cells and DCs
   - Splenic sinuses: lined by ECs – drain into pulp veins which drain into trabecular veins