thymus

Question 1

What is meant by immune system?

Answer 1

• It is the capacity of the body to protect itself against possible harmful foreign substances (microorganisms and foreign substances

Question 2

list Lymphoid organs classifications

Answer 2

I) Primary (Central):
Development and maturation of B & T
lymphocytes. (bone marrow
and thymus)

II) Secondary (Peripheral):
Mature lymphocytes respond to Antigen.
1. Lymph nodes
2. Spleen
3- Mucosal associated lymphoid
tissues (MALT):
a- Gut Associated Lymphoid Tissues (GALT).
b- Bronchus Associated Lymphoid Tissues ( BALT)

Question 3

illustrate Lymphoid Organs
Classification according to capsule

Answer 3

• Capsulated : Surrounded by
  connective tissue capsule.
  1. Lymph nodes
  2. Spleen
  3. Thymus
• Non-encapsulated :
  Solitary lymphatic nodules Aggregated lymphoid tissue beneath m.m.:
  1. Tonsils (palatine, pharyngeal &lingual)
  2. Peyer’s patches
  3. Appendix

Question 4

To which classification the thymus belongs ?

Answer 4

Primary-Capsulated lymphoid organ.

Question 5

illustrate Structure of the thymus

Answer 5

• Stroma
  1. Capsule: irregular collagenous CT
  2. Trabeculae: delicate septa (incomplete lobules)
• The capsule & trabeculae contain BVs , efferent lymphatic vessels and nerves.
  There is NO:
  Reticular fibers (cellular reticulum made by epithelioreticular cells that serve as stroma).
  Afferent lymphatics.
• Parenchyma
  Each lobule is formed of
  1. Cortex
  2. Medulla

Question 6

from which cortex consist ?

Answer 6

The cortex
* It is the outer dark region of the lobule
* Strong basophilic in H&E
* Site of maturation of T-lymphocytes
* Contains:
1. Epithelioreticular cells ERCs (large nursing cells).
2. Thymocytes (immature T cells).
3. Macrophages (phagocytose T cells that react with self Ag)

Question 7

what’s Epithelial reticular cells (ERCs) ?

Answer 7

• Have features of both epithelial and
  reticular cells.
• 6 types are recognized; 3 types in the
  cortex: Types I, II and III and 3types in
  the medulla.
• Provide framework for developing T
  cells.

Question 8

discuss Epithelial reticular cells of the cortex

Answer 8

Type I
* Separate the cortex from the ct capsule.
* Occluding junction between these cells →blood thymus barrier.

Type II
* Present in the mid cortex.
* Form meshwork.
* LM: Stellate, pale cytoplasm, large ovoid euchromatic nucleus.
* EM: Long processes filled with tonofilaments and joined by
desmosomes.
* Function: divide the cortex into isolated areas for developing T
cells

Type III
* Present at the corticomedullary junction.
* Occluding junctions are present between adjacent cells.
* Form functional barrier between cortex and medulla.
* Function:
1. Synthesize factors and hormones necessary for maturation of T lymphocytes (thymosin & Thymopoietin).
2. Create isolated cortical compartment thus prevent contact between T-cells and antigen (blood-thymus barrier)

Question 9

Why epithelioreticular cells are
called epithilio?

Answer 9

1. Developed from endoderm.
2. Contains bundles of tonofilaments in their cytoplasm
3. Desmosomes are present between their processes.
4. Occluding junctions are present between adjacent cells

Question 10

about Thymocytes (immature T lymphocytes) :
1.Derived from the ………..
2. Are ………….. in cortex
3. Surrounded by the processes of the …………… separating them from ……… during maturation.
4. As they mature, they migrate towards …………….
5. 90% T cells die in the cortex by …………..
6. Mature T lymphocytes migrate to medulla and enter blood stream to
thymus dependent zones; ………………

Answer 10

1. bone marrow
2. immature T-lymphocytes
3. epithelial reticular cells / antigens
4. medulla.
5. apoptosis.
6. paracortex in the lymph node
   and PALS in the spleen
   (peri-arteriolar lymphatic sheath)

Question 11

about the medulla of thymus :
1. It is the …… area of the thymus lobule.
2. Contains: …………

Answer 11

1. central pale
2. 1.Epithelial reticular cells (Types IV, V, VI) (larger& paler than those in the cortex)
   2.Mature T-lymphocytes
   3.Hassal’s Corpuscle

Question 12

illustrate Hassal’s Corpuscles

Answer 12

• Acidophilic structure present in the medulla of thymus
• Formed of concentric layers of epithelioreticular cells (type VI)
  around a central hyaline acidophilic mass
• Formed as a result of degeneration and calcification of ERCs.
• They increase in number with age.
• Function: secrete several cytokines that control activity of local dendritic cells and promote the development of T cells.

Question 13

illustrate Blood-thymus barrier

Answer 13

• Exists in the cortex making it an immunologically protected region.
• Formed of:
  1. Endothelium of thymic capillaries (continuous type) joined by tight junctions.
  2. Complete thick basal lamina of blood capillaries.
  3. Perivascular C.T. and macrophages
  4. Basal lamina of epithelialoreticular cells.
  5. Complete layer of ERC (Type I)
• Function: protect immature T lymphocytes from Ag in blood

Question 14

illustrate the blood supply of thymus

Answer 14

• Arteries enter the thymus through the capsule to the trabeculae
• At cortico-medullary j they enter the cortex make anastomosing arches
• Enter the medulla→ drain into small venules in the medulla
• Have NO afferent lymphatics

Question 15

illustrate Function of the thymus

Answer 15

• Proliferation and differentiation of T-cells in the cortex immunocompetent venules blood stream thymicdependent zones
• Thymic reticular epithelial cells secrete several hormones or factors which stimulate the differentiation of T-lymphocytes