L3: Lymphatic tissue Flashcards
What is the lymphatic system comprised of?
- lymphatic vessels, which transport interstitial fluid (lymph) back to the blood circulation
- The lymphoid tissue which house lymphocytes and other cells of the body’s immune defense system
What are the types of organs in the Lymphatic system?
1ry lymphatic organs: Sites where lymphocytes mature and become immunocompetent “has function” (B cells in bone marrow and T cells in the thymus).
2ry lymphatic organs: Mature lymphocytes are distributed via blood Or lymph to secondary lymphoid organs (e.g., lymph nodes, spleen) where they await activation.
What is a description of the thymus?
A flattened bilobed organ lying behind the sternum.
What are the Steps of development of the thymus?
It continues to enlarge up to puberty but then undergoes a slow involution becoming replaced by adipose tissue.
What is the Thymus formed of?
Formed of:
1) Stroma
2) Parenchyma
“Any parenchymal organ (those with no lumen)”
What is the structure of the stroma of the thymus?
- Capsule: thin loose C.T.
- Trabeculae: thin, divide gland into 2 lobes then, subdivided into incomplete lobules.
” + reticular fiber network that forms the floor of cells”
What is the structure of the parenchyma of the thymus?
Each lobule is formed of:
- Cortex: outer dark part (high content of lymphocytes). “As it is the site of development”
- Medulla: central pale part of the thymus lobule (due to low content of lymphocytes).
What are the cells found in the cortex of the thymus?
- Lymphoblasts (thymocytes): In the outer cortex.
- Have dark nuclei “inactive” and are derived from the CFUs (Colony forming units) in the bone marrow. - T- lymphocytes:
- Deep in of the cortex.
- Numerous, derived from the lymphoblasts.
- Migrate to the medulla. - Few macrophages:
- Reticular “has processes” epithelial cells “endothelial characters” : Branched cells with a pale nucleus. “Secrete thymic hormones”
What are the cells found in the medulla of the thymus?
- T- lymphocytes:
- Less numerous
- They have completed their development in the cortex, then migrate to the medulla. - Reticular epithelial cells. “Less numerous”
- Few macrophages “less numerous”
- Hassall’s corpuscles: A central hyaline acidophilic mass “Remaining of the cytoplasm and cell membrane of T-sales” surrounded by concentric layers of reticular epithelial cells.
What is the function of the thymus gland?
- Production of T- lymphocytes: In the cortex, then migrate to the medulla post capillary venule blood circulation the lymph node and the spleen where they settle to produce T- lymphocytes
throughout life. - It is essential for the normal development of the lymphoid tissue “t-cells” in early life
What is the histological structure of the lymphoid follicles or lymphoid nodule “Structural and functional unit of lymphatic system”?
- Spherical basophilic mass of small lymphocytes mainly B lymphocytes and few T lymphocytes with a rich network of reticular fibers supporting the cells.
- It is not encapsulated with connective tissue. “Unlike the thymus”
What is the size of lymphoid follicles?
Lymphoid nodules vary widely in size, measuring a few hundred micrometers to one mm in diameter.
What are the sites of lymphoid nodules?
- They are found free in many connective tissues in the body.
- They are also present within lymph nodes, spleen, and tonsils, but not in the thymus which contains only T cells.
What are the types of lymphoid follicles?
Primary and secondary
What is the shape of primary follicles?
Primary follicle without germinal center
What is the shape of secondary follicles?
- Has germinal center: Which is a pale central area formed as a result of exposure to infection or antigen, so the small B- lymphocytes (have dark nuclei) develop into activated medium sized lymphocytes (have pale nuclei) which aggregate in the center of the primary follicle forming a pale area called germinal center.
What is the definition of the lymph node?
Lymph nodes are bean-shaped, encapsulated structures, generally 2–10 mm in diameter “ Unlike lymphoid follicles which are 1 mm at max” , distributed throughout the body along the course of the lymph
What is a lymph node formed of?
- Stroma.
- Parenchyma.
Mention the structure of the stroma of lymph nodes
- Capsule:
✓ Dense fibrous “C.T Like most capsules”
✓ Penetrated by afferent lymphatics at the convex surface of the lymph node.
✓ Thickened at the hilum through which arteries and nerves enter and veins and efferent lymphatics leave the node - Trabeculae:
✓ They arise from the capsule.
✓ They divide the cortex into cortical compartments and the medulla into irregular areas. - Reticular fibers:
✓ They form network holding the parenchymal cells in its
meshes.
✓ They appear only with silver stain.
✓ The network is more condensed at the site of lymphoid follicles and the medullary cords.
Mention the structure of the parenchyma of lymph nodes
- The cortex:
✓ It is formed of lymphatic follicles (primary or secondary)
surrounded by lymph sinuses. - Lymph sinuses:
✓ They are spaces surrounding the cortical lymphatic follicles and the medullary cords.
✓ They contain lymph, lymphocytes and macrophages. - The medulla:
✓ It is formed of irregular medullary cords (as follicles but
arranged in the form of cords) surrounded by lymph sinuses.
What is the function of lymph nodes?
- Filtration of lymph by the macrophages of lymph sinuses.
- Humoral immunity by B-lymphocytes which produce antibodies.
- Cell mediated immunity by T-lymphocytes which when activated form killer cells to destroy the antigens.
What is the structure of MALT?
Large and diffuse collections of lymphocytes, plasma cells, and lymphoid nodules present in the mucosa of the digestive, respiratory, and genitourinary tracts
Give examples for MALT.
As the tonsils, the Peyer patches in the ileum and aggregates of lymphoid follicles in the appendix
What is the definition of tonsils?
Masses of lymphatic tissue “Lymphatic follicles + diffuse lymphatic tissue” present in the lamina propria “epithelium + C.T” of a mucous membrane.
What are types of tonsils?
According to their location
• Palatine tonsil. “اللوز”
• Pharyngeal tonsil. “اللحمية”
• Lingual tonsil.
What is the function of tonsils?
Quick defense mechanism through formation of antibodies against infective agents taken with food or air
What is the definition of palatine tonsil?
Two masses of lymphatic tissue embedded in the C.T. under the mucous membrane of oro-pharynx
What is the structure of palatine tonsil?
1- Non-keratinized stratified squamous: Cover its surface & dips into the underlying lymphatic tissue forming primary and secondary tonsillar crypts.
2- Lymphatic tissue: types:
- Lymphatic nodules → Primary or secondary.
- Diffuse lymphatic tissue: formed of lymphocytes and plasma cells. It occupies the areas between the lymphatic nodules.
3- Capsule: Dense C.T. separates the tonsils from the muscular wall of the pharynx.
What’s the definition of pharyngeal tonsil?
Single mass of lymphatic tissue present in nasopharynx.
What is the structure of pharyngeal tonsils?
Covered by pseudo-stratified columnar ciliated epithelium which forms folds. “Small folds not crypts”
What does hypertrophied pharyngeal tonsil result from and what is it called?
Hypertrophied pharyngeal tonsils resulting from chronic inflammation are called adenoids.
What is the definition of lingual tonsils?
They are small collections of lymphatic tissue in the C.T
What is the definition of the spleen?
The spleen is the largest single accumulation of lymphoid tissue in the body.
What is the structure of the spleen?
Stroma:
Capsule:
✓ Thick dense fibrous connective tissue capsule, containing smooth muscle fibers “To push blood in case of emergency” and penetrated by afferent lymphatics.
Trabeculae:
✓ Divide the spleen into lobules, containing smooth muscle fibers
Reticular network→ similar to that of lymph node.
Parenchyma: It is formed of splenic lobules.
Each splenic lobule includes:
- White pulp:
• White pulp is a lymphatic follicle with central artery which is eccentric in position.
• The white pulp may contain germinal center.
• They are scattered all over the spleen.
- Red pulp:
• It occupies the areas between the white pulps.
• The red pulp is formed of splenic cords and blood sinusoids. - Blood sinusoids: Irregular blood channels surrounded by macrophages. “For filtration”
- Splenic cords (Billroth cords): Irregular masses of lymphatic tissue mixed with blood cells (RBCs & WBC), fill spaces between the blood sinusoids.
What is the function of the spleen?
Humoral and cell mediated immunity.
The only lymphoid organ involved in filtration of blood.
It is also the main site of destruction of aged erythrocytes.
Storage of blood
Compare between the lymph node and spleen according to:
Number Capsules parenchyma Sinuses Function
Number: Numerous, widely distributed in the body - Single organ, in the abdomen.
Capsule: Dense C.T. - Dense C.T. with smooth muscles
Parenchyma:
Cortex → lymph follicles (no central artery).
Medulla → medullary cords
-
White pulp →lymph follicles (with central artery)
Red Pulp →blood & diffuse lymph tissue
Sinuses: Lymph sinuses (contain lymph) - Blood sinusoids (contain blood)
Function:
Filtration of lymph
Production of small
-
lymphocyte → humoral & cell mediated immunity
Filtration & storage of blood
Destruction of old RBCs
