Development/Microanatomy of lymphoid organs Flashcards
Type of lymphatic tissues
- Peripheral Tissue- Peyers patches, appendix
- Diffuse lymphatic tissue- cells scattered in the connective tissue.
- Seen in digestive tract, skin, respiratory system - Organized lymphatic tissue
- Encapsulated tissues/organs
- Seen in thymus, spleen, lymph nodes
Cells of the lymph system
- Fixed cells
- Outer epithelial reticular cells (collagen III)- provide a netlike structure to give the organ some shape and structure - Migratory cells
- Lymphocytes (b cells, T cells, NK cells)
- B cells require specific antigens
- T cells and NK cells- don’t need to have a specific antigen
Prenatal blood formation
- 1st 3rd of pregnancy: Yolk sac- initial seeder of stem cells. These stem cells will end up in liver and spleen
- 2nd 3rd of pregnancy: Liver is primary expansion organ for stem cells. Spleen is also participating although minimally. Stem cells will go to bone marrow.
- Last 3rd of pregnancy: Bone marrow conducts further stem cell development. Some will stay and become B-cells. Others will go to thymus and become T-cells
Primary lymphoid tissue
- Allow cells to develop and then train them to some sort of degree for specifics. Constantly checked and if criteria is not met (will be ~90%) they will be apoptotically dealt with by macrophages.
- Bone marrow and Thymus
Thymus
- Comes from pharyngeal pouches (stratified squamous epithelium)
- Will be larger earlier in life and produce a lot of lymphocytes but will reduce in size as the animal ages. Will still produce some lymphocytes but will be mostly filled with fat.
- Made up of lobes of lobules. Lobules have medulla and cortex
Parts of thymus
Medulla
- Contains a lot less lymphocytes (because as they mature, lymphocytes move towards medulla and more and more get destroyed for not meeting criteria)
- Contains macrophages for destroying lymphocytes
- Also has Hassel’s corpuscles- somewhat unknown function. Could be involved with criteria detection of lymphocytes or assist in lymphocyte functionality
Cortex
- Densely filled with most immature lymphocytes; the lymphocytes will move towards the medulla region as they mature
Blood-thymus barrier- epithelial reticular cells and lymphocytes surround blood capillaries
Importance of the blood-thymus barrier
Limits what is actually exposed to the developing lymphocytes which ensures that there is specific lymphocyte later on. Don’t want them marking self for destruction
Thymus-immunological function
- Medulla region of thymus contains macrophages that scan developing lymphocytes and ensures that they meet specific criteria. If they don’t then they will be destroyed.
- 90% of lymphocytes are destroyed and never leave the thymus
Hemopoiesis or hematopoiesis location
- Red marrow- hemopoietically active
- Yellow marrow- fat containing component
- Small cells- islets/cords with vascular sinuses that allow for exchange between the hemopoietic and circulation compartments
Purpose of secondary lymphatic organs
- Respond to infections
- “Percolators” located at strategic locations
Lymph nodes general structure (most domestics)
- Afferent vessels- lymph enters near cortex
- Nodules arranged peripherally in cortex
- Medullary region in middle
- Encapsulated
- B-lymphocytes most common
- Efferent vessels- located at hilus; lymph exit
- Subcapsular sinus
Lymph nodes general structure (pig)
- Afferent vessels- enter at hilus
- Efferent vessels- exit at cortex
- Nodules in center
- Smaller sinus’
Paracortex cell type
More T cells
Nodules of secondary lymphatic organs
Three parts:
1. Mantle- original naïve cells (located close to capsule outer edge)
- Light zone- more mature cells (part of germinal center)
- Dark zone- proliferating cells (part of germinal center)
Lymphocyte movement
- Move from tissues into the lymph node through the afferent lymphatic vessels
- Move through the lymph node (cortex to medulla)
- Exit lymph node through efferent lymphatic vessels, travel around the body to the thoracic duct when lymphocytes end up in veins entering the heart and continue through arteries where they can leak out into other tissues.
- Will eventually end up back at the post-capillary venules inside the lymph nodes which are leaky and allow the lymphocytes escape blood and be back in lymphatics
