Immune System pt.1 (Primary Lymphoid Tissue) Flashcards
What type of organ is the thymus?
primary lymphoid organ
Where is the thymus located?
- in the neck
- extends into the thorax
What forms the framework of the thymus?
epithelial cells (instead of connective tissue!)
What forms the thymus?
epithelial cells seeded with developing lymphocytes from the bone marrow
What type of organ is the thymus?
lobed (comprised of lobes)
What in the thymus is covered by what? What do they form?
- lobes are covered by connective tissue capsule
- capsule extends into the lobes (septa) -> divides the parenchyma into lobules
What are the primary lymphoid organs?
- thymus
- bone marrow
What do primary lymphoid organs have in common?
the organs produce and mature immune system cells.
produces new cells (nothing else)
Where are new immune system cells produced?
bone marrow
What occurs in the thymus?
T-lymphocytes undergo maturation
What do secondary lymphoid organs have in common?
location in which immune responses occur
response against invadors occur
State examples of secondary lymphoid organs.
- lymph nodes
- spleen
- Bayer’s patches
What is the thymus surrounded by?
capsule of dense connective tissue
What regions do each thymus lobe have?
cortical region and medullary region
What surrounds the thymus lobules? Through what?
- blood vessels
- transverse the fine reticular connective tissue surrounding the lobules
What connective tissue surrounds the thymus lobules?
fine reticular connective tissue
Explain the ‘life span’ of the thymus.
- most prominent in young animals
- regresses later in life
What happens to the parenchyma of the involuted thymus?
it is replaced by adipose and connective tissue
State the two parts of the thymus lobules and where they are located.
outer= cortex
inner= medulla
State the difference in cell composition of the cortex and medulla.
- composition is the same!
cortex is darker as it is more concentrated in cells.
What cells are named after the thymus?
T-lymphocytes
What cells form the framework of the cortex of the thymus?
stellate epithelial reticular cells
State the composition of the thymus cortex.
- stellate epithelial reticular cells form the framework
- large, light-staining nucleus
- several long processes
- cells flatten at the periphery of the lobule
What do the processes of stellate epithelial reticular cells in the thymus do?
branch and connect to cell processes extending from other epithelial reticular cells
Describe what occurs at the periphery of the thymus lobules. What do these cells produce?
- epitheial reticular cells flatten
- form a sheet around the margin of the lobule
- produce thymosin, thymulin, thymic humoral factor and thymopoetin
- proteins influencing immunity and the maturation of lymphocytes
What proteins influence immunity and the maturation of lymphocytes? What produces them?
- thymosin, thymulin, thymic humoral factor and thymopoetin
- hormones!
- produced by flattened stellate epithelial reticular cells
What do epithelial cells typically do at the basement membrane? How does this compare to the thymus?
- typically line the basement membrane by layers
- reticular cells do not have a basement membrane
- do not have layers
- go by a 3-dimentional network
State the maturation process of T-lymphocytes.
- T-lymphocytes produced in the bone marrow
- travel through blood vessels
- mature in the thymus
What happens if there are no reticular cells?
no reticular cells –> no chemicals –> no development for t-lymphocytes –> serve no function
What occupies the space between the epithelial reticular cells (in the thymus)?
thymocytes (maturing T-lymphocytes)
Where do blast thymocytes migrate to? What do they do?
- migrate from bone marrow
- via blood vessels
- to cortex periphery
- undergo mitotic division
What happens as thymocytes continue to mature?
- they move from the outer thymic cortex towards the medulla
What happens to most thymic lymphocytes?
undergo apoptosis in the cortex
What percentage of thymic lymphocytes are actually released into the circulation? Where are they released? What happens with the rest?
- 1-3%
- released into the lumen of medullary blood vessels
- rest= apoptosis
What is interspersed among lymphocytes? What is their function?
- macrophages
- remove spent lymphocytes
- have remnants of aptotic cells in their cytoplasm
What is the name of the not-yet mature T-lymphocyte?
thymocyte
What direction do maturing lymphocytes travel in?
towards the medulla
How do medullary epithelial reticular cells compare to the cortical epithelial cells?
- larger
- form the framework in the medulla of the thymus
What is lighter staining and why?
- medulla is lighter staining than the cortex
- fewer small lymphocytes and macrophages fill the space of the framework
What is also found in the medulla? What is its function?
- unique thymic corpuscles (Hassall’s corpuscles)
- unknown function
State the composition of the Hassall’s corpuscles.
- degenerated central wall
- surrounded by layers of keratinized cells
- can be calcified
Why is the medulla less dense than the cortex?
- most T-lymphocytes are killed when migrating (apoptosis)
What are the different types of bone marrow?
red bone marrow
yellow bone marrow
What is red bone marrow? What does it do?
- highly cellular
- specialized to produce blood cells and platalets
State the ‘life span’ of the red bone marrow.
birth:
- principal source of blood cells
- found throughout the entire animal skeleton
adult:
- mostly limited to the sternum, ribs, vertebrae, skull, ilia and the ends of long bones
State the composition of yellow marrow.
- rich in adipose tissue
- occupies the remainted of the adult skeleton
What does bone marrow produce?
- erythrocytes
- immune system cells
What direction does bone marrow change with aging?
red –> yellow
Compare red and yellow bone marrow.
red bone marrow:
- lots of immune system cells
- erythrocytes in different development stages
yellow bone marrow:
- same composition yet much less concentrates
- high concentration of adipose tissue
State the composition of red bone marrow.
- extravascular hematopoietic tissue
- vascular sinusoids
- rich in blood cells in various stages of formation
- white blood cells in various development stages
- connective tissue cells
- supported by a reticular meshwork
What cells are present in red bone marrow?
- hematopoietic cells
- adipocytes (lipocytes)
- megakaryocytes (platalet production)
What is hematopoietic tissue composed of?
- many different blood cells in many different development stages
- reticular connective tissue
What type of capillaries does red bone marrow contain? What are they?
- vascular sinusoids
- type of capillary with holes in the wall
- allowing new bone marrow cells into the circulation
What type of cell series (Cell line) do cells undertake to turn from a morphologically indistinct stem cell to a specific mature blood cell.
erythroid (red blood cell) series
or
granulocytic (myeloid; white blood cell) series
What cell series do red blood cells undergo?
erythroid
What cell series do white blood cells undergo?
granulocytic (myeloid)
What is a proerythrocyte? Other name?
- large, round cell with a basophylic cytoplasm
- rubiblast
What does the proerythrocyte give rise to? How? Other name?
- multiple divisions
- basophyllic erythrocytes (prorubicytes)
What does the basophylic erythrocyte give rise to? How do they compare? Other name?
- polychromatophillic erythroblasts
- rubicytes
- smaller cells
- more condensed chromatin
What are the characteristics of orthochromatophilic erythroblasts (normoblasts, metarubricytes)?
nucleus:
- round
- highly condensed
- deeply stained
cytoplasm:
- eosinophylic
- shows slight blue tinges
What is the final step of erythrocyte maturation?
- nucleus is extruded
- anucleate reticulocyte is left
State the maturing erythrocyte names of cells in order.
proerythrocyte (rubriblast)
basophylic eyrthroblast (prorubricyte)
polychromatophillic erythroblast (rubricytes)
orthochromatiphillic erythrocyte (normoblast, metarubicyte)
anucleate reticulocyte
erythrocyte
How do the cells compare when an erythrocyte is maturing?
size: precursor cells are much larger!
nucleus: precursor cells have a much more highly concentrated nucleus
nucleus: nucleoli is lost during erythythrocyte development
What happens to the nucleus when the erythrocyte matures?
decreases in concentration
How do immature cells of the bone marrow look?
- large
- eurochromatic nucleus
- nucleoli
What form of cell predominates?
older cells > immature forms
