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139 Cards in this Set

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1. Where is the thymus located

2. What does it look like

3. When is it largest
1. in the mediastinum of the thoracic inlet

2. soft multi-lobulated tissue

3. largest prior to puberty (replaced by fat as age)
even in the adult, the thymus continues to have ______ activity
endocrine
what is unique about nude mice
thymus is congenitally absent

no functional immune system
what organ has a function to prevent autoimmune disease
thymus

thymic stromal cells select T-cells for exclusion in order to provide self-tolerance
Premature involution of the _____ is associated with reduced immune fxn
thymus
describe the structure of the thymus
composed of 2 lobes covered by a thin CT capsule

numerous lobules incompletely divided by septa originating from the capsule
how does the thymus appear in cross section
thymic lobules appear to be distinct but are in fact interconnected (incomplete lobulation) with each other through bridges of parenchymal material.

lobules vary in size and are made up of a number of small nodules or follicles

lobules are clearly divided into dark staining cortex and a lighter medulla by the differential density of cells in each region.
describe thymic follicles
irregular in shape and fused together (esp toward interior)

each follicle consists of a medullary and cortical portion
why are thymic nodules have such distictly different staining of the cortex/ medulla
due to the differential density of cells

the cell types (thymocytes) are the same but the distribution is not equal

there are impt physiological differences between those in the cortex and those in the medulla
how is the thymus different than the lymph nodes
there are no germinal centers

it is not a site of antibody production
Understand the layout of cells within the thymus
thymus
thymus
understand the blood thymus barrier
Hassal's Corpuscles
Hassall's Corpuscle
Bursa of Fabricius
Bursa of Fabricius
Bursa of Fabricius
Bursa of Fabricius
Bursa of Fabricius
spleen
spleen
spleen
spleen
Spleen

A. Trabeculae
B. RBC
C. Smooth Muscle
spleen

periarteriolar lymphocyte sheath
spleen

a. trabeculum
b. periarteriolar lymphocyte sheath
c. capsule
lymph vessel

notice the 1 way valves
lymph vessel
understand the relationship between capillaries and lymphatic vessels
lymph node
lymph node
why are pigs screwed up (anatomically...)
hemal lymph node
you don't need to cut it out during surgery/necropsy and send it for histopath b/c you will make UT look like a sub-par vet school
MALT (mucousa associated lymphoid tissue)

peyer's patch in ileum
parasite/ MALT
MALT (mucousa associated lymphoid tissue)
MALT (mucousa associated lymphoid tissue)
MALT (mucousa associated lymphoid tissue)
BALT
BALT
BALT
What negative consequence can be associated with BALT?
Diffusely lines all airways

Lots of lymphoid hyperplasia around the airways can result in difficult breathing.
Where is BALT concentrated
concentrated at branch points b/c that is where particles build up


lymph follicles (nodules) with GCs
What are 2 examples of GALT and describe them
Tonsils (palatine)
- deep crypts
- stratified squamous epithelium
- lymphoid nodules with germinal centers

Peyer's Patch (ileum)
- lymphoid domes, nodules, germinal centers
- intraepithelial lymphocytes
- M cells
what makes tonsilar epithelium unique
most squamous epithelium has tight jxns to keep things from getting across, but this has crypts

this allow some things to get across
GALT primarily produces what antibody
IgA
what makes M cells unique
M cells (microfold cells)

have invaginations that penetrate up into the epithelium and allow lymphocytes up into this area (so easier to detect antigen...or screw up your gut if you carry the HLA-DQ2, HLA-DQ8, or the MY09B gene)
describe thymic blood supply
blood vessels enter though capsule

travel ialong septa to the corticomedullary border & enter the parnechyma

arterioles entering the thymus send capillaries to the cortex, which branch at the periphery and return

at corticomedullary jxn, postcapillary venules are found (represent specialized sites of transit of mature lymphocytes into the blood)
how is thymic stroma different than many other organs
it is cellular in nature rather than fibrillar

epithelio-reticular cells are branching (tree-like) in shape & put forth processes supported by intracellular cytoskeletal elements

these processes come into contact with each other and are held together at points of contact (desmosomes)

the process form a completely cellular scaffolding (cytoreticulum) which distinguishes it from other, truly fibrillar parenchyma of other organs
the thymic cortex is composed of
lymphoid cells supported by a network of finely branced epithelial reticular cells which is continuous with a similar network in the medullary portion

the network of epithelial cells connects with blood vessels
where are the largest lymphocytes located in the thymic cortex
in the periphery of the cortex
where do the earliest events in thymocyte development occur in the thymus?
in the cortex
where does T cell receptor rearrangement & postive selection take place in the thymus
in the cortex
thymocytes that reach the _____ have already successfully undergone T cell receptor gene rearrangement & positive selection & have been exposed to a limited degree of negetave selection
medulla
what area of the thymus is specialized to allow thymocytes to undergo additional rounds of negative selection to remove auto-reactive T cells
the medulla
name the 3 principal cell types in the thymus
thymocytes (lymphocytes of the thymus)

few macrophages

specialized epithelio-reticular cells (stellate cells) which provide supporting framework (stroma) for the parenchyma
most of the lymphocytes in the thymus are _______ (B or T cells)
T cells
in order to acqiure immunocompetence, what must happen to lymphocytes
they must reside in the thymus for a period of time
how do you know whether you are looking at a macrophage or thymocyte?
macrophages are larger with cacuolar nuclei

frequently have coarse granules of undigested material in their cytoplasm
describe epithelio-reticular cells
obscured by the lymphocytes that sit on them

form a physical, cellular boundary b/w the parenchymal thymocytes and the rest of the body

@ periphery of cortex, subjacent to teh capsule and around all blood vessels, the processes form a continuous cellular layer constitutin the blood thymus barrier
- prevents exposure of immature lymphocytes to blood borne antigens
what forms a physical, cellular boundary b/w the parenchymal thymocytes and the rest of the body?
epithelio-reticular cells
what prevents exposure of immature lymphocytes to blood borne antigens
epithelio-reticular cells in concert with the lining epithelium of the capillaries
what forms the blood-thymus barrier

where is it
the processes of the epithelio-reticular cells

at periphery of the cortex, subjacent to the capsule & around all blood vessels
hassall's corpuscles
degenerate epithelio-reticular cells

appear as concentric eosinophilic whorls of materials

no function

each consists of a concentraic mass of epithelial cells joined by desmosomes and containing intracellular bundles of filaments
where to B cells get their name from
bursa of fabricius
- the site of hematopoiesis in birds
- particularly the source of B cells
describe the bursa of fabricius
unique to birds

site of hematopoiesis

source of B cells

in mammals, bone marrow does this

located in the caudal dorsal part of cloaca as a diverticulum (atrophies with age like thymus)

has blind, crypt-like invaginations

appears like the thymus

unlike tonsil, overlying epithelium is columnar (not squamous) and there are no germinal cells
is the bursa of fabricius more like the thymus or tonsil
looks similar to thymus & atrophies with age

unlike tonsil
- has columnar (not squamous) overlying epithelium
- no germinal centers
infectious bursal disease
viral dz that destroys the bursa
- reduces immune fxn
- increases susceptibility to other infectious dz
is the spleen part of the circulatory system or the lymphatic system
yes




(it is though of as part of the circ sys but traditionally described with lymphatic organs b/c large pop of lymphocytes so significant immune fxn)
what is the pulp of the spleen?
the parenchyma

most of it is soft, dark red masss (red pulp)
- large, irregular, thin-walled blood vessels (splenic sinusoids) interposed b/w sheets and strands of reticular CT (spenic cords)
- red b/c of blood

the balance of the spleen is composed of lymphoid tissue (white pulp)
- small irregularly sized pale yellow to white nodules & cords
describe the splenic capsule
composed mostly of collagenous and elastic CT interspersed with smooth muscle (responsible for splenic contraction)

tear or rupture in the capsule results in significant internal hemorrhaging

the capsule sends septa (trabecula) part way into the interior andfrom these arise a delicate network of reticular fibers, to from a stromal network on which the lymphocytes rest
what is one impt fxn of the spleen (esp significant for racing horses and marine mammals)
act as a reservoir for erythrocytes

the contractionj of the capsule expels these into circulation when extra oxygen carrying capacity is needed
how does the spleen differ from lymph nodes (as far as its lymphatic fxn goes)
it is inserted directly into the blood stream
where in the spleen does storage and blood filtration occur
in the red pulp (composed of sinuses, splenic cords, and the marginal zone)
why does the spleen become enlarged when under some general anesthetics?
relaxation of muscle tone
describe the spleen's hematopoietic fxns
impt during gestation

after birth (in mammals) stops erythropoietic fxn except when extra erythropoietic production is needed (extramedullary hematopoiesis)
describe the spleen's lymphoid fxns
has significant reticuloendothelial fxns

produces lymphocytes and antibody
what is the spleen's most impt fxn?
acts as a vascular filter (clearing blood of aged and diseased cells and foreign particles

is the site of immune rxns to blood-borne antigens

antibodies are produced and antibody coated blood cells are removed from circulation by the spleen

immune fxns occur in white pulp (composed of periarteriolar lymphocyte sheath which are rich in T cells as well as lymphoid follicles and germinal centers which are rich in B cells)
what is the fxn of the white pulp, what is it composed of?


what is the fxn of the red pulp, what is it composed of?
White Pulp = immune fxns

composed of
- PALS (rich in T cells)
- lymphoid follicles & germinal centers (rich in B cells)


Red pulp = storage & blood filtration

composed of
- sinuses
- splenic cords
- marginal zone
why is the spleen a common site of infection/ cancer/ emboli/ etc.
b/c of its fxn of storage and filtration, plus the reticuloendothelial fxns
______ are the first part of the arterial input inot the spleen
central arteries

branches of splenic artery enter via the capsule & divide into trabecular arteries

as these leve septum and enter the interior of the spleen, it immediately acquires a continuous coating of lymphocytes and is thus called central arteries (the sleeve of lymphocyte is the PALS)
_____ collectively constitute what is called the "white pulp" of the spleen
all the PALS
Where in the spleen can the special conditions required for proliferation of B lymphocytes be met
the PALS (white pulp)

will see germinal centers at intervals (number and size of them will vary based on the immune state of the animal)
______ separates the red and white pulp
the marginal zone
what happens to the central artery once it reaches the marginal zone
divide into smaller vessels

fine branches of central artery (penicillar arteries - characterized by cuboidal epi) branch again to form arterial capillaries

as they exit white pul, are surrounded by a sheath of phagocytic cells and reticular fibers (now called sheathed arteries)

may empty the blood iirectly into splenic sinusoids (closed circulation) or into reticular CT of splenic cords (open circulation)
the white pulp surrounds ________ as a periarterial lymphoid sheath
the central arteries
describe the PALS
has a diffuse framework of reticular fibers

resident lymphocytes of the diffuse region are supported by it

mostly T lymphocytes with less numbers of macrophages and plasma cells

nymph nodules and germinal centers (Malpighian bodies) formed by B lymphocytes are present along the course of the central arteries

germinal cneters are clones of B lymphocytes that are isolated so that these cells can develop properly

consequently the stroma in GCs isnt' fibrillar in nature (similar to the thymus)
Does red or white pulp constitute teh bulk of splenic volume
red
describe the red pulp
constitutes the bulk of splenic volume

supporting reticulum of red pulp is continuous with the septa (consist of reticular fibers invested by reticulocytes)

arranged into anatomizing splenic cords separated by venous sinuses

sinuses are blood spaces lined with endothelium and contain circulating blood cells

cords are the masses of cells in between sinuses containing erythrocytes & many other cell types (esp macrophages)

phagocytosis of aged erythrocytes occurs in cords (hemosiderin present)
where in the spleen does phagocytosis of aged erythrocytes occur?
the cords of the red pulp
where in the spleen is hemosiderin present
the cords of the red pulp (b/c that's where phagocytosis of aged RBC occurs)
how are sheathed capillaries different from other blood vessels
most blood vessels are lined by simple squamous epithelium

sheathed capillaries have endothelium composed of fusiform cells oriented paralell to the long axis

surrounded by a sheath of reticulocytes and macrophages bound together by reticular fibers

the cells of the walls are phagocytic
true or false

all capillaries arising from the penicillar arteries become sheathed capillaries
false
what cell types are the main resident population of the splenic cords
macrophages & reticular cells
blood cells emptied into the splenic cords re-enter the blood vessels through ________
the endothelium of the sinusoids
describe the endothelial cells of the splenic sinusoids
elongated (in cross section, may appear cuboidal) oriented along the long axis of the sinusoids

no jxnal complexes

incomplete basement membrane (forming narrow circular bands around the endothelial cells with large intervening fenestrations)
macrophages ingest aged erythrocytes, platelets, and other matter as they pass through _______
the splenic cords
true or false

cells of the blood can move from a splenic sinus to an adjacent cord and vice versa
true
true or false

the endothelium lining the splenic sinusoids is phagocytic
true
why do lymph nodes become enlarged?
b/c of proliferation of lymphocytes (mostly B cells)
name 2 common sites for metastatic cancers to be found
lymph nodes

spleen
what is interstitial fluid
lymph once it enters a lymph capillary
most interstitialfluid returns via ______
venous circulation

1% becomes lymph and passes through 1 or more LN before returning to venous circulation
describe high-endothelial vessels
in deep cortex of lymph nodes

low cuboidal epithelium

lymphocytes which reach the lymph node via blood stream migrate through this epi as part of their recirculation
what 2 things result in edema
excessive lymph production

failure to flow
describe lymph vessels
simple endothelial lined tubes that do no branch or anastomose

at their periphery, the basement membrane is incomplete and lacks pericytes

can be difficult to see histologically

larger vessels have complete basement membranes, pericytes, and some smooth muscle
what is the medulla made up of
meduallary cords of lymphatic tissue
- plasma cells
- macrophages
- B cells

medullary sinuses
- vessel like spaces separating the cords
- lymph flows into the sinuses from cortical sinuses and into efferent lymphatic vessels
describe a reactive germinal center
dark and light zones

dark
- mostly T cells & proliferating B cells

light
- where B cells mature
what is the importance of the paracortical zone
where T cells mainly interact with dendritic cells and macrophages to promote the immune response
as lymph moves through the __a____ & ___b____, antigens are detected by and processed by _____c____ & ____d____
a. subcapsular
b. cortical sinuses

c. macrophages
d. lymphocytes
describe the main cell types of the cortex
outer cortex
- mainly B cells arranged as follicles (may develop germinal center)

deeper cortex
- mainly T cells
how are pig LN different
lymph nodules (including germinal centers) are located in the medulla and medullary cords are located in the cortex
where do you find hemal LN
in ruminants

retroperitoneal along vertebral column

with some visceral organs

along the jugular furrow
describe a hemal LN
strx similar to LN but with some characteristics of the spleen (capsule and trabecula contain smooth muscle and sinuses are filled with blood rather than lymph)
diffuse vs nodular lymphatic tissue
diffuse
- unorganized collection of lymphocytes
- usually assoc with all mucosal surfaces (even skin)
- density of lymphocytes assoc with the degree of antigenic stimulation &/or infection (so mine are currently over-run)
- lymphatic tissue outside of specifically lymphatic organs is, by definition, diffuse
- lymphoid organs (spleen/ LN) also have diffuse lymphatic tissue located b/w more defined strx (nodules)

nodular tissue
- always occurs w/in a background of diffuse lymphatic tissue (even spleen & LN)
- typical ex of nodular lymphatic tissue is germinal center (highly ordered collection of B cells found in some lymphoid organs)

when diffuse lymphatic tissues associated with mucosal surfaces become particularly dens but is not encapsulated, they can become so well defined to be recognized as a specific strx (tonsil/ peyer's patch)
______ protects the body against pathogens which may enter the body via the mucosa
MALT
give an example of MALT
tonsil

in resp airway
MALT produces ____ which can be taken up by epithelial cells and released onto epithelial surface
IgA
what are tonsils
accumulations of llymphoid tissue surrounding the openings of the digestive and resp tracts

palatine tonsils are ofspecial significance and are located in teh lateral wall of the oropharynx and covered by strat squam epi
- surrounded by thick hemicapsule of CT (not a capsule) which delimits them from the pharyngeal muscle and facilitates their removal
why do deep crypts help the tonsils fxn
exposure to antigens relies on contact of antigens with cells of the immune system across the epithelium

the deep crypts result in increase of surface area to facilitate this

the epithelium may specialize in places to form open meshwork of cells with an incomplete basal lamina (reticulated epi) which allows infiltration of epi by lymphocytes and macrophages
tonsilar lymphoid nodules consist mainly of

other areas are occupied by
B lymphocytes


T lymphocytes, activated B lymphocytes (germinal centers), and other cells of the immune system
where do the most prominent accumulations of diffuse lymphocytes or activated lymph follicles of the GI sys occur
in the ileum & the appendix in the form of peyer's patches


dome shaped protrusions into the lumen of the ileum (germinal centers deep in submucosa)
describe the epithelium of peyer's patch domes
appears columnar and contains cells with deeply invaginated basal surfaces (M cells)

immune system cells can enter these invaginations (intraepithelial pockets) where they are exposed to materials which have been endocytosed by the epithelial cells and then released into the invaginations

goblet cells are rare or absent in the epithelium which covers the domes