17a – Thymus and Spleen Flashcards
Thymus
- White to tan, lobulated organ within anterior mediastinum
- Ruminants and pigs have a large cervical lobe that extends along the cervical trachea
Thymus structure
- Composed of epithelial tissue and lymphoid tissue
- Lobules
o Cortex: immature T cells
o Medulla: mature T cells
Thymus function
- Necessary microenvironment for proliferation and maturation of T cells
Thymus changes from birth to puberty
- Large at birth
- Begins involution at or near puberty
o Remains active for life
o *Gradually replaced by loose connective tissue and fat
What are some miscellaneous diseases of the thymus?
1.Thymic aplasia/hypoplasia
2. Lymphocytolysis/thymic atrophy
*more susceptible to opportunistic pathogens and more severe infections in both cases
3. Thymic hemorrhage/hematoma
Thymic aplasia/hypoplasia
- Loss or functional impairment of T cells and impaired cell-mediated immunity
- Congenital immunodeficiency
o Severe combined immunodeficiency (SCID)
o Foals, mice, dogs (Jack Russell Terriers, Basset Hounds)
Some deficiencies involve failure of both T and B cells. What are the morphological changes?
- Lymph node hypoplasia
- Lack of splenic white pulp
- Thymic hypoplasia
What is lymphocytolysis/thymic atrophy caused by?
- Malnutrition
- Drugs/toxins
- Viral infections
- *normal with advancing age
What does lymphocytolysis/thymic atrophy result in?
- Varying degrees of ACQUIRED (secondary) immunodeficiency
o Increased severity of infectious diseases
o Increased susceptibility to opportunistic pathogens
Thymic hemorrhage/hematoma
- In dogs, sudden death is occasionally seen due to HYPOVOLEMIC SHOCK resulting from MASSIVE thymic and mediastinal hemorrhage
What are some causes of thymic hemorrhage?
- Trauma
- Ruptured aortic aneurysms
- Ingestion of anticoagulant rodenticide
What are the general features of a primary neoplasia of the thymus?
- Space occupying mass in cranial mediastinum
- Dyspnea
- *thymic (mediastinal) lymphoma: thoracic effusion is common=thoracic aspirates are often used for diagnosis
- 2 main differentials
o Thymic lymphoma
o Thymoma
Thymic lymphoma (T-cell neoplasma)
- Neoplastic proliferation of T-lymphocytes
- Often younger animals (cats, calves, dogs)
o Cats: wide age distribution, young cats=FeLV
o Cattle: yearlings, usually beef, no KNOWN viral association - Malignant behaviour
Thymic lymphoma histologically
- Sheet-like infiltrates of NEOPLASTIC LYMPHOCYTES
Thymoma
- LESS common
- Neoplastic proliferation of EPITHELIAL cells
- Dogs, sheep, goats
- Slow growing, encapsulated
- Rarely metastasize
Thymoma histologically
- Neoplastic proliferation of THYMIC EPITHELIAL ELEMETNS
o Accompanied by varying amounts of non-neoplastic lymphoid tissue
In dogs and humans, thymoma can result in
- Paraneoplastic syndrome of myasthenia gravis
o Autoimmune attack of ACh receptors of the NMJ
In cats, thymoma may cause
- Cutaneous paraneoplastic syndrome
o Ex. feline thymoma-associated exfoliative dermatitis
Spleen
- Present in left cranial part of abdomen within the greater omentum
- Attached to greater curvature of stomach
- Covered by a fibromuscular capsule
- Parenchyma is incompletely dissected by fibromuscular trabeculae
- Varies in size and shape among species
Spleen: red pulp structure
- Sinusoids/vascular spaces
- Splenic cords
- Supporting macrophages
- Lymphocytes
- Plasma cells
- Blood cells
Spleen: red pulp function
- Filters blood: removal of foreign material (phagocytosis)
- RBC storage (ex. horses)
o Spleen size can decrease by contraction of capsule to release stored blood into circulation in response to hypovolemia or E stimulation - Hematopoiesis (EMH) under certain circumstances in an adult
o Normal in young and rodents+mink adults
Spleen: white pulp structure
- Periarterial lymphatic sheaths (PALS): T-cells
- Lymphoid nodules: B-cells
- Peripheral marginal zone: phagocytic macrophages and dendritic cells
Spleen: white pulp function (secondary lymphoid organ)
- Immune response
o Production of B cells and plasma cells to produce Ab and memory lymphocytes
Immune response in the spleen
- Starts in red pulp where macrophages/dendritic cells trap and process blood borne particles/viruses/bacteria/protozoa and PRESENT them as processed antigen
- T and B cells
- Production of Ab, memory cells, etc
- *splenectomised animals are more susceptible to hemoparasites (ex. Mycoplasma haemofelis)
What are the 5 diseases of the spleen?
- Miscellaneous diseases
- Circulatory diseases of the spleen
- Inflammation of spleen
- Adaptations of growth
- Primary and secondary splenic neoplasia
What are the 5 types of miscellaneous diseases of the spleen?
- Siderofibrosis (gamna-grandy bodies) of splenic capsule
- Splenic amyloidosis
- Splenic contraction
- Splenic rupture
- Splenic torsion
Siderofibrosis=gamna-gandy bodies of splenic capsule
- Incidental finding/senile change
o Possible sequela of prior hemorrhage
Siderofibrosis-gamnagandy bodies of splenic capsule GROSSLY
- Granular white-yellow deposits within splenic capsule
Siderofibrosis-gamnagandy bodies of splenic capsule HISTOLOGICALLY
- Accumulations of
o Hematoidin: yellow pigment
o Hemosiderin: gold-brown pigment
o Mineral deposits: blue
o Fibrous tissue - *within affected capsule
Splenic amyloidosis
- Most common form of amyloidosis is secondary amyloidosis: CHRONIC
o Deposition of amyloid protein AA (acute phase protein) secondary to chronic inflammation
Splenic amyloidosis GROSSLY
- Splenomegaly (not always)
- Beige to orange discolouration
- +/- firm prominent white pulp areas
Splenic amyloidosis HISTOLOGICALLY
- Deposition of amyloid around follicular arteries (detect with Congo red)
Splenic contraction
- Due to contraction of smooth muscle in capsule/trabecula
What is splenic contraction induced by?
- Catecholamine release
- Circulatory shock
- Acute splenic rupture
What does splenic contraction look like GROSSLY?
- Small dry spleen with wrinkling of the capsule
What does incomplete splenic contraction look like GROSSLY?
- Numerous dark red to black, raised, soft, blood filled areas of various size with intervening areas of depression
- *may be indistinguishable from acute splenic infarcts
Splenic rupture
- Fairly common, often seen in dogs and cats
- Primary: trauma
- Secondary to splenomegaly OR splenic neoplasia which results in thinning of splenic capsule
What is the result of splenic rupture?
- Death by exsanguination OR healing by scarring
- *potential sequelae include hemoabdomen and splenosis
Splenic rupture leading to splenosis
- Seed of splenic explants on peritoneal/serosal surfaces forming accessory spleens
What do accessory spleens look like GROSSLY?
- 1+ small red nodules within the omentum
- *can be mistaken for hemangiosarcoma metastases
What do accessory spleens look like HISTOLOGICALLY?
- Look like small (normal) spleens
Splenic torsion
- Dogs and pigs
- With or without torsion of the stomach (GDV)
- If whole spleen twists around the gastrosplenic ligament=
o Severe congestion and hemorrhagic (venous) infarction due to occlusion of splenic vein
o *may lead to hemodynamic shock
Splenic torsion GROSSLY
- Splenomegaly
- Blue to black
- Folded back on itself (C-shaped)
What are some circulatory disturbances of the spleen?
- Active hyperemia
- Passive congestion
- Splenic infarction
- Splenic hematoma
Active hyperemia of spleen is seen with
- Acute systemic infection
- Bacterial sepsis
Passive congestion of spleen is caused by
- *disturbances in systemic and portal circulation
o Can be seen with shock=Vascular pooling - Barbiturate administration (ESPECIALLY horses and dogs)
- Hemolytic anemia
Acute hyperemia and passive congestion GROSSLY
- Splenomegaly
- Red-purple to black (increased amounts of unoxygenated blood)
- *Oozes blood on cut surfaces
Passive congestion of spleen HISTOLOGICALLY
- Vascular spaces are dilated and contain erythrocytes
- Germinal centers are WIDELY separated and trabeculae are THINNED
Splenic congestion from barbiturate euthanasia in a horse leads to
- Marked splenomegaly
