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SYSTEMIC PATHOLOGY - 1 > 17a – Thymus and Spleen > Flashcards

17a – Thymus and Spleen Flashcards

1
Q

Thymus

A
  • White to tan, lobulated organ within anterior mediastinum
  • Ruminants and pigs have a large cervical lobe that extends along the cervical trachea
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2
Q

Thymus structure

A
  • Composed of epithelial tissue and lymphoid tissue
  • Lobules
    o Cortex: immature T cells
    o Medulla: mature T cells
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3
Q

Thymus function

A
  • Necessary microenvironment for proliferation and maturation of T cells
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4
Q

Thymus changes from birth to puberty

A
  • Large at birth
  • Begins involution at or near puberty
    o Remains active for life
    o *Gradually replaced by loose connective tissue and fat
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5
Q

What are some miscellaneous diseases of the thymus?

A

1.Thymic aplasia/hypoplasia
2. Lymphocytolysis/thymic atrophy
*more susceptible to opportunistic pathogens and more severe infections in both cases
3. Thymic hemorrhage/hematoma

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6
Q

Thymic aplasia/hypoplasia

A
  • 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)
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7
Q

Some deficiencies involve failure of both T and B cells. What are the morphological changes?

A
  • Lymph node hypoplasia
  • Lack of splenic white pulp
  • Thymic hypoplasia
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8
Q

What is lymphocytolysis/thymic atrophy caused by?

A
  • Malnutrition
  • Drugs/toxins
  • Viral infections
  • *normal with advancing age
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9
Q

What does lymphocytolysis/thymic atrophy result in?

A
  • Varying degrees of ACQUIRED (secondary) immunodeficiency
    o Increased severity of infectious diseases
    o Increased susceptibility to opportunistic pathogens
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10
Q

Thymic hemorrhage/hematoma

A
  • In dogs, sudden death is occasionally seen due to HYPOVOLEMIC SHOCK resulting from MASSIVE thymic and mediastinal hemorrhage
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11
Q

What are some causes of thymic hemorrhage?

A
  • Trauma
  • Ruptured aortic aneurysms
  • Ingestion of anticoagulant rodenticide
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12
Q

What are the general features of a primary neoplasia of the thymus?

A
  • 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
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13
Q

Thymic lymphoma (T-cell neoplasma)

A
  • 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
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14
Q

Thymic lymphoma histologically

A
  • Sheet-like infiltrates of NEOPLASTIC LYMPHOCYTES
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15
Q

Thymoma

A
  • LESS common
  • Neoplastic proliferation of EPITHELIAL cells
  • Dogs, sheep, goats
  • Slow growing, encapsulated
  • Rarely metastasize
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16
Q

Thymoma histologically

A
  • Neoplastic proliferation of THYMIC EPITHELIAL ELEMETNS
    o Accompanied by varying amounts of non-neoplastic lymphoid tissue
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17
Q

In dogs and humans, thymoma can result in

A
  • Paraneoplastic syndrome of myasthenia gravis
    o Autoimmune attack of ACh receptors of the NMJ
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18
Q

In cats, thymoma may cause

A
  • Cutaneous paraneoplastic syndrome
    o Ex. feline thymoma-associated exfoliative dermatitis
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19
Q

Spleen

A
  • 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
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20
Q

Spleen: red pulp structure

A
  • Sinusoids/vascular spaces
  • Splenic cords
  • Supporting macrophages
  • Lymphocytes
  • Plasma cells
  • Blood cells
21
Q

Spleen: red pulp function

A
  • 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
22
Q

Spleen: white pulp structure

A
  • Periarterial lymphatic sheaths (PALS): T-cells
  • Lymphoid nodules: B-cells
  • Peripheral marginal zone: phagocytic macrophages and dendritic cells
23
Q

Spleen: white pulp function (secondary lymphoid organ)

A
  • Immune response
    o Production of B cells and plasma cells to produce Ab and memory lymphocytes
24
Q

Immune response in the spleen

A
  • 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)
25
Q

What are the 5 diseases of the spleen?

A
  • Miscellaneous diseases
  • Circulatory diseases of the spleen
  • Inflammation of spleen
  • Adaptations of growth
  • Primary and secondary splenic neoplasia
26
Q

What are the 5 types of miscellaneous diseases of the spleen?

A
  • Siderofibrosis (gamna-grandy bodies) of splenic capsule
  • Splenic amyloidosis
  • Splenic contraction
  • Splenic rupture
  • Splenic torsion
27
Q

Siderofibrosis=gamna-gandy bodies of splenic capsule

A
  • Incidental finding/senile change
    o Possible sequela of prior hemorrhage
28
Q

Siderofibrosis-gamnagandy bodies of splenic capsule GROSSLY

A
  • Granular white-yellow deposits within splenic capsule
29
Q

Siderofibrosis-gamnagandy bodies of splenic capsule HISTOLOGICALLY

A
  • Accumulations of
    o Hematoidin: yellow pigment
    o Hemosiderin: gold-brown pigment
    o Mineral deposits: blue
    o Fibrous tissue
  • *within affected capsule
30
Q

Splenic amyloidosis

A
  • Most common form of amyloidosis is secondary amyloidosis: CHRONIC
    o Deposition of amyloid protein AA (acute phase protein) secondary to chronic inflammation
31
Q

Splenic amyloidosis GROSSLY

A
  • Splenomegaly (not always)
  • Beige to orange discolouration
  • +/- firm prominent white pulp areas
32
Q

Splenic amyloidosis HISTOLOGICALLY

A
  • Deposition of amyloid around follicular arteries (detect with Congo red)
33
Q

Splenic contraction

A
  • Due to contraction of smooth muscle in capsule/trabecula
34
Q

What is splenic contraction induced by?

A
  • Catecholamine release
  • Circulatory shock
  • Acute splenic rupture
35
Q

What does splenic contraction look like GROSSLY?

A
  • Small dry spleen with wrinkling of the capsule
36
Q

What does incomplete splenic contraction look like GROSSLY?

A
  • 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
37
Q

Splenic rupture

A
  • Fairly common, often seen in dogs and cats
  • Primary: trauma
  • Secondary to splenomegaly OR splenic neoplasia which results in thinning of splenic capsule
38
Q

What is the result of splenic rupture?

A
  • Death by exsanguination OR healing by scarring
  • *potential sequelae include hemoabdomen and splenosis
39
Q

Splenic rupture leading to splenosis

A
  • Seed of splenic explants on peritoneal/serosal surfaces forming accessory spleens
40
Q

What do accessory spleens look like GROSSLY?

A
  • 1+ small red nodules within the omentum
  • *can be mistaken for hemangiosarcoma metastases
41
Q

What do accessory spleens look like HISTOLOGICALLY?

A
  • Look like small (normal) spleens
42
Q

Splenic torsion

A
  • 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
43
Q

Splenic torsion GROSSLY

A
  • Splenomegaly
  • Blue to black
  • Folded back on itself (C-shaped)
44
Q

What are some circulatory disturbances of the spleen?

A
  • Active hyperemia
  • Passive congestion
  • Splenic infarction
  • Splenic hematoma
45
Q

Active hyperemia of spleen is seen with

A
  • Acute systemic infection
  • Bacterial sepsis
46
Q

Passive congestion of spleen is caused by

A
  • *disturbances in systemic and portal circulation
    o Can be seen with shock=Vascular pooling
  • Barbiturate administration (ESPECIALLY horses and dogs)
  • Hemolytic anemia
47
Q

Acute hyperemia and passive congestion GROSSLY

A
  • Splenomegaly
  • Red-purple to black (increased amounts of unoxygenated blood)
  • *Oozes blood on cut surfaces
48
Q

Passive congestion of spleen HISTOLOGICALLY

A
  • Vascular spaces are dilated and contain erythrocytes
  • Germinal centers are WIDELY separated and trabeculae are THINNED
49
Q

Splenic congestion from barbiturate euthanasia in a horse leads to

A
  • Marked splenomegaly

SYSTEMIC PATHOLOGY - 1 (24 decks)

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