Bone Marrow 2

Question 1

How does the thymus develop as an animal ages?

Answer 1

The thymus is at its maximal mass at birth and regresses (involutes) with age

Question 2

Why is the thymus considered a “composite” organ?

Answer 2

Because it contains both lymphoid and epithelial tissues

Question 3

What does the lymphoid component of the thymus do?

Answer 3

It matures T-cells (few T-cells survive this process since the thymus is picky)

Question 4

What does the epithelial component of the thymus do?

Answer 4

It supports the development of T-cells

Question 5

What two diseases can a deficiency of the thymus result in?

Answer 5

Immunodeficiency or autoimmunity

Question 6

What are the disorders of a “too-small” thymus (three)?

Answer 6

• Severe Combined Immunodeficiency Disorder (SCID) (lymphocytes never populated the thymus to begin with)
• Involution (increased loss)
• Lymphoid atrophy (increased loss)

Question 7

What are the disorders of a “too-large” thymus?

Answer 7

Neoplasia (thymoma or lymphoma)

Question 8

What is Severe Combined Immunodeficiency Disorder (SCID)?

Answer 8

A genetic defect in the common lymphoid stem cell line leading to defective cell-mediated and humoral immune responses

(I think basically this is where you don’t make functional T-cells and B-cells, therefore your adaptive immune response is garbage)

Question 9

Which animals can get SCID?

Answer 9

Dogs, mice, humans, and horses

Question 10

What horse breeds commonly get SCID?

Answer 10

Arabian horses

Question 11

What are the classic necropsy findings in SCID?

Answer 11

Bronchopneumonia in combination with an extremely small thymus, lymph nodes, and spleen

Question 12

What does thymic atrophy result in?

Answer 12

An acquired immunodeficiency

Question 13

Which viruses can cause thymic atrophy in dogs, cats, cows, and horses?

Answer 13

Dogs: Parvovirus and distemper virus

Cats: Panleukopenia virus

Cows: BVDV

Horses: EHV-1 in aborted foals

Thymic atrophy in puppies and kittens should steer you towards parvovirus or panleukopenia

Question 14

What are four non-viral causes of thymic atrophy?

Answer 14

• Malnutrition*
• Toxins
• Chemotherapeutic agents
• Stress

Question 15

What are the main differences between thymic lymphoma and thymomas?

Answer 15

Thymic lymphoma

• Tumour composed of malignant T-cells
• Aggressive, seen in younger animals
• More often seen in cats and cows (cows is associated with sporadic form, not BLV. In cats it’s more FeLV associated)

Thymoma

• Tumour derived from the epithelial components
• Uncommon
• Occurs in older animals
• Usually benign
• Can cause autoimmune diseases (ex. myasthenia gravis)

Question 16

How can you differentiate between thymic lymphomas and thymomas grossly?

Answer 16

Trick question - you can’t really tell from gross examination. You need to use the signalment, clinical signs, and histopathology to differentiate

Question 17

If you are suspicious of a lymph node pathology, what should you check first?

Answer 17

You should always trace it back to the site of lymph drainage

Question 18

What are the disorders of a “too-small” lymph node (five)?

Answer 18

• SCID
• Aging
• Lack of antigenic stimulus
• Viral (BVD, CDV)
• Cachexia and malnutrition

Question 19

What is the term for a large lymph node?

Answer 19

Lymphadenopathy or lymphadenomegaly

Question 20

What are the disorders of a “too-large” lymph node (five)?

Answer 20

• Reactive lymphoid hyperplasia (ex. bacterial bronchopneumonia)
• Acute lymphadenitis (ex. strangles)
• Chronic lymphadenitis
• Primary neoplasia (ex. lymphoma)
• Metastatic neoplasia (ex. metastatic melanoma)

Question 21

How can you tell the difference between reactive lymphoid hyperplasia and neoplasia?

Answer 21

With reactive lymphoid hyperplasia you should still see a distinct cortex and medulla. In neoplasia, you usually lose all distinguishing characteristics

Question 22

What are the disorders of a “too-small” spleen?

Answer 22

Blood related:

• Contraction from excitement or shock

Lymphocyte related:

• Cachexia
• Aging
• Splenic lymphoid hypoplasia (due to SCID)

Question 23

What are the disorders of a “too-large” spleen (four)?

Answer 23

• Bloody spleen (uniform splenomegaly with blood)
• Meaty spleen (uniform splenomegaly without blood)
• Splenic nodules with bloody consistency
• Splenic nodules with firm consistency

Question 24

Name three differentials for uniform splenomegaly with blood

Answer 24

1. Congestion (due to torsion or barbiturate administration)
2. Acute hemolytic anemia
3. Acute hyperemia (caused by acute septicemia, ex. anthrax)

Question 25

What does uniform splenomegaly with blood look like grossly?

Answer 25

These spleens are big, dark red to blue-black, and ooze blood on section