Thymus

Question 1

What is the embrologic origin of the thymus?

Answer 1

third (occasionally fourth) pharyngeal pouch

Question 2

What age-related changes occur in the thymus?

Answer 2

• grows from birth to puberty
• reaches max size during puberty
• involutes, becomes replaced with fibrous/fatty tissue, following pubtery

*this involution is accompanied by decreased function

Question 3

What are causes of thymic involution other than age?

Answer 3

• HIV infection
• severe stress

Question 4

What are the functions of the thymus?

Answer 4

-T cell maturation and development of T cell central tolerance

Question 5

What developmental abnormalities of the thymus exist?

What is their significance if any?

Answer 5

Thymic hypoplasia (DiGeorge syndrome)

-del 22q11

-severe cell-mediated immunte deficits, accompanied by hypoparathyroidism

Thymic cysts:

• uncommon
• benign and frequently incidental findings
• thymic neoplasms can appear similar to cysts; should rule out if experiencing symptoms

Thymic hyperplasia:

-grossly normal size, microscopic hyperplasia of the follicle

-due to germinal centers containing B cells

-can occur in myasthenia gravis and other autoimmune disorders

Question 6

What is the most common primary cancer of the thymus?

Answer 6

thymoma (neoplasm of thymic epitheliual cells)

Question 7

What is the common presentation of a thymoma?

Answer 7

-adult >40 y/o; rare in children

• frequently found incidentally in work-up of autoimmune disease (myasthenia gravis)
• signs of impingment of mediastinal structures (40%)
• most commonly located in anterosuperior mediastinum