Thymus cortex vs Thymus medulla
How to tell Thymus cortex and Thymus medulla apart when a board question makes them look alike.
Cortex does positive selection; medulla does negative selection and has Hassall corpuscles.
Side by side
Thymus cortex is densely lymphocytic and is the site of positive selection of T cells recognizing self-MHC.
Thymus medulla is lighter, contains Hassall corpuscles, and mediates negative selection with AIRE-dependent self-antigen expression.
Thymus cortex is densely lymphocytic and is the site of positive selection of T cells recognizing self-MHC.
Thymus medulla is lighter, contains Hassall corpuscles, and mediates negative selection with AIRE-dependent self-antigen expression.
The repair move
Before naming the answer, force the stem through the separator: Cortex does positive selection; medulla does negative selection and has Hassall corpuscles.
Try a free question
CoreA histology slide of a 6-year-old child's thymus shows a densely packed lymphocytic outer region (with cortical thymic epithelial cells expressing MHC) where developing T cells undergo POSITIVE SELECTION (those whose TCRs bind self-MHC moderately survive). Which region of the thymus is this?
Show answer and explanation
Answer: A. Thymus CORTEX. Thymus CORTEX is the densely packed lymphocytic outer region where developing T cells (thymocytes) undergo POSITIVE SELECTION. Cortical thymic epithelial cells (cTECs) present self-MHC to double-positive (CD4+CD8+) thymocytes. Those with TCRs that bind self-MHC moderately survive; those that fail to bind die by neglect (death by apoptosis).
Why the other choices are tempting
- B. Thymus medulla is lighter staining and is the site of NEGATIVE selection (deletion of self-reactive T cells) with AIRE-dependent tissue-restricted antigen expression by medullary thymic epithelial cells (mTECs).
- C. Hassall corpuscles are unique structures in the thymic medulla (whorls of epithelial cells); function is partially understood but contribute to medullary mTEC and Treg development.
- D. Thymic capsule is the connective tissue covering.
- E. Adipose tissue infiltration occurs with thymic involution in adults.
Flashcards
Tap a card to flip it.
What clue supports Thymus cortex?
Choose Thymus cortex when the stem gives this pattern: Thymus cortex is densely lymphocytic and is the site of positive selection of T cells recognizing self-MHC.
What clue supports Thymus medulla?
Choose Thymus medulla when the stem gives this contrasting pattern: Thymus medulla is lighter, contains Hassall corpuscles, and mediates negative selection with AIRE-dependent self-antigen expression.
Quick answers
What is the fastest way to tell Thymus cortex from Thymus medulla?
Cortex does positive selection; medulla does negative selection and has Hassall corpuscles.
What is the key clue for Thymus cortex?
Thymus cortex is densely lymphocytic and is the site of positive selection of T cells recognizing self-MHC.
What is the key clue for Thymus medulla?
Thymus medulla is lighter, contains Hassall corpuscles, and mediates negative selection with AIRE-dependent self-antigen expression.