Cortex- Ectodermal Cells

Medulla- Endodermal Cells

Enter junction between medulla and cortex

move out to subcapsular zone (DN T Cells)

move in to cortex (DP T Cells) for positive selection

Move to medulla (DP T Cells) for negative selection

Thymus fails to develop

Express neither CD4 nor CD 8

Beginning to rearrange receptor genes:

1. Gamma, Delta, and Beta

Note: Beta has 4 chances to rearrange b/c 2 D segments on each chromosome (80% success)

Cell surface receptor on thymocytes that interacts with thymic epithelial cells

Keeps Thymocytes on T Cell development

Has an intra and extra cellular domain that get cleaved. Intra domain initiates transcription of chains.

Follows Double Negative Thymocyte

Expresses CD4 and CD8

Rearranges Gamma, Delta, and Alpha chains

Failures die in thymic cortex- 98%

Pre T Cell Receptor

First Checkpoint

When Beta is chosen over Gamma and Delta, it binds with a surrogate Alpha Chain, a CD3 Complex, and two Zeta chains

A Beta has FOUR chances

Alpha Chain Rearrangement

Second Checkpoint

Have MANY opportunities to be successful

Tries to find a match with an MHC molecule

Has 3-4 days

Not subject to allelic exclusion

Only involves Alpha:Beta Cells

Takes place in Cortex

Develop into Single Positive Thymocytes

Mediated by epithelial cells only

lack expression of MHC I or MHC II in early development because of a nonfunctional TAP

Cells become deficient in either CD4 or CD8

Eliminate T Cells that bind too strongly

Mediated by several types of cells (dendritic cells and macrophages are most important)

Involves non thymic proteins (AIRE)

Induces some epitheleal cells in medulla of thymus to express other outside proteins

In periphery of all people

Express CD25

Contacts MHC II and inactivates differentiation of CD4 T Cells that are self reactive

Kinase crucial in determining coreceptor

Stops expression of coreceptor not selected