The second phase of T-cell development

1.Positive selection: Selection for those cells whose T-cell receptors engage self-MHC.

2.Negative selection: Selection against those cells whose T-cell receptors bind too strongly to self-peptide/MHC combinations.

3.Lineage commitment: Commitment of thymocytes to effector cell lineages, including CD4 helper or CD8 cytotoxic populations

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Cork Cambium

Another meristematic tissue called cork cambium or phellogen develops, usually in the cortex region

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Cork Cambium

Phellogen cuts off cells on both sides. The outer cells differentiate into cork or phellem, while the inner cells differentiate into secondary cortex or phelloderm.

Phellogen, phellem, and phelloderm are collectively known as periderm

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Secondary Growth in Roots

In the dicot root, the vascular cambium is completely secondary in origin.

It originates from the tissue located just below the phloem bundles, a portion of pericycle tissue

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