There are three pathways for the synthesis of JAs, including the octadecane pathway starting from α-linolenic acid (18:3) and the hexadecane pathway starting from hexadecatrienoic acid (16:3). 

All three pathways require three reaction sites: the chloroplast, peroxisome, and cytoplasm. 

TLS Online TPP Program

#Id: 7219

#Unit 5. Developmental Biology

T-Cell Development

Commitment of hematopoietic precursors to the T cell lineage

The initiation of antigen receptor gene rearrangements

The selection and expansion of cells that have successfully rearranged one of their T-cell receptor genes (-selection). 

TLS Online TPP Program

#Id: 7220

#Unit 5. Developmental Biology

TLS Online TPP Program

#Id: 7221

#Unit 5. Developmental Biology

Time course of appearance of  thymocytes and  thymocytes during mouse fetal development.

TLS Online TPP Program

#Id: 7222

#Unit 5. Developmental Biology

Allelic exclusion Most T cells fully rearrange and express a TCR chain from only one of their two TCR alleles.

TLS Online TPP Program

#Id: 7223

#Unit 5. Developmental Biology

Thymic selection The most distinctive property of the mature T cells is that they recognize only foreign antigen combined with self-MHC molecules.

TLS Online TPP Program

#Id: 7224

#Unit 5. Developmental Biology

Two distinct selection processes are required

Positive selection, which selects for those thymocytes bearing receptors capable of binding self-MHC molecules, resulting in MHC restriction

Negative selection, which selects against thymocytes bearing high-affinity receptors for self-MHC/peptide complexes, resulting in self-tolerance.