Thymus development

The thymus is the primary lymphoid organ for the generation of T cells. Its formation during the second half of mouse gestation depends on cellular contributions from all germinal layers. Originating from the third pharyngeal pouch endoderm, the early thymus anlage is populated by neural crest-derived mesenchymal cells, endothelial cells, precursor, and mature hematopoietic cells. Collectively, these cell populations create a microenvironment able to generate functionally competent naïve T cells. Spontaneous and engineered gain- and loss-of-function mutations in mice combined with cell fate mapping and single-cell transcriptomic analyses have revealed the molecular mechanisms that govern the fate commitment, differentiation, position, state, and function of a large number of separate, cell populations resident in the embryonic thymus. This information provides to date a comprehensive, albeit yet still incomplete, understanding of how the thymus is formed and how its function is established during embryogenesis.