Figure 1.

Phenotypic markers and pathways in TEC development. In current models of TEC development, bipotent TEC progenitors with a cTEC-like phenotype give rise to both cTEC and mTEC lineages. Events that occur between bipotent TEC and the generation of mature cTEC are not known. In contrast, SSEA-1⁺ mTEC stem cells have been reported to mark the emergence of the mTEC lineage. While these cells have been shown to give rise to Aire⁺ mTEC, whether they are able to give rise to all currently known mTEC subsets has not been examined. Most relevant to this, the origins of CCL21⁺ mTEC that also reside within mTEC^lo are not known, and their status as either immature progenitors or a functionally mature mTEC^lo subset requires further study. Downstream of Aire⁺ mTEC^hi, a terminal differentiation process occurs which gives rise to several TEC subsets and structures, the inter-relationships and functional properties of which remain to be fully determined.