The murine thymus originates from the third pharyngeal pouch at day E9.5 of embryonic development PF-01367338 molecular weight and is solely derived from the endoderm [7]. Specification of the thymus involves the sequential upregulation of important transcription factors (Hoxa3, Pax-9, Pax-1, Eya1, Rae2, chordin, and BMP; (reviewed in [8]) eventually leading to the expression of the thymic-specific

transcription factor Foxn1 [9, 10]. From E11.5 onwards, the first precursor T cells migrate into the thymic anlage and noncanonical NF-κB signaling becomes important for full differentiation of the medullary microenvironment, culminating in the upregulation of auto-immune regulator (Aire) [11-13] that enables medullary TECs to express self-antigens [2, 3]. In the adult thymus cross-talk remains important, as the process of differentiation but also maintenance of medullary TECs, via ligation of RANK and CD40 by ligands expressed on thymocytes [11, 12, 14]. Mature cortical and medullary TEC originate from a common thymic epithelial

progenitor cell (TEPC) [15, 16]. Although full differentiation of mature TECs from a clonal precursor population has been demonstrated, the precise phenotypical characterization of that precursor as well as its genotype are still lacking, making it difficult to identify this TEC in the adult KU 57788 thymus. Despite this, expression of placenta-expressed transcript 1 (Plet-1) does identify a subset of TEPCs with the ability to generate differentiated progeny. Especially, fetal Plet-1+ TECs are able to give rise to a functional thymus when transplanted under the kidney capsule [17-19]. However, although present on TECs in the adult thymus, Plet-1+ cells seem to lose their precursor potential after E15 of embryonic development [20]. So far, no exclusive marker for TEPCs has been identified in the adult thymus. Still, the regenerative

capacity of the involuted thymi has been revealed in different murine models (reviewed in [21]), suggesting the presence of an adult TEPC population. Leucine-rich repeat-containing G protein-coupled receptor (Lgr)5 is a marker for stem cells in the adult intestine of mice [22]. Single Lgr5+ cells from adult murine intestine were able to expand and form a new crypt/villus structure second in-vitro [23, 24]. Although Lgr5+ cells in the crypt are a transient state of the BMI+ stem cells, they still give rise to epithelial cell subsets of the intestine [25, 26]. Lgr5 together with Lgr4 responds to the wingless type (Wnt) agonist R-spondin, together these receptors fine-tune Wnt signaling [27, 28]. Mice with a targeted deletion of Lgr5 die immediately after birth due to fusion of the tongue with the floor of the oral cavity [29]. In addition, Lgr5-deficient embryos tend to have premature paneth cell differentiation in the small intestine [30]. Lgr5+ transcripts have been reported in the E13.