Figure 1. Initial seeding of the embryonic thymic anlage.

(a) Correlation of the number of tail somites with the timing of embryonic development. Each circle represents an individual embryo. Area between broken lines corresponds to the tail somite stages (10-12) of initial thymus seeding.

(b) Typical dissected thymic lobes from E11.5 embryos stained with VE-Cadherin (VE-Cad, red), cytokeratin (CK, green) and DAPI (blue). VE-Cadherin⁺ endothelial cells were completely removed by dissection in most explants (top), but not all (bottom).

(c) Frequency of seeded embryonic thymic rudiments, as determined by emergence of T cell potential of dissected thymuses at tail somites 4-9 (n=18), 10-12 (n=17) and 13-16 (n=9), corresponding to E11.0-E11.75 embryos. p values were calculated with two-sided Fisher’s exact test.

(d) Representative FACS profile of T cells (CD4⁺CD8⁺ and/or Thy1.2⁺CD25⁺) derived from culture of an individual E11.5 (TS11) thymus on OP9-DL1 stroma.

(e) Expression (2^–ΔCt) of Dll4 in PLET1⁺ thymic epithelial cells isolated from E11.5 and E12.5 embryos. Mean (s.d.) expression levels (relative to the average of Actb, Hmbs and Tbp) of 2 biological replicates are shown. **p<0.01.