Supplementary Figure 1: Staging of fetal thymus at E11.
From: Initial seeding of the embryonic thymus by immune-restricted lympho-myeloid progenitors
Related to main Figure 1. (a) Staging criteria for E11.0, 11.25, 11.5 and 11.75 embryos with the corresponding somite (S) pairs indicated. Stage criteria (in red) include morphological assessment of branchial arches (1st and 3rd row), nasal processes (2nd and 3rd row) and limb bud (4th row) development. Abbreviations: BA: branchial arches, numbered; M: Maxillary; m: mandibulary. (b) Tail somite (TS) counting to stage thymus development. Somite development exhibits a faithful correlation with organ development in the embryo, however some somites especially trunk somites have already initiated their differentiation at E11, resulting in ambiguous and inconsistent somite counts. By correlating somites in the tail, which are still visible at E11, with morphology assessment of branchial arches, nasal processes and limb bud development, TS counts representing the corresponding E11 developmental stages were documented (see Fig. 1a). TS counting should only be undertaken in embryos after E11, when the cloaca (*) is protruding, as similar TS number may be obtained from the base of the limb bud at E10.25-10.75 when the cloaca is not yet protruding. (c) At E11.5 the 3rd pouch derived primordia constituted by the thymus (light red) and parathyroid (dark red) rudiments are attached to the pharyngeal endoderm and surface ectoderm. Separation between the thymus and parathyroid primordia starts around E12.0-E12.5. Scheme adapted from27. (d) Sagittal sections were performed to reveal the fetal thymic lobes which are located in the third branchial arches (arrows) and can be visualized by cytokeratin staining (CK, green; DAPI, blue). I, II and III indicate the 1st, 2nd and 3rd branchial arches, respectively. (e) At E11.5 the cytokeratin (CK, red left panel, green right panel) positive fetal thymus is not vascularized but surrounded by blood vessels as confirmed by Vwf eGFP (green left panel, blue right panel) and VE-Cadherin (VE-Cad, red; right panel) staining. Arrows indicate the location of the thymus rudiment.
