Detection of CB-derived blood vessels at the site of thymic implant in SCID CB/Thy chimeras mismatched for sex or HLA-A2. (a and b) Confocal microscopy of a thymic graft from a CB/Thy SCID chimera transplanted with CB and thymus mismatched by sex. Human blood vessels, here identified by the expression of vWF (red), are shown to comprise Y chromosome-positive cells of CB origin, as detected by FISH (green, arrowheads). (c and d) Confocal microscopy of a thymic graft from a CB/Thy SCID chimera transplanted with CB and thymus mismatched for the expression of HLA-A2. The transplanted thymic tissue was from an HLA-A2⁺ donor, whereas CB cells were from an HLA-A2⁻ donor. Human blood vessels of thymic origin (arrows) are identified by the coexpression of vWF (red) and HLA-A2 (green). In addition, a number of human vWF⁺ blood vessels lacking expression of HLA-A2 are observed (arrowheads), indicating the participation of CB cells to the revascularization of the graft. Bar in b = 42.3 μm; bar in a, c, and d = 63.5 μm.