Figure 2. Tip cells have weak TIE2, but strong ANG-2 expression.

(A and B) TIE2 expression in lectin- and TIE2-double-stained retinas from adult mice (A) and postnatal mouse pups (B) (n = 3 each; arrows: TIE2-positive ECs; arrowheads: TIE2-negative ECs). (C) Specimen of human melanoma (n = 6) and healthy skin specimens (n = 3) were stained for CD34 and TIE2. The resting vasculature in the control specimen uniformly coexpressed TIE2 and CD34 (arrows, top row). In contrast, TIE2-positive (arrows) and TIE2-negative ECs (arrowheads) were detected in the vasculature of melanomas (bottom row). (D) Co-localization of CD34 and TIE2 in the spheroidal EC xenografting assay (30, 31). Lumenized vessels stained positive for TIE2 (arrow indicating yellow co-localization; left). ECs in non-lumenized vascular structures stained for CD34 but not for TIE2 (arrowhead, right). (E) Comparative TIE2 expression of migrating and confluent HUVECs (scratch wound assay; image shown in pseudocolors; n = 3). (F) Quantitative assessment of mean TIE2 expression in migrating and confluent ECs (*P < 0.05). (G) FACS analysis of confluent and subconfluent ECs for uPAR and TIE2 expression. (H) Quantitative assessment of the mean EC subpopulation of TIE2-negative and uPAR-positive ECs under non-permeating conditions. sub, subonfluent; con, confluent. (I) Abundant Ang2 mRNA expression in tip cells of the developing retina. Whole mount retinas were analyzed by in situ hybridization against Ang2, followed by immunoreactivity against collagen IV. Merged signals were pseudocolored using Adobe Photoshop CS software. Scale bars: A, 75 μm; B, 300 μm (insets, 50 μm); C, 75 μm; D, 75 μm; E, 100 μm (insets, 20 μm); I, 20 μm.