Figure 6.

 In vitro functional assays employing Del1 transfected yolk sac cells. (A) The parental yolk sac cell line YS-B under routine culture conditions. Phase contrast (original magnification, 100×). (B) YS-B cells after 24 hr on Matrigel, showing a pattern of vascular-like morphogenesis. Cells were stained with toluidine blue. Bright-field (40×). (C) Negative control transfectants form a vascular-like network on Matrigel after 24 hr. Light areas represent organized cells; photographed under dark-field illumination at 50×. (D) Transfectant clone L13, after 24 hr on Matrigel, shows some evidence of network formation, although less than YS-B cells not expressing Del1. (E) Transfectant clone L10, after 24 hr on Matrigel, reveals no evidence of network formation; cells instead produce numerous aggregates. Dark-field illumination (50×). (F) Native yolk sac YS-B cells grown on a matrix produced by negative control transfectants make a complex structural network. Light areas represent organized cells; photographed under dark-field illumination at 30×. (G) Native YS-B cells grown on a matrix produced by Del1 transfectants. Cells are forming a dense monolayer, with no evidence of organization. Photographed under dark-field illumination at 30×. (H) Aggregates of negative control transfected yolk sac cells were placed onto polymerized Matrigel. After 24 hr, cells showed sprouting angiogenesis. Photographed under phase contrast at 100×. (I) Aggregates of Del1-transfected yolk sac clone L10 were placed onto polymerized Matrigel as in H. Photographed after 24 hr (100×); these cells show no evidence of sprouting.