Suppression of hematopoiesis inhibits vascular network formation of ECs in P-Sp culture system. (A-D) P-Sp explants were cultured on OP9 stromal cells in the presence of anti-B220 mAb (20 μg/mL) as a control (A-B) or anti–c-Kit neutralizing antibody, ACK2 (20 μg/mL) (C-D), and the development of HCs and ECs was observed. (A,C) Phase contrast view of P-Sp cultures. The dotted line shows the presumptive border between the vascular bed (vb) and the vascular network area (vn). (B,D) PECAM-1 staining of culture plates after 10 days of culture. Bar in panel A indicates 50 μm (A,C). Original magnification, ×4 (B,D). (E) Total number of CFU-c's observed in the P-Sp culture system in the presence of anti-B220 mAb (20 μg/mL) or ACK2 (5 or 20 μg/mL). Results are expressed as the mean number of CFU-c's in 3 experiments (± SEM). (F) Length of elongated ECs (sprouting) from the vb. Results are expressed as mean length at 10 random points. Standard deviations are indicated.