PDGFRβ and EGFR inhibition through the use of chemical inhibitors or neutralizing antibodies in vivo leads to a blockade of pericyte recruitment to EC tubes and concomitant cranial and abdominal hemorrhage phenotypes in developing quail embryos. Two chemical inhibitors and 2 neutralizing antibodies were identified based on their ability to interfere with PDGFR signaling (imatinib and α-PDGF-BB) and EGFR signaling (gefitinib and α-HB-EGF) and administered individually or in combination to quail at 72 hours of embryonic development at a doses of 100nM for the chemical inhibitors and 20 μg/mL for the neutralizing antibodies. The quail were then allowed to develop for 144 hours, at which time the eggs were cracked and the embryos assessed for vascular phenotypes. (A-B) Embryos treated with individual reagents developed mild cranial hemorrhages, while those embryos treated with both gefitinib/imatinib or α-PDGF-BB/HB-EGF, to block PDGFR and EGFR signaling simultaneously, led to more severe hemorrhage phenotypes (Table A). (C-D) CAM tissue from control, gefitinib/imatinib double treatment, and α-PDGF-BB/HB-EGF double treatment embryos was isolated and double stained for the quail EC-specific marker QH1 (green) and PDGFRβ (red). (C) Representative images are shown demonstrating pericyte association with microvascular beds. Arrows denote representative nonassociated pericytes. (D) The number of nonassociated pericytes per high-powered field was quantified, showing an increase in the number of nonassociated pericytes with blood vessels in vivo after treatments to inhibit PDGFR and EGFR signaling. n ≥ 5; P ≤ .01.