Proliferation pattern of capillary endothelial cells in chorioallantoic membrane development indicates local growth control, which is counteracted by vascular endothelial growth factor application

Abstract

The density and distribution of whole mount BrdU‐anti‐BrdU labeled endothelial cells (days 6–15) in the chick chorioallantoic membrane (CAM) was analyzed with computer‐assisted microscopy. A significant loss of proliferative activity was noted after day 10: the density of labeled nuclei (in 10⁻² mm⁻²) decreased from a median 7.78 (days 6, 8, 10) to 2.42 (days 12, 14, 15). CAMs initially showed random patterns of labeled endothelial cells, but changed to clearly focal patterns after day 12. A regular arrangement of labeled nuclei was never seen. After application of vascular endothelial growth factor (VEGF) to the day 13 CAM, a significant increase in proliferative activity (11.50) and a random distribution of labeled endothelial cells was observed on day 15. Development of CAM precapillary vessels was assessed in terms of length density (in mm⁻¹, mean ± standard deviation), which was augmented threefold from day 6 (1.22 ± 0.05) to day 14 (3.54 ± 0.23) and then remained nearly constant. VEGF application from day 13 to 15 raised arterial length per unit area to 4.53 ± 0.77. It is concluded that normally a local regulation of endothelial proliferation and differentiation develops in the CAM, which doubles capillary endothelial cell density but simultaneously adapts to the decreasing need for endothelial cells, and thus maintains the quasi two‐dimensional vessel pattern. However, proliferative foci persist in the capillary layer after day 10, and precapillary vessel density continues to increase until day 14. VEGF enhances DNA synthesis in all capillary endothelial cells. This leads to the disappearance of proliferative foci, to multiple capillary layers, and to an excessive formation of precapillary vessels. We propose that CAM endothelial cell proliferation is regulated by factors like endothelial cell density and extension. The proliferative pattern in capillaries, and the length density of precapillary vessels should be used for the evaluation of angiogenesis in the CAM assay. We recommend the CAM after day 12 for evaluating putative angiogenic mitogens. ©1995 Wiley‐Liss, Inc.