Microvascular responses to chronic hypoxia by the chick chorioallantoic membrane: a morphometric analysis.

Numerous studies have demonstrated an increased capillarity in response to hypoxia in a variety of tissues. We studied the effects of hypoxia on the number and morphology of pre- and postcapillary vessels in the chick chorioallantoic membrane (CAM). Measurements of CAM microvessels were made from in vivo photographs after incubation in 15% oxygen (hypoxia) for 7 days (Days 7-14 of development). Quantitation of arteriolar and venular number, diameter, and length within defined areas was performed using a digitizing tablet with a 0.001-in. resolution, or 1.2 microns on photographs enlarged to 30x. The 15% oxygen environment produced a 54% increase in overall vessel density, with arterioles increasing 78% and venules 34%. The increases were primarily among vessels less than 10 microns in diameter. Moreover, the total vessel length per area of CAM also was increased by the 15% oxygen. Among vessels less than 80 microns in diameter, the low oxygen regimen stimulated a preferential increase in the number of arterioles, evidenced by a significant upward shift in the arteriole: venule ratio. Control vs 15% oxygen groups showed no statistical differences for the diameters and lengths of individual arterioles and venules. Thus, the observed increase in the total vessel length per area following 15% oxygen reflects the increased number of vessels. These data demonstrate that chronic exposure of the CAM to low oxygen stimulates an increase in the density of the pre- and postcapillary vessels which favors the arterioles.