Regional hypoxia elicits regional changes in chorioallantoic membrane vascular density in alligator but not chicken embryos

Hypoxic incubation increases vascularization in the chick chorioallantoic membrane (CAM). The effect of regional hypoxia on the vascular density of American alligator (Alligator mississippiensis) and chicken (Gallus gallus) CAMs was studied to determine if hypoxic proliferation of blood vessels is localized or global across the CAM. Eggs were incubated under normoxic conditions with a portion of the eggshell covered with non-toxic beeswax to induce external regional hypoxia. CAMs were examined under a microscope with a 'bulls eye' coverslip and a vascular density index (VDI) was determined. The hypoxic portions of the alligator CAMs were more vascular than the normoxic portions (VDI = 200.9 versus 157. 8, respectively). Presumably this response is maladaptive by causing increased blood flow to the poorly oxygenated portions, i.e. increased 'shunt'. Thus, we hypothesize increased vascularity due to hypoxic incubation is due to local release and subsequent rapid local breakdown or uptake of angiogenic factors. In contrast, the hypoxic and normoxic portions of the chick CAMs exhibited virtually no difference in VDI (VDI= 211.5 versus 217.9, respectively). We suggest the air cell and air space of the chicken eggs allows for circulation of gas in ovo, eliminating the possibility of regional internal hypoxia.