Tissue PO2 and growth rate in early chick embryos

We determined dry mass, frequency distribution of tissue PO2 and microvascular density of head and trunk of early chick embryos between day 4 and day 6 of incubation. During this period the percentage of the dry mass of the head on total dry mass increased from 31% to 48% indicating that the head grows faster than the trunk. Tissue PO2 values ranged between zero and arterial PO2. About half of the numbers were less than 5 Torr. Mean tissue PO2 was significantly higher in the head than in the trunk. This was paralleled by a significantly higher microvascular density in the head. The high frequency of low tissue PO2 values found at each day at all measuring sites suggests that the embryonic tissue of both head and trunk extracts as much oxygen from the capillary blood as possible. Consequently, growth rate strongly depends on oxygen availability. Since the arterial PO2 is the same in the head and in the trunk (Meuer and Baumann, 1988), the diversity of tissue PO2 and growth rate found in this study is probably caused by differences in the structure of the microvascular bed resulting in variations of tissue blood perfusion, capillary transit time and diffusion distance.