Morphometric evaluation of chorioallantoic oxygen transport in the chick embryo.

A morphologic and morphometric study was conducted to elucidate the mechanism of O2 transport across the inner shell membrane and chorioallantoic blood-gas barrier in chick embryos. The chorioallantois and shell membranes in 16-day-old incubating chicken eggs were fixed in situ, systematically sampled, and prepared for electron microscopic examination. Scanning and transmission electron micrographs are presented to describe the pathway for respiratory gas exchange. Using stereologic methods the dimensions of important gas exchange parameters were measured and from those data the O2 diffusing capacity of the chorioallantois (DCA) was calculated. Our average DCA value of 6.8 microliter O2 . min-1 . Torr-1 is quite similar to previous physiologic estimates. We found that the rate-limiting factor in chorioallantoic O2 uptake is O2-hemoglobin binding in erythrocytes, which is ten times slower than diffusion across the thin (harmonic mean thickness = 0.47 micrometer) blood-gas barrier. Our analysis provides strong support for the chorioallantoic gas exchange model of Piiper et al. (1980), and implies that the inner shell membrane provides a negligible resistance to O2 movement at this developmental age.