Morphometry and estimated bulk oxygen diffusion in larvae of Xenopus laevis under chronic carbon monoxide exposure

To understand the mechanisms that allow tadpoles of the African clawed frog Xenopus laevis to develop under conditions of impaired convective transport (hemoglobin poisoning with carbon monoxide), whole animal surface area and volume were measured and bulk oxygen diffusion was modeled at four developmental stages (from initiation of heartbeat to pre-metamorphic climax). Surface area [8.5 mm2 at stages Nieuwkoop-Faber (NF) 33-34 to 70.2 mm2 at stages NF 50-51] and volume (1.8 mm3 at stages NF 33-34 to 35.7 mm3 at stages NF 50-51) measured from volumetric analysis from dual plane images of each animal were not significantly different between treatments. Bulk oxygen radial diffusion was estimated by modeling the larvae as a set of adjacent cylinders with different radii. The model was used to predict the oxygen tension at the water-skin interface at which the oxygen tension in the center of the animal is nil (0.7 kPa at stage NF 33-34 and 14.0 kPa at stage NF 50-51), suggesting that bulk oxygen diffusion is sufficient to meet the metabolic demand up to stages NF 46-47 irrespective of the oxygen tension at the water-skin interface. At NF 50-51 an anoxic core in the animal would appear if bulk oxygen diffusion were the only means of oxygen transport at oxygen tensions below 15 kPa. However, the relative volume of the anoxic core would only exceed 10% of the total volume of the animal only at oxygen tensions below 5 kPa. Therefore, the ten-fold increase in mass between NF 50-51 and metamorphosis would prove insufficient for embryonic oxygen requirements via simple diffusion, and therefore would require additional transport mechanisms.