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 transpor t (hemoglobin poisoning with carbon monoxide), whole animal surface area an d volume were measured and bulk oxygen diffusion was modeled at four develo pmental stages (from initiation of heartbeat to premetamorphic climax). Sur face area [8.5 mm(2) at stages Nieuwkoop-Faber (NF) 33-34 to 70.2 mm(2) at stages NF 50-51] and volume (1.8 mm(3) at stages NF 33-34 to 35.7 mm(3) at stages NF 50-51) measured from volumetric analysis from dual plane images o f each animal were not significantly different between treatments. Bulk oxy gen radial diffusion was estimated by modeling the larvae as a set of adjac ent cylinders with different radii. The model was used to predict the oxyge n tension at the water-skin interface at which the oxygen tension in the ce nter 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 a t 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 co re would only exceed 10% of the total volume of the animal only at oxygen t ensions below 5 kPa, Therefore, the ten-fold increase in mass between NF 50 -51 and metamorphosis would prove insufficient for embryonic oxygen require ments via simple diffusion, and therefore would require additional transpor t mechanisms.