Influence of the oxygen pressure in the culture medium on the oxygenation of different types of multicellular spheroids.

Six different types of spheroids of both human and rodent origin were cultured, using the liquid-overlay technique. Oxygen gradients were measured with micro-electrodes, when the spheroids were attached to thin cover-glasses. The gradients were measured from the upper surface towards the center of the spheroids. Stable and reproducible gradients were obtained. Large variations were seen in the steepness of the gradients, depending both on the type and on the size of the spheroids. An interesting phenomenon was discovered. When some types of spheroids were cultured in a medium with a low oxygen pressure (medium equilibrated with gas containing 4-5% O2), the gradients continuously changed and became flatter. Detailed studies showed that most of the changes occurred within 2 days after the transfer to the low oxygen pressure. After 2 days, no further dramatic changes took place. This phenomenon was seen in two types of human glioma (U-118 MG and U-178 MG) and two types of embryonic, hamster lung-cell (V-79-379A and CHEL) spheroids. In the cases of human-osteosarcoma (U-393 OS) and human thyroid-cancer (HTh7) spheroids, no such changes could be seen. The low oxygen tension in the culture medium was chosen to mimic the environmental conditions in solid tumors.