Immunohistochemical staining for Hif-1α and VEGF after exposure to normal (20%) or impaired (2%) oxygen at several time points. (a) To evaluate the staining, a semiquantitative quickscore (1–9) was used which combines positivity (P) with a range from 1 to 6 and intensity (I) with a range from 0 to 3. There is a strong induction of both proteins in the acute phase (0–24 hours), but after prolonged hypoxia a new balance occurs. Hif-1α is not expressed under normal oxygen (20%) conditions, whereas VEGF has a low constitutional expression. (b–d) Immunohistochemical staining under hypoxic conditions: (b) Hif-1α staining at 0 hrs—there is no Hif-1α present; (c) Hif-1α staining after 24 hrs—almost all cells are positive; (d) Hif-1α staining after 72 hrs—some cells are positive.