This reduced colonization was explained by increasing apoptotic cells detected by TUNEL staining in hypoxic environment of the prevascular fetal thymus primordium of Hif-1α deficient mice. On the other hand, by using in vivo bromodeoxyuridine labeling indicated that the frequencies of proliferating cells were affected in E11.5 fetal thymus primordium of Hif-1α deficient mice.