Radiation-induced apoptosis in the mammary epithelium is not dependent on systemic factors. (a) Timeline for whole organ culture (WOC) experiments. Mice were treated with placebo or E+P at 6 to 8 weeks of age. At tissue harvest, contralateral fourth mammary glands were placed in either serum-free control media or media supplemented with E+P. Mammary glands were maintained in WOC for 3 days followed by a 5-Gy dose of irradiation 6 hours prior to harvesting and fixing the tissue. (b) Mammary glands from mice treated with E+P in vivo had a significantly greater apoptotic response to radiation compared with the placebo-treated controls. The increase due to E+P in vivo was evident regardless of whether the tissues were maintained in serum-free control media or media supplemented with E+P (*P < 0.01). (c) TUNEL-positive staining is shown for placebo-treated glands in control media (i), E+P-treated glands in control media (ii), placebo-treated glands in E+P-treated media (iii), and E+P-treated glands in E+P media (iv). Panels (i-iii) were taken at × 400. Panel (iv) was taken at × 200 to show that TUNEL-positive cells were present throughout all structures of the E+P-treated glands. E+P, estradiol and progesterone; IR, ionizing radiation; TUNEL, terminal uridine deoxynucleotidyl transferase dUTP nick-end labeling.