A, DNA laddering in glands milked twice daily (M) or unmilked (U) for 3, 7, 14 and 21 days. Results are representative of 2 animals at each time point. Mammary tissue from 10 day lactating (L) and 4 day involuting (I) mice (Mse) were used as controls. B and D, TUNEL detection of apoptotic cells in goat tissue 14 days after cessation of milking (B), and in mouse mammary tissue on day 10 of lactation (C) and 4 days after litter removal on day 10 (D). Arrows, TUNEL-positive cells. Scale bar, 50 μm; applies to all panels.