Fig. 1.

Apoptosis in ICR mice occurs at the lactation-involution interface. (A)Whole mounts of mammary gland at different times of development stained with Haematoxylin show the ductal structures in 8 week virgin mice (V8) and the extent of proliferation after 12 days of pregnancy (P12) and 9 days of lactation (L9). Regression of the tissue is visible by day 2 of involution (I2) and extensive by day 5 (I5). (B) 5 μg genomic DNA extracted from the same developmental samples was subjected to agarose gel electrophoresis. Apoptotic DNA ladders appear within 1 day of involution and are maximal for the next 2 days. Apoptosis subsequently continues but is less extensive on day 4 and 5 and is largely absent by day 12.