Figure 2

Hormonal signals and the activation of transcription factors mediating mammary gland development. During the pregnancy stage, alveolar density continuously increases. Towards the end of pregnancy, starting at day 13, Stat5a/b are induced by tyrosine phosphorylation and can be detected in their DNA binding configuration. Activated Stat5 persists during lactation, but rapidly declines after cessation of suckling [23]. Activated Stat3 is not detectable during the lactation phase, but is rapidly induced to maximal levels several hours into the weaning period [24]. Within the same time, the levels of bcl-x and bax increase [25]. Milk protein synthesis (WAP) and alveolar density decrease during involution.