Figure 1

Stages of mammary gland development. (a) The development of the mouse mammary gland can be divided into five stages (pregnancy, parturition, lactation, weaning, involution) and crucial hormonal components required for each transition have been identified [22]. Placental lactogen (PL) concentrations increase during pregnancy, and prolactin (Prl) and oxytocin (TO) concentrations rise during lactation, sharply declining after weaning. P13, pregnancy day 13;i1, involution day 1;i2, involution day 2;i3, involution day 3. (b) Histological whole mount analysis of mouse mammary gland from virgin mice, pregnant, lactating and involuting mice. Mammary tissues from virgin mice (upper left panel), mice at day 11 of pregnancy (upper right panel), day 2 of lactation (lower right panel) and day 7 of involution (lower left panel) were fixed and stained with Carmine Red. Extensive cellular proliferation occurs during pregnancy; quiescent, terminally differentiated secretory alveolar cells are responsive to the lactogenic hormones and responsible for milk production during lactation; massive cell death characterizes the involution phase.