The mammary gland is an organ comprised of branched ductal network terminated by secretory alveoli and embedded in mesenchymal stroma. Mammary alveoli, built by myoepithelial and epithelial cells with apico basal polarity, obtain their functional differentiation only during lactogenesis. For this reason they represent a good model for studies on differentiation process of epithelium.

Stem cells are cells with a capacity to self-renew and to generate daughter cells that can differentiate into different cell lineages to form all the cell types that are found in the mature tissue. The activity of mammary stem cells and their mitotic progeny is fundamental to normal mammary growth, differentiation and regeneration in successive cycles of pregnancy, lactation and involution. The isolation and characterization of mammary stem cells is fundamental to understanding mammary gland development and tissue homeostasis as well as breast oncogenesis.

The aim of this review is to present a concrete model of apoptosis, namely the physiological death of mammary epithelial cells, and put emphasis on the role of Bcl-2 related proteins in the regulation of this process. The `paper is based on the literature and our own observations related to the regulatory mechanisms of mammary gland remodelling.