APOPTOSIS AND MAMMARY-GLAND INVOLUTION - REVIEWING THE PROCESS

Apoptosis is a process of programmed cell death. Mammary gland involut ion is a tissue remodelling process. Mammary epithelial cell apoptosis is an integral component of tissue remodelling but it is only one ele ment. Equally important are the factors which degrade basement membran e and extracellular matrix. Both operations are required for completio n of mammary gland involution. The primary apoptotic process occurs fi rst and is temporally distinct from the second stage of involution typ ified by lobular-alveolar collapse. Local factors related to milk accu mulation trigger the first stage, but loss of systemic hormonal stimul ation governs the second stage. Changes in the expression patterns of cell cycle control genes and bcl-2 family member genes are found in th e first stage. Proteinase gene activation dominates the second stage. These findings support a two stage model of mammary gland involution. Both mammary epithelial cell apoptosis and mammary gland remodelling a dvance through a process which includes both loss of survival factors and gain of death factors. This review focuses on signalling pathways and genetic controls which are activated and repressed during mammary gland involution.