Three major phases associated with the physiological processes of the mammary gland characterize this new classification.

Lobular Development between Puberty and 25 Years of Age
Lobular development is important during the early reproductive stage (from puberty to 25 years of age). At this time, the high cellular activity of the lobules explains the higher frequency of fibroadenomas at this period. Later, the first lobules are gradually replaced by more mature and less active lobules during the cyclical period, and especially after pregnancy. At the end of reproductive life, involution particularly affects the lobules.

Cyclical Hormonal Modifications between Puberty and 50 Years of Age
The normality of different histological structures of the mammary gland seems to very much depend on a normal, balanced relationship between epithelial and stromal elements. Both these elements are under hormonal control, and the hormonal modifications occurring at each cycle have been much analyzed.

However, the interpretation of these in vitro studies remains difficult because the correlation with in vivo studies is not straightforward; a repeated histological breast biopsy in the same women is not justified. The cyclical modifications of different mammary constituents vary considerably between women, as well as within the same woman after breastfeeding when profound and powerful hormonal modifications occur. Because of the inevitable variations, the likelihood of minor abnormalities is rather high after 35 years.

Breast Involution between 35 and 55 Years

Involution starts by 35 years of age, sometimes earlier. The process of lobular involution associated with cyclical changes occurs over a long period (20 years). If the balance between these two processes is not respected, minor aberrations are highly probable. This period of fluctuating involution extending over 20 years thus explains the high frequency of minor breast aberrations. It appears that normal epithelial involution of the lobule is dependent on the continuing presence of the specialized stroma around it. Should the stroma disappear too early, the epithelial acini remain and may form microcysts. Since microcyst formation is associated with involution of stroma, breast cysts are rare in adolescent girls.

In conclusion, these three periods have distinct clinical presentations, but overlapping and interacting processes also lead to complex clinical situations.