**1. Introduction**

The mammary gland plays an essential role during the early postnatal life of young mammals, providing them nutrients, water and electrolytes, and immune protection until they reach the size and maturity to survive independently. The mammary gland has a stroma rich in adipose cells and glandular epithelium that origins a lobulealveolar system, in which terminal there are alveolar epithelial cells involved in milk production. The mammary gland development, as well as its fundamental structure is very similar among different species, with little differences in function, architecture, and number of glands. For example, in rodents, the branches are few and disperse, whereas ruminants have more branches and they are concentrated in the terminal of alveoli [1]. This gland undergoes cyclic changes that make it reach its maximal development in lactation. This is a unique model of cyclic morphogenesis in adults, with four characteristic steps replicated in each pregnancy and which ends with its involution in menopause. The four phases of mammary gland maturity in adulthood are proliferative phase, secretory differentiation phase, secretory activation phase, and lactation phase [2, 3]. Although breast development begins during embryogenesis, it is during pregnancy when terminal maturation of the gland occurs, developing a lobule-alveolar system characterized by branching of the galactophorous ducts and the differentiation of terminal buds to alveoli. Growth of mammary gland is stimulated during pregnancy by the mammotrophic combination of steroids (estrogen, progesterone, and corticosteroids) and polypeptide hormones (prolactin, growth

### *Lactose and Lactose Derivatives*

hormone, and placental lactogen). The four phases of breast differentiation that occur after conception have clear histological differences, as can be seen in **Figure 1**. In the first phase, there is an increase in the amount of glandular tissue, associated with an increase in the number of acini. The second phase is defined by the beginning of lipid synthesis and by its accumulation inside mammary epithelial cells (MEC). The third phase is characterized by the presence of differentiated MEC, capable of producing and secreting all the constituents of milk, which results in a dilation of the alveoli and the presence of an eosinophilic secretion that occupies the acinar lumen. The final phase of mammary gland development is called the lactation phase, and it is the stage in which breastfeeding is established [2]. At this stage, milk secretion is continuous, which is associated with a greater degree of dilation of the alveoli and the presence of milk secretion in the acinar lumen. The process of mammary gland involution starts after weaning and develops in two stages. The first step is a reversible first phase, which lasts a few days, and is due to the release of local breast factors that trigger apoptosis of the secretory epithelia. The second phase, the remodeling, involves the replacing of the lobule-alveolar structures by adipose tissue, degradation of the extracellular matrix and its basal lamina, and the remodeling of adipose tissue [4].

**Figure 1.**

*Histological characteristics of mammary morphogenesis in adults. (a) virgin; (b) proliferative phase; (c) secretory differentiation phase; (d) secretory activation phase; (e) lactation phase; and (f) early involution. Hematoxylin and eosin staining, bar 50 μm [5].*
