This imunofluorescence image is obtained from mammary gland of lactating mouse two days post-parturition. The mouse carried a transgene composed of the smooth muscle -actin promoter conjugated to enhance green fluorescent protein (GFP) reaction. Three-dimensional organization of myoepithelial cells is visualized in green color due to the expression of the promoter transgene in myoepithelial cells. The tissue was also stained red with antibody against smooth muscle -actin conjugated directly with CY3 fluorescent dye. The orange color staining results from overlapping of the green and red staining. The cells on the surface of the terminal duct lobular unit are stained orange, while those deeper in the tissue stained only green because the antibody did not penetrate deep into the tissue. Note a small intralobular duct that merges into the larger lactiferous duct. 600. (Courtesy Dr. James
J. Tomasek, University of Oklahoma Health Science Center.) abrupt involution and apoptosis during the last few days of menstrual cycle before onset of menstruation.
Mammary glands undergo dramatic proliferation and development during pregnancy.
The mammary glands exhibit a number of changes in preparation for lactation. These can be examined by the trimester of pregnancy.
 First trimester is characterized by elongation and branching of the terminal ductules. The lining epithelial and myoepithelial cells proliferate and differentiate from breast progenitor cells found in epithelium of terminal ductules. Myoepithelial cells proliferate between the base of the epithelial cells and the basal lamina in both the alveolar and ductal portions of the gland.
 Second trimester is characterized by differentiation of alveoli from the growing ends of the terminal ductules. The development of the glandular tissue is not uniform, and variation in the degree of development is seen even within a single lobule. The cells vary in shape from flattened to low columnar. Plasma cells, lymphocytes, and eosinophils infiltrate the intralobular connective tissue stroma as the breast develops (Plate 103, page 894). At this stage, amount of glandular tissue and mass of the breast increases mainly due to the growth of the alveoli (Fig. 23.35).
 Third trimester commences maturation of the alveoli. The epithelial glandular cells become cuboidal with nuclei positioned at the basal cell surface. They develop an extensive rER; secretory vesicles and lipid droplets appear in the cytoplasm. The actual proliferation of the interlobular stromal cells declines and subsequent enlargement of the breast occurs through hypertrophy of the secretory cells and accumulation of secretory product in the alveoli.
The changes in glandular tissue during pregnancy are accompanied by a decrease in the amount of connective tissue and adipose tissue.
Both merocrine and apocrine secretion are involved in production of milk.
The secreting cells contain abundant granular endoplasmic reticulum, a moderate number of large mitochondria, a supranuclear Golgi apparatus, and a number of dense lysosomes (Fig. 23.36). Depending on the secretory state, large lipid droplets and secretory vesicles may be present in the apical cytoplasm. The secretory cells produce two distinct products that are released by different mechanisms.
 Merocrine secretion: The protein component of the milk is synthesized in the rER, packaged into membrane-limited secretory vesicles for transport in the Golgi apparatus, and released from the cell by fusion of the vesicle’s limiting membrane with the plasma membrane.