Schematic drawing of the human breast as seen during lactation.
The breast is composed largely of branched tubuloalveolar glands contained within an extensive connective tissue stroma and variable amounts of adipose tissue. (Modified from Waiwick R, Williams PL, eds. Gray's Anatomy. 35th ed. Edinburgh: Churchill Livingstone, 1973.)
ends in a lactiferous duct that opens through a constricted orifice onto the nipple. Beneath the areola, the pigmented area surrounding the nipple, each duct has a dilated portion, the lactiferous sinus. Near their openings, the lactiferous ducts are lined with stratified squamous epithelium. The epithelial lining of the duct shows a gradual transition from stratified squamous to two layers of cuboidal cells in the lactiferous sinus and finally to a single layer of columnar or cuboidal cells through the remainder of the duct system. Myoepithelial cells of ectodermal origin lie within the epithelium between the surface epithelial cells and the basal lamina. These cells, arranged in a basket-like network, are present in the secretory portion of the gland but are more apparent in the larger ducts.
The morphology of the secretory portion of the mammary gland varies with the menstrual cycle
In the inactive gland, the glandular component is sparse and consists chiefly of duct elements (Fig. 22.33). During the menstrual cycle, the inactive breast undergoes slight cyclic changes. Early in the cycle, the ducts appear as cords with little or no lumen. Under estrogen stimulation, at about the time of ovulation, the secretory cells increase in height, lumina appear in the ducts as small amounts of secretions accumulate, and fluid accumulates in the connective tissue.
Mammary glands undergo dramatic proliferation and development during pregnancy
The mammary glands exhibit a number of changes in preparation for lactation. The changes in the glandular tissue are accompanied by a decrease in the amount of connective tissue and adipose tissue. Plasma cells, lymphocytes, and eosinophils infiltrate the fibrous component of the connective tissue as the breast develops. 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. As the cells proliferate by mitotic division, the ducts branch and alveoli begin to develop. In the later stages of pregnancy, alveolar development becomes more prominent (Fig. 22.34). The actual proliferation of the stromal cells declines, and subsequent enlargement of the breast occurs through hypertrophy of the secretory cells and accumulation of secretory product in the alveoli.
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
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