Functional Anatomy of Lactation

The lactating mammary gland consists of an arborizing ductal network that extends from the nipple and terminates in grape-like lobular clusters of alveoli forming the lobuloalveolar unit, which is the site of milk secretion. A stylized diagram of these structures is shown in Figure 1. Alveoli are composed of a single layer of polarized secretory epithelial cells that possess specialized features indicative of highly developed biosynthetic and secretory capacities, including numerous mitochondria, an extensive rough endoplasmic reticulum network, and a well-developed Golgi apparatus. Secretory components including lipid droplets and casein containing secretory vesicles are found juxtaposed to the apical membrane of these cells. The epithelial cells are connected to each other through a junc-tional complex composed of adherens and tight-junctional elements that function to inhibit transfer of extracellular substances between the vascular system and milk compartments during lactation (Figure 2). The basal portion of alveolar epithelial cells is surrounded by a meshwork of myoepithelial cell processes that contract to bring about milk

Capillary Bed

Capillary Bed

Myoepithelial Cells

Duct ^^ ft JKil Alveolus 1 Adipocytes^

Myoepithelial Cells

Figure 1 (A) Camera lucida drawing of a section of the breast of a woman who died 2 days after last suckling her infant. The drawing clearly shows collecting ducts and the grape-like lobuloalveolar units, which are engorged with milk. (From Dabelow A (1941) Morphology Journal 85: 361-416.) (B) Cross-sectional diagram showing the relationship of the lobuloalveolar unit composed of milk secreting alveoli and ducts to the other cellular compartments of the mammary gland. Arrows indicate milk secretion by the alveolar epithelial cells into the lumen.

ejection and by a connective tissue stroma that supports and separates the lobules. The stromal component also contains lymphatics and becomes extensively vascularized during lactation to sustain

Lumen

Figure 2 Diagram of a mammary epithelial cell showing pathways for milk secretion described in the text. SV, secretory vesicle; RER, rough endoplasmic reticulum; BM, basement membrane; N, nucleus; PC, plasma cell; FDA, fat-depleted adipocyte; J, junctional complex containing the tight and adherens junctions; GJ, gap junction; ME, myoepithelial cell; CLD, cytoplasmic lipid droplet; MFG, milk fat globule. (Redrawn from Neville MC, Allen JC and Watters C (1983) The mechanisms of milk secretion. In: Neville MC and Neifert MR (eds.) Lactation: Physiology, Nutrition and Breast-Feeding, p. 50. New York: Plenum Press.)

Figure 2 Diagram of a mammary epithelial cell showing pathways for milk secretion described in the text. SV, secretory vesicle; RER, rough endoplasmic reticulum; BM, basement membrane; N, nucleus; PC, plasma cell; FDA, fat-depleted adipocyte; J, junctional complex containing the tight and adherens junctions; GJ, gap junction; ME, myoepithelial cell; CLD, cytoplasmic lipid droplet; MFG, milk fat globule. (Redrawn from Neville MC, Allen JC and Watters C (1983) The mechanisms of milk secretion. In: Neville MC and Neifert MR (eds.) Lactation: Physiology, Nutrition and Breast-Feeding, p. 50. New York: Plenum Press.)

the biosynthetic demands of alveolar epithelial cells. In nonpregnant, nonlactating animals the stroma contains a large adipose component.

The nipple, which is the termination point of the mammary ductal network, is innervated by the fourth intercostal nerve. Afferent sensory stimuli from suckling are transmitted to the spinal cord and the brain, resulting in release of prolactin and oxytocin from the pituitary. Prolactin, secreted from the anterior pituitary, acts directly on alveolar epithelial cells to foster synthesis and secretion of milk components. Oxytocin, secreted from the posterior pituitary, stimulates contraction of the myoe-pithelial cells that surround the alveoli and ducts. This process, called the 'letdown reflex,' forces the milk from the alveoli through ductules into ducts draining several clusters of alveoli. In the human, the small ducts converge into 15-25 main ducts that drain sectors of the gland and open directly on the nipple. The secretory product is stored in the alveolar space until myoepithelial cell contractions force it through the ducts toward the nipple, where it is available to the suckling infant.

New Mothers Guide to Breast Feeding

New Mothers Guide to Breast Feeding

For many years, scientists have been playing out the ingredients that make breast milk the perfect food for babies. They've discovered to day over 200 close compounds to fight infection, help the immune system mature, aid in digestion, and support brain growth - nature made properties that science simply cannot copy. The important long term benefits of breast feeding include reduced risk of asthma, allergies, obesity, and some forms of childhood cancer. The more that scientists continue to learn, the better breast milk looks.

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