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LACTATION | Physiology
L LACTATION Contents Physiology Dietary Requirements
Physiology J L McManaman and M C Neville, University of Colorado, Denver, CO, USA ª 2005 Elsevier Ltd. All rights reserved.
Lactation is a uniquely mammalian physiological process in which the caloric and nutrient reserves of the mother are transformed into a complex fluid capable of supporting the nutritional demands of newborns for sustained periods. Milk, the product of lactation, is a mixture of solutes whose composition reflects the activities of distinct secretion and transport processes of the mammary gland and mirrors the differing nutritional requirements of mammalian neonates. In humans, this fluid is capable of providing the fullterm infant with all the nutrients required for the first 4–6 months of life as well as offering significant protection against infectious disease. Although artificial formulas are widely utilized for human infant nutrition in developed countries, many components of human milk, including critical growth factors, long-chain polyunsaturated fatty acids, antiinfectious oligosaccharides and glycoconjugates, and the protein lactoferrin, are not duplicated in formula. Although it is likely that such substances are beneficial even to healthy infants in well-protected environments, they are particularly important to infants living in conditions of inadequate sanitation, as well as to preterm infants and infants with feeding problems. Despite the obvious importance of milk to neonatal nutrition and the selective advantage of lactation in mammalian evolution, the physiological mechanisms underlying milk secretion and utilization are not well understood and the molecular mechanisms involved in the production of individual milk components are still poorly characterized. In this article, the
functional anatomy of the mammary gland is described, followed by a brief description of human milk composition and a review of the transport mechanisms involved in the secretion of individual milk components. We then summarize the functional differentiation of the mammary gland and the initiation of lactation—a process that involves a series of carefully programmed functional changes that transform a prepared, but nonsecretory, gland into a fully functioning organ during the first week postpartum in humans.
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 junctional complex composed of adherens and tightjunctional 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
Physiology J L McManaman and M C Neville, University of Colorado, Denver, CO, USA ª 2005 Elsevier Ltd. All rights reserved.
Lactation is a uniquely mammalian physiological process in which the caloric and nutrient reserves of the mother are transformed into a complex fluid capable of supporting the nutritional demands of newborns for sustained periods. Milk, the product of lactation, is a mixture of solutes whose composition reflects the activities of distinct secretion and transport processes of the mammary gland and mirrors the differing nutritional requirements of mammalian neonates. In humans, this fluid is capable of providing the fullterm infant with all the nutrients required for the first 4–6 months of life as well as offering significant protection against infectious disease. Although artificial formulas are widely utilized for human infant nutrition in developed countries, many components of human milk, including critical growth factors, long-chain polyunsaturated fatty acids, antiinfectious oligosaccharides and glycoconjugates, and the protein lactoferrin, are not duplicated in formula. Although it is likely that such substances are beneficial even to healthy infants in well-protected environments, they are particularly important to infants living in conditions of inadequate sanitation, as well as to preterm infants and infants with feeding problems. Despite the obvious importance of milk to neonatal nutrition and the selective advantage of lactation in mammalian evolution, the physiological mechanisms underlying milk secretion and utilization are not well understood and the molecular mechanisms involved in the production of individual milk components are still poorly characterized. In this article, the
functional anatomy of the mammary gland is described, followed by a brief description of human milk composition and a review of the transport mechanisms involved in the secretion of individual milk components. We then summarize the functional differentiation of the mammary gland and the initiation of lactation—a process that involves a series of carefully programmed functional changes that transform a prepared, but nonsecretory, gland into a fully functioning organ during the first week postpartum in humans.
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 junctional complex composed of adherens and tightjunctional 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