Anatomy and Physiology of Lactation

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The defining characteristic of all animals of the class Mammalia is the provision of milk, a fluid the composition of which exactly mirrors the needs of the young of the species. Although the location and external form of the mammary glands differ among species, the general phases of development and the mechanisms of milk production are remarkably similar. For this reason, researchers have been able to draw on a myriad of work in animals to understand this process. In humans, milk is produced and stored in differentiated alveolar units, often called lobules, in the breast. The amount of milk produced is regulated by prolactin and local factors. Removal of milk from the breast is accomplished by a process called milk ejection, brought about by a neuroendocrine reflex that ends with the secretion of oxytocin from the posterior pituitary gland and its interaction with myoepithelial cells that surround the ducts and alveoli. In humans, the small ducts coalesce into 15 to 25 main ducts that drain sectors of the gland. The main ducts dilate into small sinuses as they near the areolus, where they open directly on the nipple.

In this article, the developmental stages of the human mammary gland are described, followed by a discussion of the major principles of human milk secretion and ejection. Finally, interactions between lactation and maternal metabolism are outlined briefly, and the interaction of lactation with fertility is discussed. The transition from pregnancy to lactation, called lactogenesis, is described in the article by Neville et al (p. 35) in this issue.

Section snippets

OVERVIEW OF THE FUNCTIONAL ANATOMY OF THE BREAST

The breast, shown in Figure 1, contains a tubuloalveolar parenchyma embedded in a connective-tissue and adipose-tissue stroma. In the mature breast of nonpregnant, nonlactating women, 10 to 15 branching ducts form a treelike pattern that extends from the nipple to the edges of a specialized fat pad on the anterior wall of the thorax. Lobules of varying complexity extend from these ducts. These lobules are the origin of the acinar structures that develop into the milk-secreting organ.

STAGES OF MAMMARY DEVELOPMENT

The development of the mammary gland occurs in five stages: (1) embryogenesis, (2) pubertal development, (3) development in pregnancy, (4) lactation, and (5) involution (Table 1, Fig. 3). The switch between pregnancy and lactation is called lactogenesis. Embryogenesis begins at 18 to 19 weeks' gestation, when a bulb-shaped mammary bud can be discerned and extends from the epidermis into the dense, subepidermal mesenchyme50 (Fig. 4A). At the same time, a loose condensation of mesenchyme extends

MILK COMPOSITION

Milk is a complex fluid composed of several phases that can be separated by physical forces.42 On sitting for several hours or with short-term, low-speed centrifugation, membrane-bound globules (the milk-fat globules) containing the milk lipids rise to the surface, where they form the cream layer overlying the skim milk. In human and bovine milk, the fat accounts for approximately 4% of milk volume23 and contains milk components, such as cholesterol, phospholipids, and steroid hormones.

THE REGULATION OF MILK SYNTHESIS, SECRETION, AND EJECTION

Milk is synthesized continuously into the alveolar lumen, where it is stored until milk removal from the breast is initiated. Two levels of regulation must exist: (1) regulation of the rate of synthesis and secretion and (2) regulation of milk ejection. Although both processes ultimately depend on sucking by an infant or other stimulation of the nipple, the mechanisms involved, central and local, are different. Prolactin mediates the CNS regulation of milk secretion, but its influence is

OXYTOCIN, MILK EJECTION, AND SUCKLING

Milk removal from the breast is accomplished by the contraction of myoepithelial cells, the processes of which form a basketlike network around the alveoli where milk is stored, in concert with sucking by the infant. When infants are suckled, afferent impulses from sensory stimulation of nerve terminals in the areolus travel to the CNS, where they promote the release of oxytocin from the posterior pituitary. In women, oxytocin release often is associated with such stimuli as the sight or sound,

EFFECTS OF LACTATION ON MOTHERS

In many species, such as high-producing dairy cattle and laboratory mice, lactation entails massive adjustments of maternal metabolism so that a large portion of the metabolic output can be directed to milk synthesis. These adjustments reach a fine point in several species, including some whales, seals, and hibernating bears, that do not eat during much of the period of lactation.48 In these species, an accelerated starvation that directs nutrient fluxes to milk production occurs. By contrast,

SUMMARY

Milk secretion is a robust process that proceeds normally in at least 85% of women postpartum.41a Anecdotal evidence suggests that, with assistance in the techniques of breastfeeding, at least 97% of women can successfully breastfeed their infants. The causes of lack of success in breastfeeding are not well understood because, at least in Western societies, when infants fail to thrive on the breast, formula substitution is easy. Although this article is not the place to discuss possible

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    Address reprint requests to Margaret C. Neville, PhD Department of Physiology Box C240, Room 3802 University of Colorado Health Sciences Center Denver, CO 80262 e-mail: [email protected]

    Preparation of this article was supported in part by NIH Grant #HD19547.

    *

    Department of Physiology, University of Colorado Health Sciences Center, Denver, Colorado

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    Copyright © 2001 W. B. Saunders Company. Published by Elsevier Inc. All rights reserved.