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Abstract
To produce milk, four secretory processes are synchronized in the alveolar cell of the mature, functional mammary gland: (1) exocytosis, (2) fat synthesis and secretion, (3) secretion of ions and water, and (4) transcytosis of immunoglubulins and other substances from the interstitial space. Milk is synthesized continuously into the alveolar lumen, where it is stored until milk removal from the breast is initiated. Prolactin mediates the central nervous system regulation of milk secretion, but its influence is modified greatly by local factors that depend on milk removal from the breast. Oxytocin mediates milk let-down by stimulating the contraction of myoepithelial cells that surround the alveoli and ducts. Lactogenesis includes all the processes necessary to go from the undifferentiated mammary gland in the early pregnant animal to full lactation sometime after parturition. The most important factors in initiation of lactogenesis stage II appear to be progesterone withdrawal. The metabolic demands of breastfeeding require an increase in maternal metabolism. Postpartum suppression of fertility is thought to be the result of an alteration in pulsatile gonadotropin releasing hormone secretion from the hypothalamus. Women who wish to ensure against pregnancy during lactation usually are advised to use other contraceptive means.
Figures and Tables
Fig. 1
The pathways for milk synthesis and secretion by the mammary epithelial cell. I=Exocytosis of milk protein, lactose, and other components of the aqueous phase in Golgi-derived secretory vesicles. II=Milk fact secretion by way of the milk fat globule. III=Direct movement of monovalent ions, water, and glucose across the apical membrane of the cell. IV=Transcytosis of components of the interstitial space. V=The paracellular pathway for plasma components and leukocytes. Pathway V is open only during pregnancy, involution, and in inflammatory states such as mastitis. Abbreviations : SG, secretory granule; RER, rough endoplasmic reticulum; BM, basement membrane; MFG, milk fat globule; CLD, cytoplasmic lipid droplet; N, nucleus; PC, plasma cell; FDA, fat depleted adipocyte; TJ, tight junction; GJ, gap junction; D, desmosome; ME, myoepithelial cell. (From Neville MC. Anatomy and physiology of lactation. Pediatr Clin North Am 2001;48:13-34.)
Fig. 2
Mammary alveolus. Milk is secreted into alveoli (a). The ductile (d) through which the milk is ejected by contraction of the myoepithelial cells is surrounded by supporting structures that include vasculature and a rich stroma composed of fibroblasts and adipocytes, and plasma cells (PC). (From Neville MC. Anatomy and physiology of lactation. Pediatr Clin North Am 2001;48:13-34.)
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