Article

Initiation of human lactation: Secretory differentiation and secretory activation

School of Biomedical, Biomolecular and Chemical Sciences, M310, Faculty of Life and Physical Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, 6009, WA, Australia.
Journal of Mammary Gland Biology and Neoplasia (Impact Factor: 5). 01/2008; 12(4):211-21. DOI: 10.1007/s10911-007-9054-4
Source: PubMed

ABSTRACT Theories for the origin of milk have been recorded since the time of Ancient Greeks. In those times it was believed that milk was derived from special vessels that connected the uterus to the breasts. The "chyle theory" on the origin of milk was another prominent theory which persisted well into the nineteenth century before the realisation that milk components were derived from blood and some milk constituents were actually synthesized within the breasts. The demonstration that milk ejection was the expulsion of milk that had already been secreted and that milk secretion was a separate continuous process, set the background for the development for the current understanding of milk synthesis and secretion. Today we know that there are two stages in the initiation of lactation- secretory differentiation and secretory activation. Secretory differentiation represents the stage of pregnancy when the mammary epithelial cells differentiate into lactocytes with the capacity to synthesize unique milk constituents such as lactose. This process requires the presence of a 'lactogenic hormone complex' of the reproductive hormones, estrogen, progesterone, prolactin and some metabolic hormones. Secretory activation on the other hand, is the initiation of copious milk secretion and is associated with major changes in the concentrations of many milk constituents. The withdrawal of progesterone triggers the onset of secretory activation but prolactin, insulin and cortisol must also be present. This review describes the works of pioneers that have led to our current understanding of the biochemical and endocrinological processes involved in the initiation of human lactation.

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    • "Oxytocin (OT) is a neuroendocrine hormone that is essential for normal breastfeeding physiology, as it stimulates breast myoepithelial cell contraction, which transfers milk to the areola for the infant (Pang and Hartmann, 2007). OT has been implicated in maternal behavior and in forming and maintaining social bonds, particularly in its interaction with dopamine (Pedersen et al., 1994; Pedersen, 1997; Numan et al., 2005; Aragona et al., 2006). "
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    • "OT has received less interest than gonadal or stress hormones as a potential etiologic factor in PPD, although it has attracted attention for its involvement in breastfeeding difficulties often present in PPD (Stuebe et al., 2012). It is well known that OT is critically implicated in milk letdown (Pang and Hartmann, 2007). The documented association between breastfeeding difficulties and PPD (Dennis and McQueen, 2009; Taveras et al., 2003; Watkins et al., 2011) is worthy of attention (Skalkidou et al., 2010). "
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    • "The ratio of adipocytes to epithelia decreases to facilitate parenchymal expansion during this phase. A complex milieu of hormones marked by a drop in progesterone activates epithelial changes leading to lactation, which is typically maintained by the suckling response of the newborn (Neville et al. 2002; Pang and Hartmann 2007). Postlactation mammary involution, studied after natural or forced weaning, involves rapid regression of the differentiated gland and depends on programmed cell death pathways that eliminate up to 50% – 80% of the secretory epithelium within 1 wk of weaning (Walker et al. 1989). "
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