Lactogenesis : The Transition from Pregnancy to Lactation - 17/08/11
Résumé |
Lactogenesis is the onset of milk secretion and includes all of the changes in the mammary epithelium necessary to go from the undifferentiated mammary gland in early pregnancy to full lactation sometime after parturition. Based on work in ruminants, Hartmann22 and Fleet and colleagues17 divided lactogenesis into two stages.
Stage I occurs during pregnancy, when the gland becomes sufficiently differentiated to secrete small quantities of specific milk components, such as casein and lactose. In humans, stage I occurs at approximately midpregnancy and can be detected by the measurement of increased plasma concentrations of lactose and ⍺-lactalbumin.4 After lactogenesis stage I has been achieved the gland is sufficiently differentiated to secrete milk, but secretion is held in check by high circulating plasma concentrations of progesterone27 and, possibly, in some species such as humans, estrogen. The secretion product, often called colostrum, which can be extracted from the breasts of pregnant women,2, 30 contains relatively high concentrations of sodium; chloride; and protective substances, such as immunoglobulins and lactoferrin. Casein is not present10, 54 and the lactose concentration is low2 at this time.
Stage II is the onset of copious milk secretion associated with parturition. In many species, such as cows, goats, and rats, this stage begins before birth of the young, brought about by the sharp decrease in plasma progesterone that also initiates parturition. In humans, the progesterone level does not decrease prepartum but decreases approximately 10-fold during the first 4 days after birth, accompanied by a programmed transformation of the mammary epithelium, which leads to transfer to the infant of 500 to 750 mL/d of milk by day 5 postpartum.10, 46 This transformation requires a concerted change in several processes, including changes in the permeability of the paracellular pathway between epithelial cells; changes in the secretion of protective substances, such as immunoglobulins, lactoferrin, and complex carbohydrates; and an increased rate of secretion of all milk components. Lactogenesis stage II can be monitored by changes in milk composition and volume in women and other species in which large milk samples can easily be obtained.47 The terms colostrum and transitional milk, traditionally used to describe the mammary secretion product during the first 4 days postpartum and from days 4 to 10 postpartum, respectively, do not define clear-cut temporal changes in milk composition and are not useful distinctions. Rather, the changes in milk composition that occur postpartum should be viewed as part of a continuum wherein rapid changes in composition occur during the first 4 days postpartum followed by slow changes in various components of milk throughout the course of lactation.2
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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: peggy.neville@uchsc.edu Preparation of this article was supported in part by NIH grant no. HD19547 (MCN). |
Vol 48 - N° 1
P. 35-52 - février 2001 Retour au numéro
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- Anatomy and Physiology of Lactation
- Margaret C. Neville
- Nutrient Composition of Human Milk
- Mary Frances Picciano
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