Physiology and Endocrine Changes Underlying Human Lactogenesis II
Abstract
Lactogenesis stage II, the onset of
copious milk secretion, takes place during the first 4 d postpartum in
women and involves
a carefully programmed set of changes in milk
composition and volume. The evidence is summarized that progesterone
withdrawal
at parturition provides the trigger for
lactogenesis in the presence of high plasma concentrations of prolactin
and adequate
plasma concentrations of cortisol. Although the
process is generally robust, delayed lactogenesis does occur with
stressful
deliveries and in poorly controlled diabetes.
Failure of early removal of colostrum from the breast is associated with
high
milk sodium and poor prognosis for successful
lactation in many women. We speculate that this problem may result from
accumulation
of a substance in the mammary alveolus that
inhibits lactogenesis, even in the face of appropriate hormonal changes
after
parturition.
The breast is one of the few organs that
undergoes most of its development postnatally. At birth the breast is
represented
by a nipple, a few small ductal elements and an
underlying fat pad. Only with the onset of puberty and the secretion of
estrogen
does the gland initiate a complex developmental
process. Ducts grow out into the fat pad guided by an interesting
structure,
the terminal end bud (1),
which guides the elongation, branching and spacing of the ducts. In
women, with the onset of menses, and in those rodents
in which there is a significant luteal phase, alveolar
structures begin to sprout from the sides of the duct stimulated by
progesterone and probably prolactin. The mature breast
resembles a flowering tree in springtime with lobular alveolar
complexes,
called terminal duct lobular units (TDLU)4 by pathologists, sprouting regularly from the major ducts. The breast reaches a stage of quiescence marked by some waxing
and waning of the TDLU driven by the hormonal changes of the menstrual cycle (2–5).
The next stage of development begins in
pregnancy. As the levels of progesterone, prolactin and placental
lactogen rise, the
TDLU undergo a remarkable expansion so that each
lobule comes to resemble a large bunch of grapes. During mid-pregnancy,
secretory
differentiation begins with a rise in mRNA for many
milk proteins and enzymes important to milk formation. Fat droplets
begin
to increase in size in the mammary cells, becoming a
major cell component at the end of pregnancy. This switch to secretory
differentiation is called stage I lactogenesis (6, 7).
The gland remains quiescent but poised to initiate copious milk
secretion around parturition. This period of quiescence
depends on the presence of high levels of circulating
progesterone; when this hormone falls around the time of birth, stage
II lactogenesis or the onset of copious milk secretion
ensues. As long as prolactin secretion is maintained and milk is
removed
from the gland, the mature function of the breast,
milk secretion, is maintained. After weaning, the TDLU involute with the
apoptosis of a large proportion of the alveolar cells
and a remodeling of the gland so that it returns to the mature quiescent
state (8).
This brief outline of a complex series of
hormonally regulated developmental events shows that mammary development
is governed
by a series of switches. Most of these developmental
switches are instigated by external hormonal influences, but
lactogenesis
stage I may be simply the consequence of the inherent
program of alveolar development and involution is most often initiated
by a failure of milk removal. In this short article we
discuss the developmental switch represented by lactogenesis stage
II. The changes in milk composition and volume
attendant on this switch in women are outlined first, followed by a
discussion
of the role of hormones in its initiation. Finally,
the scanty findings concerning the role of milk removal and a hypothesis
for a role of an inhibitor of lactogenesis are
presented as a guide to future research.
Stage II lactogenesis in women
The top left panel in Figure 1shows the time course of the increase in milk volume that occurs during the 1st wk postpartum (9–11).
Milk transfer to the suckling infants starts at a volume of <100
mL/d on d 1 postpartum, begins to increase ∼36 h after
birth and levels off at an average of 500 mL at ∼4
d. Milk composition also changes dramatically during this period, with
a fall in the sodium and chloride concentrations
and an increase in the lactose concentration that start immediately
after
birth and are largely complete by 72 h postpartum (12). These changes precede the onset of the large increase in milk volume by at least 24 h and are explained by closure of the
tight junctions that block the paracellular pathway (7). Next, the concentrations of secretory immunoglobulin A and lactoferrin increase dramatically and remain high to ∼48 h after
birth (13).
Their concentrations fall rapidly after d 2, in part because of
dilution as milk volume secretion increases, but their
secretion rate is still substantial (2–3 g/d for
each protein throughout lactation). Oligosaccharide concentrations are
also
high in early lactation, comprising as much as 20
g/kg of milk on d 4 (14, 15), falling significantly to a level of ∼14 g/L on d 30. These complex sugars are also considered to have substantial protective
effect against a variety of infections (16). Thus, during the first 2 d postpartum, large molecules with significant protective power dominate in the mammary secretion;
the total nutrient value is low, simply because the amount of milk transferred to the infant is small.
The rate of
secretion of milk volume and macronutrients in milk during the first 8 d
postpartum. Data were derived from a
study of 12 multiparous women who weighed
their infants before and after every feed for the first 7 d postpartum
and gave
frequent milk samples (9). These data illustrate the high level of coordination required to produce milk of a consistent composition during early
lactation.
The substantial volume increase occurring between 36 and 96 h postpartum is perceived as the coming in of the milk and reflects
a massive increase in the rates of synthesis and/or secretion of almost all the components of mature milk (12), including but not limited to lactose, protein (primarily casein) (17, 18), lipid, calcium, sodium, magnesium and potassium (Fig. 1). Considering the secretion patterns for each of the milk components shown in Figure 1,
the coordination achieved by the mammary epithelial cell among the
activity of the various pathways that contribute all
these different milk components is a marvel. The
question we need to ask next is: “How is this remarkable degree of
coordination
achieved?”
Regulation of stage II lactogenesis
It has long been known that abrupt changes
in the plasma concentrations of the hormones of pregnancy set
lactogenesis in motion,
although the precise hormones that accomplish the
process have, in the past, been a subject of some debate. It is clear
that
a developed mammary epithelium, the continuing
presence of levels of prolactin near 200 ng/mL and a fall in
progesterone are
necessary for the onset of copious milk secretion
after parturition (19).
That the fall in progesterone is the lactogenic trigger is supported by
evidence from many species. For example, exogenous
progesterone prevents lactose and lipid synthesis
in the mammary gland after removal of the source of progesterone,
namely,
the ovary, in pregnant rats (20, 21), mice (22) and ewes (23). In humans removal of the placenta, the source of progesterone during pregnancy in this species, has long been known to
be necessary for the initiation of milk secretion (24, 25). Furthermore, retained placental fragments with the potential to secrete progesterone have been reported to delay lactogenesis
in humans (25). Thus, without a fall in progesterone, lactogenesis does not occur.
However, other hormones must be present
for this trigger to be effective. Either prolactin or placental lactogen
are necessary
for mammary development in pregnancy, and, with the
fall in placental lactogen after removal of the placenta, prolactin is
necessary for sustained lactation in most species,
although cows may be an exception. Bromocriptine and other analogs of
dopamine
(drugs that effectively prevent prolactin
secretion) inhibit lactogenesis when given in appropriate doses (26, 27). Furthermore, prolactin was not necessary for lactogenesis in mice that had not yet given birth, the placental lactogen
from the placenta providing the necessary stimulation of prolactin receptors (22).
These data support the concept that a surge in prolactin is not the
trigger for lactogenesis. It has long been known that
glucocorticoids are necessary for milk secretion
and lactogenesis, a postulate recently confirmed in our laboratory in
mice
(22).
However, a surge of glucocorticoids is not necessary and a high dose of
glucocorticoid does not promote lactogenesis. Insulin
is generally required for induction and maintenance
of milk protein gene expression in cultured mammary cells and glands,
and deficiencies in plasma insulin led to decreased
milk production in rats and goats. However, short-term deficiencies in
insulin did not interfere with lactogenesis in rats
(28).
Thus, the available literature rules out acute changes in the
concentrations of prolactin, glucocorticoids or insulin as
triggering lactogenesis, although glucocorticoids
and prolactin are necessary at some level for a fall in progesterone to
act as the lactogenic trigger.
In summary, interpretation of the data
available from both animal and human studies is that the physiological
trigger for
lactogenesis is a fall in progesterone; however,
maintained prolactin and cortisol are necessary for the trigger to be
effective.
The caveat is, of course, that the mammary
epithelium must be sufficiently prepared by the hormones of pregnancy to
respond
with milk synthesis. Postpartum prolactin levels
are similar in both breastfeeding and nonbreastfeeding women, so that
the
basic process occurs regardless of whether
breastfeeding is initiated (27).
Similarly, glucocorticoids are necessary at some level, but their role
is currently far from defined and, indeed, has received
little study in the past two decades. Likewise, the
role of insulin in vivo is not well-defined, although it is likely to
be important in maintaining a metabolic state that
allows flux of nutrients to the mammary gland.
Does milk removal play a role in the timing or extent of lactogenesis?
A delay in the onset of lactogenesis has been reported with poorly controlled diabetes (9, 11, 29) and stress during parturition (18).
The mechanism is unknown but the delay did correlate with high cord
blood glucose and cortisol. The delay occurred in women
who put the infant to the breast during the first 2
d postpartum, suggesting that poor milk removal is not the cause of
stress-related
delayed lactogenesis. In another study high breast
milk sodium concentrations on or before d 3 were observed in clinical
situations
in which the infant failed to latch on properly (Fig. 2) (30, 31).
These high sodium concentrations were statistically related to
impending lactation failure and could be reversed by the
use of a breast pump to obtain effective milk
removal. These observations suggest that milk removal and/or effective
suckling
are necessary to obtain junctional closure and
potentially an increase in milk volume secretion, at least in some
women. Evidence
on this point is mixed. Careful evaluation of milk
composition and breast-filling in nonbreastfeeding women by Kulski et
al.
(27) suggested that milk removal is not needed for the programmed physiological changes that bring about lactogenesis stage II.
In contrast, Chapman and Perez-Escamilla (32)
found that formula feeding before lactogenesis was associated with a
delay in the perception of lactogenesis. Furthermore,
the time of first feeding and the breastfeeding
frequency on d 2 postpartum were positively correlated with milk volume
on
d 5 postpartum (18),
suggesting that milk removal at early times after birth increases the
efficiency of milk secretion. In the last decade
local factors dependent on milk removal have been
implicated in the regulation of milk secretion during lactation (33).
The question is: “What are these local factors and when do their
effects become apparent?” It is possible to speculate
that they are present in high concentration in the
scant prepartum secretion product of the breast and that if not removed
early in the prepartum period, they contribute to
inhibition of lactogenesis stage II, even with adequate hormonal
changes.
Although Kulski and coworkers were unable to detect
any delay in the change in milk composition in their study of
nonbreastfeeding
women, it is possible that even the small changes
in the amount of milk remaining in the breast engendered by removal of 5
or 10 mL of secretion product per day by manual
expression could stimulate lactogenesis stage II.
FIGURE 2
Milk sodium in
poorly nursing infants. Three mothers of full-term infants visited the
clinic on d 3 of lactation complaining
of poor milk production and problems with
infant latch on. Milk samples were taken for measurement of sodium and
the mothers
were instructed to use a breast pump several
times daily and to take small samples of milk daily, which were later
brought
to the clinic for analysis of sodium. Sodium
concentrations in right and left samples are plotted. Note that once
pumping
was initiated the sodium concentrations
declined with a time course similar to that in reference women, shown as
a heavy black
line.
We can conceptualize the problem of failed lactogenesis as preglandular, glandular or postglandular (31).
An example of preglandular would be hormonal causes, such as retained
placenta or lack of pituitary prolactin. Glandular
causes might be surgical procedures, such as
reduction mamoplasty or, possibly, insufficient mammary tissue.
Postglandular
would be any cause for ineffective or infrequent
milk removal. This latter aspect has received insufficient attention. A
neonatal
intensive care nursery where careful correlation
between early milk removal and subsequent lactation performance is
possible
would be ideal for such a study. In these studies
milk volume would be most easily measured in women who are using a
breast
pump to provide milk for their infants. Very small
samples could be taken for determination of milk sodium and casein on a
daily basis to allow a true distinction to be made
between failure of tight junction closure and reopening of the junctions
due to lack of milk removal. Such data will be
valuable in determining how to manage the initiation of breastfeeding in
mothers
of sick infants as well as in sick mothers of well
infants. It would also be of great value to determine, using techniques
developed at the Hannah Research Institute in Ayr,
Scotland in collaboration with Prentice et al. (34),
whether a chemical present in colostrum inhibits milk secretion in an
in vitro model system. Identification of such a chemical
and precise measurements of its concentration could
provide a new index for predicting which women are likely to have
problems
initiating stage II lactogenesis.
Footnotes
-
Presented as part of the symposium “Human Lactogenesis II: Mechanisms, Determinants and Consequences” given at the Experimental Biology 2001 meeting, Orlando, FL on April 2, 2001. This symposium was sponsored by the American Society for Nutritional Sciences and was supported by educational grants from Medela, Ross Labs and Wyeth Nutrition International. Guest editors for this symposium publication were Nancy F. Butte, Baylor College of Medicine, Houston, TX and Rafael Perez-Escamilla, University of Connecticut, Storrs, CT.
