Anemia of Prematurity - Portal Neonatal

MAMMOGENESIS Section 3 of 11
The breast must undergo many years of development for proper breastfeeding to occur. A bulbshaped
mammary bud can be discerned in the fetus at 18-19 weeks' gestation. Inside the bud, a
rudimentary mammary ductal system is formed, which is present at birth. After birth, growth of the
gland parallels that of the child until puberty. The normal anatomy of the mammary gland following
pubertal development is shown in Images 1-2. The basic unit of the mammary gland is the alveolus or
acini cell that connects to a ductule. Each ductule independently drains to a duct that in turn empties to
the lactiferous sinuses. In the nipple, 15-25 openings allow milk to flow to the recipient infant.
At puberty, released estrogen stimulates breast tissue to increase in size through growth of mammary
ducts into the preexisting mammary fat pad. Progesterone, secreted in the second half of the
menstrual cycle, causes limited lobuloalveolar development. The effects of estrogen and progesterone
facilitate the formation of the characteristic structure of the adult breast, which is the terminal duct
lobular unit. Full alveolar development and maturation of epithelium, however, requires the hormones
of pregnancy.
LACTOGENESIS Section 4 of 11
Lactogenesis is the process by which the mammary gland develops the capacity to secrete milk. It
includes all processes necessary to transform the mammary gland from its undifferentiated state in
early pregnancy to its fully differentiated state sometime after pregnancy. It is this fully differentiated
state that allows full lactation. The 2 stages of lactogenesis are discussed below.
Lactogenesis has been divided into 2 stages. Stage 1 occurs by mid pregnancy. It is the process
whereby the mammary gland becomes competent to secrete milk. During stage 1, lactose, total
protein, and immunoglobulin concentrations increase within the secreted glandular fluid, sodium and
chloride concentrations decrease. The gland is now sufficiently differentiated to secrete milk, as
evidenced by the fact that women often describe drops of colostrum on their nipples in the second or
third trimester. The secretion of milk, however, is held in check by high circulating levels of
progesterone and estrogen.
Stage 2 of lactogenesis occurs around the time of delivery. It is defined as the onset of copious milk
secretion. In stage 2, blood flow, oxygen, and glucose uptake increase, and the citrate concentration
increases sharply. This increase of citrate is considered a reliable marker for lactogenesis stage 2.
Progesterone plays a key role in this stage. Removal of the placenta (ie, the source of progesterone
during pregnancy) is necessary for the initiation of milk secretion; however, the placenta does not
inhibit established lactation. Work by Haslam and Shyamala reveals that this noninhibition occurs
because progesterone receptors are lost in lactating mammary tissues. In addition, maternal secretion
of insulin, growth hormone (GH), cortisol, and parathyroid hormone (PTH) facilitates the mobilization of
nutrients and minerals that are required for lactation.
The stages of lactation can be summarized as follows (adapted from Riordan and Auerbach, 1998):
• Mammogenesis: Mammary (breast) growth occurs. The size and weight of the breast increase.
• Lactogenesis
o Stage 1 (late pregnancy): Alveolar cells are differentiated from secretory cells.
o Stage 2 (day 2 or 3 to day 8 after birth): The tight junction in the alveolar cell closes.
Copious milk secretion begins. Breasts are full and warm. Endocrine control switches to
autocrine (supply-demand) control.
• Galactopoiesis (later than day 9 after birth to beginning of involution): Established secretion is
maintained. Autocrine system control continues.
• Involution (average 40 days after last breastfeeding): Regular supplementation is added. Milk
secretion decreases from the buildup of inhibiting peptides.