Breast Physiology
Douglas Danforth, Ph.D.
The Ohio State University
Objectives:
1. List the relevant hormones involved in lactation and describe the functions of each.
2. Describe the gross anatomy and normal histology of the female breast.
3. Describe the changes that occur in the breast during puberty, pregnancy, and lactation.
4. Differentiate between milk synthesis and milk ejection. Define colostrum and how and where it
is produced.
5. Describe the reflex pathways and mechanisms for the control of prolactin and oxytocin
secretion and discuss the roles of these hormones in lactation and milk ejection.
There are several hormones that affect the growth and development of the breast as well as regulating
lactation. The role and importance of each varies throughout life.
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Perhaps the most important hormone regulating breast growth and development
(mammogenesis) is estrogen. Estrogen is a potent mitogen for breast tissue, stimulating the
growth and branching of the duct system as well as deposition of adipose and connective tissue
during puberty. Estrogen is also especially important in the stimulation of lobulo-alveolar
growth during pregnancy.
Prolactin from the anterior pituitary is essential for stimulating the synthesis of milk
(lactogenesis) during pregnancy. Basal prolactin levels are highest at the end of gestation and
fall after parturition. Suckling induces large surges of prolactin to maintain lactogenesis.
Whereas prolactin is a primary hormone regulating lactogenesis, oxytocin is essential for
lactation or milk secretion, including milk ejection from the alveoli. Ocytocin stimulates the
contraction of the myoepithelial cells surrounding the lobules. We will examine this more closely
in a minute.
Another hormone that is important in regulating mammary gland function is progesterone.
Since estrogen and prolactin are steadily rising during pregnancy, and lactogenesis is stimulated,
it is important that premature lactation be avoided. The elevated levels of progesterone during
pregnancy antagonize the actions of prolactin on lactation throughout gestation. The
precipitous decline in circulating steroids following parturition removes this inhibitory
component and allows lactation to commence.
In this composite image of the adult breast, several stages of breast development are depicted (Figure
1).
Note that all of these
structures/stages do not appear
in the breast at the same time.
The mammary gland is a
compound tubulo-alveolar
structure comprised of 15-25
irregular lobes radiating out from
the nipple. Individual lobes are
embedded in adipose tissue and
separated by dense layers of
connective tissue. Each lobe is
further subdivided into lobules
connected to the nipple by
lactiferous ducts.
Prior to pregnancy ducts with few alveoli are present. During the first months of pregnancy these alveoli
begin to develop and by mid-gestation the alveoli are enlarged and have acquired a lumen. During
lactation the alveoli dilate and become filled with milk for lactation. Finally, after breastfeeding has
stopped, the lobulo-alveolar glands regress and return to the pre-pregnant state.
Looking at this in more detail, at birth the breast consists mostly of a rudimentary duct system leading to
a small nipple (Figure 2).
In some infants a small
amount of milk is produced at
birth – called witch’s milk,
which is a response of the
fetal mammary tissue to the
lactogenic hormones of
pregnancy and the sudden
withdrawal of placental
steroids at birth. This
secretory function is transient
and the breast quickly
becomes quiescent until
puberty.
At puberty, mammary growth accelerates as ovarian cycles begin. The growth and branching of ducts in
response to estrogen results in lobule formation, and the increase in bulk is largely due to adipose tissue
deposition.
At the beginning of pregnancy there is a rapid growth and branching of the terminal portions of the
rudimentary lobules under the influence of chorionic gonadotropin. As pregnancy continues, lobules of
alveoli develop extensively and begin to take over space formerly occupied by stroma. This lobuloalveolar growth is regulated by estrogen, progesterone, prolactin, growth hormone, and adrenal
steroids.
After parturition, lactation ensues and is stimulated primarily by prolactin although other hormones
such as adrenal steroids may also participate.
In this graphic of a lactating alveolus, the arrangement of the milk secreting alveolar cell and the
myoepithelial cell can be seen (Figure 3).
Lactation or milk secretion
involves two processes;
lactogenesis and ejection.
Lactogenesis or milk
production is stimulated by
prolactin and begins during the
5th month of gestation. The
milk secreting alveolar cells
form a single layer of epithelial
cells lining the lumen of the
lobule. These cells contain
prolactin receptors and begin
to secrete the components of
milk into the lumen of the
lobule. Lactogenesis is only full
expressed after parturition.
The second process required for maintenance of lactation is removal of the milk from the lobule via milk
ejection. Oxytocin from the posterior pituitary causes contraction of the myoepithelial cells surrounding
the alveolus resulting in ejection of milk from the lobule into the lobulo-alveolar duct where it is
removed by the suckling infant.
The initial milk produced during pregnancy and immediately after parturition is called colostrum. It is a
rich source of immunoglobulins that provides passive immunity to the infant during the first days
postpartum. Normal milk has more than 100 constituents including fat, protein, lactose, minerals and
several hormones.
The suckling reflex involves a complex coordination of afferent neural stimulation and efferent
endocrine responses (Figure 4).
1. Mechanical stimulation by the nursing infant stimulates the supraoptic and paraventricular
nuclei of the hypothalamus via a multisynaptic neural pathway originating in the nipple. Suckling
inhibits dopamine and stimulates prolactin releasing factor secretion resulting in increased
prolactin production by the anterior pituitary, which stimulates lactogenesis.
2. In addition, oxytocin secretion from the posterior pituitary is stimulated which results in
elevated circulating oxytocin levels causing contraction of the myoepithelial cells and milk
ejection.
3. The third response to the suckling stimulus is the inhibition of hypothalamic GnRH secretion.
This causes a decrease in gonadotropin production by the pituitary and as such, ovarian
folliculogenesis is inhibited. As long as the infant is allowed to nurse at will, reproductive
function will be inhibited – this is sometimes referred to as natures contraceptive. Once the
infant starts to wean, the inhibition of GnRH is reduced and gonadotropin induced follicular
growth and resultant normal menstrual cyclicity can resume.