Once formed, the lactiferous ducts and interlobular duct system are stable
and unaffected by fluctuating hormone levels during the menstrual cycle, pregnancy
and lactation. The TDLUs, however, are dynamic and undergo changes with alterations
in hormone levels. These changes involve both the epithelium and the intralobular
Follicular phase: During the follicular phase of the menstrual cycle, the
TDLUs are at rest and do not show any growth. The intralobular stroma is dense
and indistinct from the dense interlobular stroma.
Luteal phase: After ovulation, the terminal duct epithelium proliferates
and the number of terminal ducts within a lobule increases and the basal epithelial
cells become vacuolated. The intralobular stroma s edematous and loose and
becomes distinct from the interlobular stroma. These changes manifest as progressive
fullness, heaviness and tenderness of the breast.
Menses: As the levels of estrogen and progesterone fall with the onset of
menstruation, there is an increase in apoptosis in the TDLU. Lymphocytes infiltrate
the intralobular stroma, which becomes dense. The TDLU finally regresses to
its resting appearance.
During pregnancy there is a striking increase in the number of terminal ducts
and the TDLUs are enlarged, in response to the rising sex hormone levels,
to form the major component of the breast tissue. The intralobular and interlobular
stromas become almost inapparent.
In the lactating breast the individual terminal ducts form acini, which show
epithelial vacuolization due to the presence of secretions that also fill
their lumina. After lactation, the units involute and return to their old
After the menopause, the TDLUs atrophy due to the low trophic hormone levels
so that only small residual foci remain. The lactiferous ducts and interlobular
duct system remain but the interlobular stroma is reduced in amount accompanied
by a relative increase in fatty tissue.