The TDLUs, however, are dynamic and undergo changes with alterations in hormone levels. These changes involve both the epithelium and the intralobular stroma. The intralobular stroma is dense and indistinct from the dense interlobular stroma. 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.
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 structure.