For decades, mammary stem cells have been portrayed as the hidden architects of the breast: rare, self-renewing cells capable of producing the specialized cell types that build and maintain mammary tissue. A new review in Experimental & Molecular Medicine brings together the latest understanding of how these cells behave during development, how they respond to hormonal and environmental signals, and why their remarkable adaptability may also contribute to breast disease. The article, by Eunji Lee, Yutong Tao, A.J. Wang and colleagues, presents mammary stem cells not as static entities, but as dynamic participants in a constantly remodeling organ.
The mammary gland is unusual because much of its structure is assembled after birth. During puberty, ducts extend through the surrounding fat pad, while pregnancy triggers the formation of milk-producing alveoli. After lactation, much of this tissue is dismantled through a process known as involution. Mammary stem and progenitor cells must therefore operate within a tissue that repeatedly grows, differentiates, expands, and regresses. Their activity is influenced by ovarian hormones, growth factors, immune cells, extracellular matrix components, and mechanical signals from neighboring tissue.
At the center of this biology is the concept of cellular hierarchy. Mammary epithelial cells are broadly divided into basal or myoepithelial cells, which help contract the ducts, and luminal cells, which line the ducts and include milk-secreting cells. Stem-like populations and lineage-restricted progenitors can occupy different positions within this hierarchy, although modern evidence suggests that the boundaries are more flexible than once believed. Under particular developmental or damaging conditions, mature cells may change identity or regain progenitor-like properties, a phenomenon known as lineage plasticity.
This plasticity is controlled by a complex molecular network. Hormone receptors, including estrogen and progesterone receptors, transmit signals that coordinate growth with reproductive cycles. Wnt, Notch, Hedgehog, and transforming growth factor beta pathways help regulate self-renewal, differentiation, and tissue architecture. Growth factors such as epidermal growth factor and fibroblast growth factor can further alter cellular behavior. Rather than functioning independently, these pathways form interconnected signaling circuits that allow mammary cells to interpret both internal hormonal changes and external conditions in their local microenvironment.
The surrounding niche is just as important as the stem cells themselves. Fibroblasts, immune cells, blood vessels, adipocytes, and the extracellular matrix all contribute signals that can either preserve stemness or encourage differentiation. The matrix provides physical support, but it also acts as a biochemical signaling platform. Changes in its stiffness, composition, or organization can affect cell polarity, migration, and gene expression. This helps explain why a genetically normal mammary cell may behave very differently in healthy tissue compared with an inflamed, fibrotic, or tumor-associated environment.
Technological advances are now revealing this complexity at unprecedented resolution. Single-cell RNA sequencing can measure gene activity in individual cells, allowing researchers to distinguish closely related states that were previously grouped together. Spatial transcriptomics adds geographic information, showing where specific cell populations and signals are located within tissue. Lineage tracing, organoid cultures, and three-dimensional models provide complementary ways to test whether a cell can truly self-renew or generate multiple descendants. Together, these approaches are challenging the idea that one universal mammary stem-cell marker can identify every regenerative cell in every biological context.
The same flexibility that supports normal development may become dangerous when regulatory systems fail. In breast cancer, altered stem and progenitor states may help malignant cells survive treatment, seed new tumors, or spread to distant organs. Cancer stem-like cells are not necessarily a fixed population; they can emerge when tumor cells are exposed to inflammatory signals, metabolic stress, low oxygen, or chemotherapy. This reversible transition, often called phenotypic plasticity, may allow a tumor to regenerate even after most rapidly dividing cells have been eliminated.
Inflammation and aging add further layers of risk. Persistent inflammatory signaling can reshape the mammary niche and activate pathways associated with survival and proliferation. Aging may reduce regenerative capacity while increasing genomic instability and changes in the extracellular matrix. Pregnancy, obesity, endocrine disruption, and tissue injury can also modify the signals surrounding mammary stem and progenitor cells. The review emphasizes that disease cannot be understood solely by examining epithelial cells; it must also account for the ecosystem in which those cells reside.
These insights could influence future approaches to breast-cancer prevention and treatment. Instead of targeting only rapidly dividing tumor cells, researchers may need to design therapies that disrupt the signals maintaining stem-like states or restore normal differentiation programs. Potential strategies include blocking abnormal Wnt or Notch activity, modifying tumor-associated inflammation, targeting supportive stromal cells, or changing the physical properties of the tumor microenvironment. However, because the same pathways are essential for normal tissue repair, effective treatments will require precise control to avoid damaging healthy regenerative cells.
The emerging picture is both more complicated and more promising than the traditional stem-cell model. Mammary stem cells appear to be defined less by a permanent label than by their ability to respond to changing conditions. Their behavior depends on developmental stage, tissue location, hormonal state, injury, and disease. By integrating molecular biology, advanced imaging, computational analysis, and functional experiments, the field is moving toward a dynamic model of mammary regeneration. Understanding that adaptability may ultimately reveal why healthy breast tissue renews itself so effectively—and why the same regenerative machinery can, under the wrong circumstances, help disease take hold.
Subject of Research: Mammary stem cells, their role in breast development and regeneration, and their regulation in disease and breast cancer.
Article Title: Dynamics, function, and regulation of mammary stem cells in development and disease
Article References: Lee, E., Tao, Y., Wang, A.J. et al. “Dynamics, function, and regulation of mammary stem cells in development and disease.” Experimental & Molecular Medicine (2026). https://doi.org/10.1038/s12276-026-01780-6
Image Credits: AI Generated
DOI: 10.1038/s12276-026-01780-6
Keywords: Mammary stem cells, breast development, tissue regeneration, cellular plasticity, breast cancer, mammary gland, stem-cell niche, organoids, single-cell analysis, tumor microenvironment
Tags: breast disease and cancerbreast tissue developmentbreast tissue differentiationdynamic behavior of mammary stem cellsenvironmental influences on mammary stem cellsextracellular matrix in breast tissuehormone-responsive mammary stem cellsmammary gland remodelingmammary progenitor cellsmammary stem cell hierarchymammary stem cellspostpartum mammary gland involution
