Breast Milk: An Unappreciated Source of Stem Cells

Research shows that breastfeeding provides multiple lifelong biologic advantages to children, including increased survival and improved neurocognitive and immune functioning. These and other advantages have been linked to breast milk components, including longchain fatty acids, epithelial growth factor, immunestimulating cells, and live maternal immune cells (lymphocytes and plasma cells), which survive gastrointestinal transport and implant in neonatal tissues. Previous research related to stem cells and breastfeeding has focused on the identification of mammary gland stem cells rather than their biologic functions. Gudjonnson and colleagues demonstrated that stemlike cells exist in mammary epithelial cells in humans, mice, and rats. However, these investigators did not evaluate inclusion of stem cells in the cellular components of breast milk, passage to the newborn, or establishment of lactation-derived stem cells in infant tissues. Newer research attempts to characterize the biological activity of mammary stem cells. Boulanger and colleagues suggested mammary stem cells play roles in mammary epithelial regeneration and hormone responsiveness. We propose that maternal stem cells secreted in milk and suckled by the infant may be an important but so far unappreciated live, functional component of breast milk. Perinatal stem cell research suggests that humans potentially contain persisting stem cell–like cells derived from perinatal placental cell transfer. Recent research establishes the extent and potential significance of physiologic bi-directional cell transport across the placenta. Differentiated XY chromosomal cells (presumably fetal) have been found in maternal blood and organs up to 30 years after birth. Such cells are hypothesized both to provide comparative survival advantages for mothers and to be involved in the pathogenesis of immune-mediated disease such as scleroderma and thyroiditis. Less well-studied is the reverse, whereby maternal cells cross the placenta and implant in perinatal tissues. Support for this hypothesis is provided by the findings that undifferentiated or pluripotent maternal stem cells have been found in human fetal cord blood, and differentiated maternal cells have been found in human newborn tissues and organs including the liver. The physiologic roles and responses to such nonself (maternal) engrafted stem cells remain uncharacterized. We suggest that the mother-to-perinatal engraftment continues postnatally by way of lactation. We suggest that future research should examine breastfeeding as a physiologic pathway by which the suckling infant receives maternal stem cells. Transportation of maternal stem cells to the breastfeeding infant may be associated with possible benefits (increased populations of progenitor cells that assist with repair of damaged tissues) or cause disease (graft versus host reactions, autoimmune processes, or stem cell–derived neoplasia). Given that breast milk contains multiple cellular and nonnutritive components and that breast epithelium contains stem cells, we suggest that breast milk may contain stem or pluripotent cells and that these cells may implant and serve various functions in the breastfed individual, and that these cells may be identified and characterized using newly available stem cell biomarkers. Investigation of proposed vertical stem cell Commentary