The cellular hierarchy and composition of human breastmilk

Abstract

During pregnancy and lactation, mammary stem cells remodel the breast into a milk‐secretory organ by proliferating and subsequently differentiating into both myoepithelial cells and lactocytes. Thus, we reasoned that the lactating gland, which is the fully mature organ, can offer a more comprehensive view of the mammary cellular hierarchy and dynamics than the resting gland. To non‐invasively access cells from the human lactating breast, we used freshly expressed breastmilk from which cells were isolated and characterized ex vivo for expression of markers of various lineages and differentiation stages. These included markers of pluripotent and multipotent stem cells, epithelial and mammary‐specific markers, hematopoietic and mesenchymal markers. Rare specimens of human lactating breast tissues were used to examine the origin of the different cell types identified in breastmilk. The ex vivo and in situ analyses were coupled by in vitro studies, which showed that along with the more differentiated cells, breastmilk contains progenitor cells and stem cells, some of which display pluripotency. These findings reveal a novel cellular hierarchy in breastmilk, characteristic of the lactating breast. Importantly, the presence of stem cells in breastmilk raises the question of the role of these cells for the breastfed infant.Grant Funding Source: Women and Infants Research Foundation; Medela AG