Breastmilk Is Unlikely to Be a Source of Mesenchymal Stem Cells

Dear Editor:

In the past decade, evidence has been accumulating supporting the presence of stem cells in breastmilk (termed breastmilk stem cells, BSCs).^1,2 Although it has been demonstrated that BSCs are primarily of epithelial origin and harbored in the lactating mammary epithelium, Kaingade et al. ³ considered BSCs or a subpopulation of them to be of mesenchymal origin (termed mesenchymal stem cells, MSCs) and conducted an in vitro investigation of endothelial growth factor and hepatocyte growth factor secretion by them. ³ In this study, the claim of MSCs in breastmilk was based on a limited number of markers used to label these cells, which were not mesenchymal-specific, and are also known to be expressed by other cell types outside the mesenchymal lineage, including mammary epithelial cells (Fig. 1). Moreover, the breastmilk-derived cell colonies morphologically displayed the classic properties of a mixed population of mammary epithelial cells and not of mesenchymal cells. ³ Finally, exclusion of the epithelial nature of these cells was not conducted by colabeling with both epithelial and mesenchymal markers. In 2010, a similar study by Patki et al. ⁴ reported expression of MSC markers by breastmilk cells; however, similar to Kaingade et al., ³ the former study's conclusions were based on the use of nonspecific markers expressed also by other cell types, with no functional tests to support the presence of MSCs in breastmilk.

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FIG. 1.

Mammary epithelial cells in the human lactating breast expressing markers known also for mesenchymal cells. CD44-green (A) and CD105-green (B) can be seen in both luminal epithelial and myoepithelial cells of alveoli. Both these markers were considered mesenchymal stem cell markers in the Kaingade et al. ³ study. Yet, they either are epithelial markers in the context of the normal lactating breast or represent cells undergoing EMT, a process that may facilitate the normal remodeling of the gland during pregnancy and lactation. ¹ Nuclei are stained in blue (DAPI). Scale bars: 10 μm.

Although the exact nature, origin, and properties of BSCs are still not fully established, state-of-art investigations using a plethora of techniques combining ex vivo breastmilk cell molecular and gene expression analyses, single cell analyses, and microscopic examinations coupled with immunohistochemical staining of resting and lactating mammary tissues as well as in vitro and in vivo studies, have revealed that the cellular composition of breastmilk is dynamic and heterogeneous.^1,2 It largely reflects the status and cellular heterogeneity of the lactating mammary epithelium when both the mother and infant are healthy during established lactation, with the majority of cells being of epithelial origin among the less than 2.5% of immune cells that protect the mammary gland and infant from infection, which originate from the maternal circulation.^1,2 Breastmilk epithelial cells constitute a cellular hierarchy comprising the mature milk-synthesizing lactocytes, few myoepithelial cells, progenitor cells with limited expansion and differentiation potential, and BSCs with multilineage potential, being capable of expanding in culture and differentiating into multiple cell lineages.^1,2 Evidence has supported the origin of the majority of BSCs from the lactating epithelium, whereas rarer cells expressing markers of the hematopoietic stem cell lineage have also been found in breastmilk samples, potentially originating from the maternal circulation.^1,2 These cells, however, are not MSCs and their origin and properties remain to be established.

Therefore, no convincing evidence currently exists supporting the presence of MSCs in breastmilk. Rather, it has been shown that breastmilk and the lactating breast contain different populations of epithelial cells, some of which express markers also known to be expressed by MSCs, such as CD44, CD105, vimentin, and others (Fig. 1). ¹ In the lactating epithelium, these markers may have a functional significance. It has been hypothesized that they may represent cells undergoing epithelial-to-mesenchymal transition (EMT), which have been shown to coexpress mesenchymal and epithelial markers. ¹ Importantly, the demonstration that epithelial cells in the lactating breast express these markers provided strong evidence that they cannot be mesenchymal (Fig. 1). ¹ In the context of the normal mammary gland, EMT has been reported in both the human and mouse, and has been hypothesized to facilitate the normal expansion of the gland and interaction between the lactating epithelium and stroma. ¹

Taken together, current evidence of the heterogeneity of breastmilk cell populations does not support the presence of MSCs in breastmilk. Further analyses are required to decipher the role and properties of EMT and the different cell types in breastmilk and the lactating mammary gland.