The Search for Stem Cell in Human Breast Milk

FAN YIPING

Publication

The Search for Stem Cell in Human Breast Milk

FAN YIPING

Abstract

Mammary stem cells (MaSC) are under stringent scrutiny these recent years, due in no small part to the fact that breast cancer is the most common cancer among females worldwide and that MaSC have been extensively studied as a system to delineate the pathogenesis and treatment of breast cancer. However, research on MaSC requires tissue biopsies which limit the quantity of samples available. Taking reference from other systems with epithelial cells? lining, studies have document the presence of the of the stem cell in the luminal discharges due to the proximity of the stem cell niche to their luminal cavities. For example, mesenchymal progenitor cells have been isolated through the collection of human menstrual blood and urothelial stem cells have been derived from urine. Extrapolating these reports to a tissue of common lineage, they suggest the plausibility of mammary stem/ progenitor cells being shed into human breast milk (HBM) either by means of sloughing or active shedding into the lumen for yet unknown purposes during a time of intense cellular turnover. In my study, I hypothesised that stem cells are shed into the HBM and aimed to isolate them from HBM. Successful derivation of these cells from HBM may aid progress of research in mammary stem/ progenitor cells by providing a novel and non-invasive source. In addition to allowing a comprehensive understanding of the various components of HBM throughout the entire lactational period, this novel cell source may contribute to the reconstruction of the mammary tissues, and the unshedding of mechanisms behind the link between MaSC and breast cancer. HBM contains a mixed population of cells of haemopoietic, mesenchymal and neuro-epithelial lineages. Further analysis of the adherent cultured cells reveals a heterogeneous population of cells with differential expression of cytokeratin (CK)5, CK14, CK18 and CK19. In addition, there was a small population of nestin-positive cells (16.0?2.6%), of which 53.1?4.2% and 55.2?2.85% co-stained with CK5 and CK 19 respectively, and only 22.3?1.5% and 26.0?2.7% co-stained with the more mature epithelial markers, CK14 and CK18 respectively. This suggests a hierachical model of mammary cells within our culture system with the nestin+ cells being the putative MaSC followed by the intermediates of nestin+CK5+ and nestin+CK19+ cells, which are in turn more immature than the nestin+CK14+ and nestin+CK18+ cells. The terminally differentiated cells in our model would be the nestin-CK14+ and nestin-CK18+ cells. In order to prospectively isolate putative MaSC for characterisation, two different approaches were undertaken. Firstly, flow cytometric sorting of side population (SP) cells revealed that 2% of cellular component of HBM were able to exclude Hoeschst 33342 dye, which selects for primitive stem cell populations. HBM SP cells co-expressed nestin but not the mature epithelial marker CK18. However, attempts to culture expand these putative MaSC in a wide range of in vitro conditions did not result in any mammary nor haemopoietic stem cells. Next, prospective isolation through selection of CD133 positive cells was done. Two percent of HBM were CD133-positive. These cells did not contain any haemopoietic activity, nor were attempts to expand them successful. The derivation of MaSC from HBM would have availed a non-invasive source of stem cells of relevance to the understanding of lactation biology, oncogenesis and regenerative medicine. While some markers of primitive cell types of hierarchical importance were detected, there was no evidence of any cell types with self-renewal and multi-lineage differentiation capacity. This may in part be due to poorly defined growth conditions, or the absence of such cell types in HBM.