The mammary gland "side population": a putative stem/progenitor cell marker?

Area of Science:

• Stem cell biology
• Hematopoiesis
• Cancer research

Background:

• Hematopoietic Stem Cells (HSCs) are crucial for blood formation and tissue repair.
• The Side Population (SP) is a cell subset identified by its ability to efflux the dye Hoechst 33342.
• The Bcrp membrane transporter is recognized as the key determinant of the SP phenotype.

Purpose of the Study:

• To investigate the role of the Side Population (SP) phenotype as a potential universal stem cell marker.
• To evaluate the stem cell potential of SP cells in mouse and human breast epithelium.
• To critically assess the evidence supporting the enrichment of stem cells within the SP population.

Main Methods:

• Flow cytometry analysis to identify and isolate the Side Population (SP) based on Hoechst 33342 dye efflux.
• Assessment of Bcrp activity as the molecular basis for the SP phenotype.
• In vitro and in vivo differentiation assays using SP cells from mouse and human breast tissue.

Main Results:

• The Side Population (SP) phenotype, mediated by Bcrp, was identified in various tissues, including mammary and breast epithelium.
• SP cells from breast tissue demonstrated characteristics of undifferentiated cells.
• These SP cells exhibited the capacity for in vitro and in vivo differentiation into luminal and myoepithelial lineages.

Conclusions:

• The Side Population (SP) phenotype, associated with Bcrp activity, is observed in diverse tissues and proposed as a universal stem cell marker.
• While SP cells in breast tissue show potential for differentiation, the evidence for stem cell enrichment remains correlative.
• Further rigorous studies are required to provide definitive proof that the SP population indeed harbors stem cells.