Pluripotent stem cells: Basic biology or else differentiations aimed at translational research and the role of flow cytometry.

Pluripotent stem cell research has revolutionized the modern era for the past 14 years with the advent of induced pluripotent stem cells. Before this time, scientists had access to human and mouse embryonic stem cells primarily for basic research and an attempt towards lineage-specific differentiations for cell therapy applications. Regarding pluripotent stem cells, expression of bonafide marker proteins such as Oct4, Nanog, Sox2, Klf4, c-Myc, and Lin28 have been considered giving a perfect readout for pluripotent stem cells and assessed using an analytical flow cytometer. In addition to the intracellular markers, surface markers such as stage-specific embryonic antigen-1 for mouse cells and SSEA-4 for human cells are needed to sort pure populations of stem cells for further downstream applications for cell therapy. The surface marker SSEA-4 is the most appropriate for obtaining pure populations of human pluripotent stem cells. When differentiated in a controlled manner using growth factors or small molecules, it is mandatory to assess the downregulation of pluripotency markers (Oct4, Nanog, Sox2, and Klf4) with subsequent up-regulation of stage-specific differentiation markers. Such assessments are done using flow cytometry. Pluripotent stem cells have a high teratoma-forming potential in vivo. Small amounts of undifferentiated PSCs might lead to dangerous teratomas upon transplantation if leftover in the pool of differentiated cells. Hence, flow cytometry is essential for sorting out PSC populations with teratoma-forming potential. The pure populations of differentiated progenitors need to be flow-sorted before differentiating them further for cell therapy applications. For example, Glycoprotein 2 is a specific cell-surface marker for pancreatic progenitors that enables one to sort the pancreatic progenitors differentiated from human PSCs. Taken together, analytical flow cytometry, and cell sorting provide indispensable tools in PSC research and cell therapy.