Question 2 What are totipotent, pluripotent, multipotent, and unipotent stem cells?

Answer 2

These are all stem cells with the ability of self-renewal and differentiation, but they vary in their differentiation potential. Totipotent stem cells can differentiate into any cell type or give rise to an entire organism. The fertilized egg and cells produced by the first few divisions are totipotent. Pluripotent stem cells can differentiate into nearly all cells, but not an entire organism. Embryonic stem (ES) cells are a subtype of pluripotent stem cells. Multipotent stem cells are limited to differentiating into closely related families of cells, often from the same germ layer from which the stem cell was derived. Unipotent stem cells can differentiate into only one specific cell type.

Hans RS. The potential of stem cells: an inventory. In: Knoepffler N, Schipanski D, Sorgner SL, eds. Human Biotechnology as Social Challenge. Burlington, PA: Ashgate Publishing; 2007:chap 3; 28.

Question 4 What are the main approaches for researchers to obtain mouse and human embryonic stem (ES) cells?

Answer 4

Mouse ES cells can be obtained by directly isolating the inner cell mass from mouse blastocyst (usually 10–20 cells per embryo). These cells can grow in the laboratory with the proper nutrients to keep their pluripotent ability. Human ES cells are also obtained from blastocyst stage embryos, which are created by in vitro fertilization in a petri dish.

Goodell MA, Nguyen H, Shroyer N. Somatic stem cell heterogeneity: diversity in the blood, skin and intestinal stem cell compartments. Nat Rev Mol Cell Biol. 2015;16:299–309.

Martello G, Smith A. The nature of embryonic stem cells. Annu Rev Cell Dev Biol. 2014;30:647–675.

Question 6 How are adult stem cells identified?

Answer 6

To identify some cells as adult stem cells, scientists use both in vivo and in vitro methods. In vivo, through lineage tracing, these cells are labeled with molecular markers and the specialized cell types that they generate are determined. These cells can also be isolated and then transplanted into other normal animals, to determine whether the cells replace their tissue of origin, or into other animals without these stem cells, to determine whether the transplanted cells can form their tissue of origin.

In vitro, stem cell properties can be enriched using methods such as clonogenic assays, side population assays, and Aldefluor assay. Stem cells can also be enriched in vivo and in vitro by the expression of a distinctive set of markers including cell surface antigens and transcription factors. However, in vitro culture conditions can alter the phenotype. Some stem cell markers may not be expressed only in stem cells and so using a combination of several markers is needed to help identify stem cells. Functional validation should be performed with serial extreme limiting dilution in vivo. All these methods have their advantages and limitations and are often combined to identify stem cells.

Goodell MA, Nguyen H, Shroyer N. Somatic stem cell heterogeneity: diversity in the blood, skin and intestinal stem cell compartments. Nat Rev Mol Cell Biol. 2015;16:299–309.

Martello G, Smith A. The nature of embryonic stem cells. Annu Rev Cell Dev Biol. 2014;30:647–675.