Types of Stem Cells

Totipotent SCs:

These cells have total potentiality to form a complete full organism. Totipotent SCs are well known for their capability of differentiating into all cell types and lineages that can form different types of tissues and organs. Totipotent SCs have unlimited capability, and in the early stages of embryonic development. They can differentiate to extra embryonic membranes and tissues, the embryo, the postembryonic tissues and final organs.

Pluripotent SCs

More specialized cells in generating different types of tissues and organs. These types of cells can differentiate into any cells type but are incapable of producing a whole organism. Pluripotent SCs can give rise to types of cells that develop from the three germ layers (ectoderm, mesoderm, and endoderm). Induced Pluripotent stem cells (iPSCs) are a type of Pluripotent stem cells generated by the genetic introduction of 4 genes encoding for transcription factors that can induce the conversion of adult cells into Pluripotent stem cells.

Multipotent SCs

These SCs are more specialized cells that are usually found in different organs of the human body. They are capable of forming cells from more than one germ layer and differentiating to different cell types within the same organ or tissue. Pluripotent and multipotent SCs share common characteristics and can be easily confused with one another. Accordingly, the classification of either type depends solely on the differentiation potential of each type of cell. Furthermore, the differentiation potential of multipotent SCs is not easily understood for several reasons. These include the fact that their physiological sites of survival referred to as “stem cell niches” usually differ from one site to another. In general, individual multipotent stem cells, that are part of a homogenous population of SCs, are usually committed to forming one cell lineage, compared to pluripotent SCs that have the potential to form more than one cell lineage.

Unipotent SCs

These cells are commonly found in specific organs and tissues of adult organisms and are limited to differentiating to one cell type only, depending on a complex structure of intrinsic and extrinsic factors. These SCs are responsible for the steady state of self-renewal through constant regeneration and repair of that organ.

Sources of Stem Cells

There are two main sources of stem cells:

Embryonic stem cells:

Pluripotent cells that exist in the early stages of embryonic development, specifically in the inner mass of blastocysts, and go on to form all the cells of the adult body. These cells no longer exist after five days of development.

Adult stem cells:

SCs that can be found in almost all body tissues including blood, brain, liver, intestine, or skin. They are more tissue specific cells committed to become a cell from their own tissue of origin, but cannot go to form all tissues of the body.

Based on the differentiation potentiality of SCs, they can be categorized into hematopoietic stem cells (HSCs) and non-hematopoietic stem cells (NHSCs). HSCs are cells that give rise to blood cell lineages and are capable of differentiating to white blood cells (WBCs), red blood cells (RBCs), and platelets. NHSCs are cells that give rise to tissues and organs other than blood.