What Are Stem Cells

What Are Stem Cells In Short

Cells that have the ability to self-renew and the capacity to differentiate into other cell types are called stem cells. In other words, stem cells can produce identical daughter cells (in terms of self-renewal) and in addition can develop into a progeny with more specialized functions (known as differentiated cells). During early life growth and under certain conditions they have the potential to differentiate into specialized cell types. Stem cells participate in the internal repair system, as they are able to multiply their number without limit and their replace other cells as long as the organism is alive. Stem cells multiply through mitosis and each resulting cell can either remain a stem cell or turn into another more specialized cell (such as muscle cell, epithelium cell, blood cell, bone cell, etc.).

What are stem cellsThis definition is applicable for fetal and embryonic stem cells, which not available during the entire lifetime of an organism. However it is also important to make a separation between adult stem cells and transient adult progenitor cells that have a limited capability for self-renewal. That’s why, when speaking about adult stem cells, it is quite important to take into account that these cells are able to self-renew during the lifespan of the organism. Another important aspect is the potency type of the stem cells. Some stem cells can produce only one type of specialized cells (unipotent stem cells), while others are able to differentiate into multiple cell types (multipotent stem cells). To sum up, in order to fully describe a cell as stem cell we have to consider it’s capacity to replicate, clonality and potency type.

The main properties that distinguish stem cells from “normal” cells are 1) they are not specialized cells that can multiply and renew themselves via simple cell division (mitosis) even after continuous intervals of inactivity and 2) under certain conditions, stem cells can be stimulated to turn into specialized cells with specific tissue or organ functions. Normally in the living organisms, stem cells regularly perform mitotic divisions in order to replace and repair damaged tissues (like in the gut and bone marrow). However there are organs, such as the heart and pancreas, in which stem cells can repair damaged cells under very specific circumstances.

Stem Cell Self-Renewal

Very often when describing stem cells replication capabilities, are used terms like “immortal”, “continuous” or “unlimited” which are not absolutely  correct. When cultured in vitro, somatic cells have the ability to multiply limited times – up to 80 cell divisions until reaching the so called  “replicative arrest”. In contrast, stem cells have the ability to divide twice the number of population doublings compared to somatic cells, without an oncogenic transformation event to occur. Thus stem cells are capable of extensive proliferation and this feature is often referred as “immortality”. Some stem cell types, like embryonic stem (ES) cells as well as adult neural stem cells (NSCs) fulfill this requirement. It is hard to define clearly what are the proliferation limits of the adult stem cells in vitro, because the factors that influence their self-renewal capacity ex vivo are still not completely clear. Most adult stem cell  self-renewal studies apply single or serial stem cell transfer into an acceptable host, as in most cases are used adult hematopoietic stem cells (HSCs). Most of the studies explore the proliferative ability of adult stem cells are probably in vivo, where they must sufficient capacity to cover the entire life span of the organism. To summarize, stem cells have significantly increased self-renewal capabilities compared to somatic cells, but still cannot be considered as “immortal”.

Stem Cell Clonality

It is even more important to state clearly that stem cells are clonogenic cells that have the ability to form more stem cells. This feature is relevant for the complete depiction of self-renewal, potential, and lineage capabilities of ste cells. This feature determines the exact lineage of stem cells and helps to clarify what actually constitutes a cell line.
Simplified, a cell line is a population of cells which can be cultivated, freezed, unfreezed and finally re-cultivated in a culture in vitro. In reality, the cells in a cell line might not derive from a single cell, but might be a mixture of a population derived from the stem cells and an additional supportive cells required for the generation of the putative stem cells. Thus the recommended reference to a cell line must explain also its ancestry.

Stem Cell Potency

As already noted multipotent stem cell can generate multiple different types of differentiated cells. Each cell type has it’s own morphological characteristics as well as gene expression profile. Many scientists consider that self-renewing cell which can produce only one differentiated cell type (the so called unipotent cells) are still stem cells, but it is even more correct to be described as progenitor cells. Compared to stem cells, progenitors are just more limited in terms of cell differentiation and self-renewal capabilities, and thus could be considered as stem cells descendants.

What Is A Stem Cell Definition

Answering “What are stem cells?” in a single sentence – stem cell are clonal, self-renewing, multipotent cells, that can generate multiple differentiated cell types.



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