What are Stem Cells?

Stem cells build tissue when and where it's needed. Stem cells are the body's silent reserves. At any given moment, many of the stem cells in your body won't be doing very much. They will only spring into action when you need either to produce more stem cells or make more of other, specialized types of cells. And they're not just found in people. All multicellular organisms, from plants to humans, need stem cells.

What are the Major Types of Stem Cells?

Stem cells are broadly classified as either adult or embryonic. Technically, most researchers prefer the term tissue stem cells for all stem cells other than those from embryos.

According to the classification of developmental stages, it can be divided into embryonic stem cells and adult stem cells. Embryonic stem cells include ES cells (Embryonic Stem Cells) and EG cells (Embryonic Germ Cells). Adult stem cells include neural stem cells (NSCs), hematological stem cells (HSCs), bone marrow mesenchymal stem cells (MSCs), epidermal stem cells (EPSCs) and so on.

--Embryonic stem cells:

Embryonic stem cells (ES cells) are obtained by extracting cells from very early embryos-at the blastocyst (hollow ball) stage-and growing them in laboratory dishes.

Embryonic stem cells are easy to isolate and purify, at least in comparison with most tissue-specific cells, which exist in vanishingly small numbers deep within tissues. Although human embryonic stem cells can multiply in the lab for years without differentiating into more specialized cells, these cells are believed capable of forming every kind of cell in the human body, given the right conditions.

--Embryonic germ cells:

Embryonic germ cell derivation is highly dependent on a feeder layer. Embryonic germ cells (EGCs) are pluripotent stem cells derived from primordial germ cells (PGCs). PGCs are progenitors of adult gametes, which diverge from the somatic lineage between late embryonic to early fetal development. First derived in the mouse, EGCs have also been derived from human, chicken, and pig. As pluripotent stem cells, EGCs demonstrate long-term self-renewal via clonal expansion in an undifferentiated state and differentiate in vitro to form embryoid bodies containing cells that represent all three germ layers as well as mixed cell populations of less differentiated progenitors and precursors.

--Adult stem cells:

Adult stem cells are also called tissue-specific stem cells because each type of adult stem cell produces only a limited set of specialized cells characteristic of a particular tissue — epidermis, blood, and so on.

--Other types:

Other types of tissue-specific stem cells are usually found deep within tissues and are harder to get at and harder to study.

Figure 1 Isolation and production of ESCs and iPSCs

Figure 2 Differentiation of pluripotent stem cells

Characteristics of Stem Cells:

--Infinite proliferation and division.

--Stem cells can divide continuously for several generations, or they can be in a quiescent state for a long time.

--Signal regulation in receptors.

--Two ways of stem cell division: differentiate cells with differentiation ability, still maintain parental characteristics and stand by as stem cells; irreversible terminal differentiation, loss of differentiation ability, and eventually senescence and death.

Importance of the Stem Cells:

Without stem cells, wounds would never heal, your skin and blood could not continually renew themselves, fertilized eggs would not grow into babies, and babies would not grow into adults. Stem cells do not have any special structure or function, but stem cells have the potential to become any kind of cell in the human body. Stem cells could help medicine in three general ways: cell-based therapies, drug discovery and basic knowledge.

References:

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[2] What are the major types of stem cells? [J]. Nature Reports Stem Cells, 2007,

[3] What makes embryonic stem cells special? [J]. Nature Reports Stem Cells, 2007,

[4] KERR C, GEARHART J, ELLIOTT A, et al. Embryonic Germ Cells: When Germ Cells Become Stem Cells [J]. Seminars in reproductive medicine, 2006, 24(304-13.

[5] AHMED O, HM S. Stem Cell Therapies in Regenerative Medicine and Diabetes Mellitus: Advances, Constraints and Future Prospects [J]. Transplant Stem Cel Biol, 2016, 3(22.

[6] Can stem cells from one tissue be grown into many other types of cells? [J]. Nature Reports Stem Cells, 2007,

[7] How can stem cells advance medicine? [J]. Nature Reports Stem Cells, 2007,