An introduction to stem cells

“Stem cells differ from other kinds of cells in the body. All stem cells — regardless of their source — have three general properties: …”

(1) “they are capable of dividing and renewing themselves for long periods”

Stem cells are said to be capable of long term self renewal if their daughter cells continue to be unspecialised stem cells, as opposed to differentiating. They can replicate continuously over a period of time to increase their numbers (proliferate), which is, for the most part, more uncharacteristic of differentiated cells.

(2) “they are unspecialized”

Stem cells do not contain structures specific to suite a specialised function as part of a tissue or organ.

(3) “and they can give rise to specialized cell types” — the national institute of health

The fact they are unspecialised means that stem cells are pluripotent (or multi-potent, in the case of adult stem cells), retaining the potential to develop into various different types of specific and specialised cells, in a sequential, multi-staged process known as differentiation

This is only set into motion when the stem cells are exposed to the appropriate physiological or experimental conditions. We know that a combination of internal signals (controlled by the genes) and external signals (such as chemicals, microenvironment and physical contact) interact to control this process, yet it is not yet entirely understood. The result is that the DNA of the cell picks up epigenetic marks, which essentially modify gene expression.

Different types of stem cell

Embryonic stem cells:

During the first five days of development, an embryo consists exclusively of embryonic stem cells, which then begin to develop into the entirety of specialised cells required to form an organism. Their ability to develop into all three basic body layers means embryonic stem cells are considered pluripotent.

In 1998, human embryonic stem cells were first extracted from embryos, which had been unused in vitro fertilisation procedures and donated to research, and grow in labs successfully.

Cell culture refers to the controlled growing of cells outside of their natural environment. In this process, cells are removed from a pre implantation stage embryo, and transferred into a laboratory culture dish, containing a nutrient culture medium.

If these embryonic stem cells are able to survive and increase in number, they’re re-plated or sub-cultured (referred to as one passage). These samples can be frozen and sent to other laboratories for further culture and testing.

A cell line is said to have been established when the stem cells, still pluripotent, have proliferated in cell culture for a period of at least 6 months, without differentiating. This is ensured through testing against the fundamental properties throughout the process, to identify and characterise the culture as embryonic stem cells.

The stem cells will remain unspecialised stem cells, so long as their conditions remain constant. However, if allowed to clump together, forming embryoid bodies, the cells will spontaneously undergo differentiation. This process can also be initiated in a controlled form by altering them chemical composition of culture medium, changing the surface of culture dish or modifying the genetic material of the cells.

Somatic / Adult stem cells:

Adult stem cells are found in small, discrete quantities in the stem cell niches of certain tissues. They are essential forgrowth, as well as the maintenance and repair of cells damaged by routine wear and tear, injury and disease. The small numbers in which they are found, make isolating adult stem cells and producing a largecell culture challenging.

Unlike embryonic stem cells, adult stem cell can remain quiescent (non dividing) for long periods until activated to re-enter proliferation. Moreover, they seem(though more recent research is suggesting otherwise) to be limited to differentiating into only different cell types relating to their tissue of origin, making them multi-potent.An example is that of hematopoietic stem cells, found in the bone marrow, which have the ability to develop exclusively into various types of blood cell.

Normal differentiation pathways have been demonstrated both in vitro and in vivo, however, a small minority of experiments have shown differentiation into cells of organs or tissues other than those expected, based upon the stem cell’s origin, although it remains unclear as to whether this occurrence is triggered by other factors, and whether is actually arises within humans.

Induced pluripotent stem cells (iPSCs):

IPSCs are essentially differentiated adult cells that have been genetically reprogrammed into stem cell-like, pluripotent cells, using viruses to introduce embryonic genes. This was first successful with human cells in 2007, thought a considerable amount of work still needs to be done before considering their use in more human treatments.

Importance in science

Some potential uses for human stem cells include:

· Developing models to enhance our understanding of growth and embryonic development

· To treat diseases and other conditions with cell-based therapies (regenerative / reparative and transplant medicine)

· Development and screening of new drugs

Stem cells are proving extremely key, and provide a potential basis for numerous developments in medicine and other areas of science, though it is clear much work and research remains before many of these ideas become achievable.

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