Sunday, 2 March 2014

What are totipotent stem cells ?

As we have explained on our what is a stem cell page, one of the main criteria used to classify a stem cell is its plasticity. Not all stem cells are created equal. There are many different types of stem cells, with varying degrees of plasticity. Plasticity refers to the amount of different specialised cells -like neurons or muscle cells - a stem cell can differentiate into. The larger this amount, the more plastic a cell is.

The three main types of stem cells in regards to plasticity are:
  • Totipotent stem cells
  • Pluripotent stem cells 
  • Multipotent stem cell

In today's article we will explain what totipotent stem cells are. Future posts will follow, dedicated to the other two types as well.

Totipotent Stem Cells
Simply put, these are the kings of stem cells. Totipotent stem cells are the most versatile of any other stem cell types. When a sperm cell and an egg cell unite, they form a zygote. The zygote is essentially a totipotent stem cell! All our cells, organs and tissues originate -directly or indirectly- from this very first, super plastic stem cell.

If we wanted to give a simple definition, we would say that totipotent stem cells are stem cells that have the capacity to give rise to an entire functional organism. There is no kind of cell, tissue or organ that totipotent stem cells can't create.

The zygote is a totipotent stem cell

The difference between totipotent and pluripotent stem cells 
You may have heard that pluripotent stem cells (like embryonic and induced pluripotent stem cells) also have the capacity to transform into the tissues and organs that an organism is comprised of. So, you may now be wondering what is the difference between a totipotent and a pluripotent stem cell.

It's true that pluripotent stem cells can give rise to all kinds of cells, organs and tissues. However, they can't give rise to an entire organism. Why? On the fourth day of human embryogenesis, the embryo forms into two separate layers. The outer layer, which becomes the placenta and an inner layer which is a mass of cells that over the next months will develop into a new human.

Although the inner cells can form nearly into all human tissues, they can't do so without the outer layer, thus they are not totipotent, but pluripotent. In other words, the main difference is that pluripotent stem cells can differentiate into all types of cells except the ones comprising the placenta.

Totipotency definition
If you are looking for a more proper definition for totipotentency, then I believe the best one is the following:
"Totipotency is the ability of a single cell to divide and produce all the differentiated cells in an organism, including extraembryonic tissues."

Interesting Facts
Sometimes, the term omnipotent is used instead to describe the same type of stem cell.

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