Animal Stem Cells

In order to understand the different types of animal stem cells you first need to understand the process by which an animal develops.

A zygote develops into an embryo, which develops into a mature animal

When an animal is created through sexual reproduction, it starts out as a single, unspecialised cell called a zygote.

During sexual reproduction, an egg cell and a sperm cell fuse to form a cell called a zygote.

This zygote then starts dividing to form more cells, and these cells start to organise themselves. During this early stage of development, the organism is called an embryo.

This embryo then goes on to develop into the adult animal (usually via multiple other stages - the process varies between species).

A zygote developing into an embryo and then into a mature animal. Note that there are many more stages that have been skipped in this diagram. Also, the process varies considerably between different animal species.

The stem cells that are found in an animal embryo have different properties to the stem cells that are found in a mature animal. Therefore they are given different names:

• The stem cells in an animal embryo are called embryonic stem cells.
• The stem cells that are found in a fully developed animal are called adult stem cells.

(The term 'adult stem cells' is actually a bit misleading. Adult stem cells can be found in all of the stages of development after the embryo. For example, in humans, adult stem cells are found in children as well as adults.)

An embryonic stem cell can differentiate into any cell type

An embryonic stem cell has the ability to differentiate into any type of cell from that organism. For example, an embryonic stem cell in a female human embryo has the ability to differentiate into a red blood cell, a nerve cell, a muscle cell, an egg cell, and so on.

An embryonic stem cell can differentiate into any cell type from that organism.

An adult stem cell can only differentiate into certain cell types

As an embryo transitions into the next stage of development (a foetus in humans), groups of embryonic stems cells in different parts of the body turn into adult stem cells.

Different types of adult stem cells are found in different organs.

Usually, the stem cells within a particular organ can differentiate into all of the cell types of that organ, but cannot differentiate into any other cell types.

For example, there are stem cells in the lining of the small intestine which can differentiate into all of the different cell types that make up that lining, but cannot differentiate into any other cell types (for example, they cannot differentiate into skin cells).

Different types of adult stem cells are found in different organs. Usually, adult stem cells can only differentiate into cell types from the organ they are found in.

New blood cells are made from adult stem cells in the bone marrow

Inside bones, there is a tissue called bone marrow. Within the bone marrow, there are adult stem cells which can differentiate into all of the different types of blood cells (including red blood cells and all the different types of white blood cells).

In the human body, billions of new blood cells are made every day. They are all made from the adult stem cells in the bone marrow.

There are stem cells in the bone marrow which can differentiate into all types of blood cells. Note that there are many more types of white blood cell than just the three shown here. Also note that platelets are only found in mammals.

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