Figure 1 (KU Medical Center, 2007)

           
Figure 2 (Domen, Wagers, & Weissman, 2006)

 

 

Table 1 shows various properties of embryonic and adult stem cells, illustrating the similarities and  differences between the two sources.

Table 1

Embryonic Stem Cells	Adult Stem Cells
Embryonic stem cells are derived from the 4-5 day old blastocyst.  In the blastocyst there is an outer layer of cells called the trophoderm which encases another group of cells called the inner cell mass, shown in Figure 1 (below).  The inner cell mass is composed of embryonic stem cells. (NIH & DHHS, Stem cell basics, 2006)	The origin of adult cells is not currently known (NIH & DHHS, Stem cell basics, 2006).  The most common theory is that the adult stem cells are stem cells from the embryo that remained undifferentiated (Princeton University, 2005).  Adult stem cells are found in many different areas of the body including the bone marrow, muscles, and brain although not every organ contains stem cells (NIH & DHHS, Stem cell basics, 2006; Harvard Stem Cell Institute, 2005).
Embryonic stem cells are a very large proportion of all the cells in the blastocyst (NIH & DHHS, Stem cell basics, 2006).	Adult stem cells are a very small fraction of all the cells in a human body, and it is estimated that only one in 10 000 – 100 000 cells in bone marrow are stem cells.  Of the adult stem cells, those that are the least specialized are most common while those which are the least specialized are very rare. (Princeton University, 2005)
Embryonic stem cells can proliferate for very long periods of time without differentiating. (NIH & DHHS, Stem cell basics, 2006)  They contain a transcription factor called Oct 4 which prevents them from differentiating (Yu & Thomson, 2006).	Adult stem cells also proliferate for long periods of time, however not generally as long as embryonic cells can in labs (Univeristy of Wisconsin-Madison, Embryonic stem cells, 2006).
Embryonic stem cells are pluripotent meaning they can differentiate into almost all cell types. This type of stem cell is the first stage and is completely undifferentiated. (NIH & DHHS, Stem cell basics, 2006)	Adult stem cells are multipotent meaning they can differentiate into many, but not all types of cells.  Generally an adult stem cell will differentiate into the type of tissue it is contained in.  That is, skin stem cells make skin cells, brain stem cells make nerve cells and so on.  It has now been found that adult stem cells do have some plasticity.  For example neural stem cells in the brain may also become blood cells or skeletal muscle cells. (NIH & DHHS, Stem cell basics, 2006)
The purpose of embryonic stem cells is to create all of the different cell types in the human body to form a complete human.	The average human needs 100 billion new blood cells every day (Domen, Wagers, & Weissman, 2006).  This is the purpose of adult stem cells, to replenish the dying specialized cells so that our bodies continue to function correctly.

 

 

 

 

 

 

 

 

 

 

Table 2 lists many types of adult stem cells, where they are found, and the cells that they generally differentiate into.  Some adult stem cells also have plasticity, and can differentiate into cell types other than those found in the tissue they reside in.  These additional cell types are listed in the last column.

Table 2

Type of Stem Cell	Found In	Generally differentiate into	Plasticity (can also differentiate into)
Hematopoetic stem cells	Bone marrow	All blood cells	Neural cells, skeletal muscle cells, cardiac muscle cells, liver cells
Bone marrow stromal cells	Bone marrow	Bone cells, cartilage cells, fat cells, connective tissue cells	Skeletal muscle cells, cardiac muscle cells
Neural stem cells	Brain	Neuron cells and two types of non-neuronal nervous system cells	Blood cells, skeletal muscle cells
Epithelial stem cells	Lining of digestive tract	Various digestive tract cells such as absorptive cells, goblet cells, Paneth cells	Unknown
Skin stem cells	Basal layer of epidermis	Keratinocytes (the protective top layer of skin)	Unknown
	Base of hair follicles	Epidermis cells, hair follicle cells	Unknown

Figure 2 illustrates the types of cells that hematopoetic stem cells and bone marrow stromal cells, both found in the bone marrow, normally differentiate into.

(NIH & DHHS, Stem cell basics, 2006) 

Photo credits (top of page, left to right): Human embryonic stem cell culture (University of Wisconsin-Madison, News and updates, 2006), Embryonic stem cells (Biotechnology Australia, 2006), Nerve cells that were differentiated from embryonic stem cells (ALS Association, 2004), Neural stem cells (Orion Biosolutions, n.d.), Scanning electron micrograph of an adult bone marrow stem cell (Basu, 2003)