Stem cells differ in their ability in differentiation considering the site their existence in human body, human age and his growth stage. Stem cells can be classified into many categories depending on its potency, its place of existence and the growth stage on which they have been obtained.

Stem cells can be classified according to their plasticity into Totipotent Stem Cells

When a sperm cell and an egg cell unite, they form a one-celled fertilized egg or zygote which is a fully potent stem cell and is called ( Totipotent). It has the potential to give rise to any and all human cells, such as brain, liver, blood or heart cells. It can even give rise to an entire functional organism. The first few cell divisions in embryonic development produce more totipotent cells. These cells can differentiate into embryonic and extraembryonic cell types.

Pluripotent Stem Cells
After four days of embryonic cells division, the cells begin to specialize into (Pluripotent) stem cells. A single (Pluripotent) stem cells has the ability to give rise to types of cells that develop from the three germ layers (mesoderm, endoderm, and ectoderm) from which all the cells of the body arise.
On the fourth day of development, the embryo forms into two layers, an outer layer which will become the placenta, and an inner cell mass which will form nearly any human tissue of the developing human body. However, these inner cells cannot do so without the outer layer; which is a Pluripotent.  As these pluripotent stem cells continue to divide, they begin to specialize further.

Multipotent Stem Cells
These are less plastic and more differentiated stem cells. They give rise to a limited range of cells within a tissue type. The offspring of the pluripotent cells become the progenitors of such cell lines as blood cells, skin cells and nerve cells. At this stage, they are (Multipotent). They can become one of several types of cells within a given organ. For example, hematopoietic stem cells differentiate into red blood cells, white blood cells, platelets, etc.

Unipotent
These cells can differentiate to only one cell type, as in production of oocytes and sperms, but have the property of self-renewal which distinguishes them from non-stem cells (e.g. muscle stem cells).

Table 1 summarizes all what we have mentioned previously regard classification of Stem Cells.

Stem cells can be classified according to the growth stage

Embryonic stem cell (ESCs)

ESCs are pluripotent and give rise during development to all derivatives of the three primary germ layers.  An embryonic stem cell is derived from a group of cells called the inner cell mass.  The cells of the outer layers of the blastocyst form the placenta and the supporting surrounding tissues to embryo while the inner cell mass form the embryo body and differentiate to give rise to all body specialized tissues and organs.
Recent studies have been shown that by using developed culture methods, ESCs can be turned into brain, heart muscle cells, blood cells, blood vessels, skin, pancreatic islet cells and bone marrow cells in vitro. It is therefore anticipated that research with embryonic stem cells will help in production of cells and tissues for replacement therapies for treating disorders such as Parkinson's disease, heart failure, blood disorders, cancer, and diabetes.

Tissue specific-stem cells (adult stem cells)
Adult stem cells is an undifferentiated (unspecialized) cells that occurs in a differentiated specialized tissue, renews itself and divide to replenish dying cells and regenerate damaged tissues. Research into adult stem cells has been fueled by their abilities to divide or self-renew indefinitely and generate all the cell types of the organ from which they originate potentially regenerating the entire organ from a few cells and accordingly these cells are pluripotent since the number of cell types that they can differentiate into is limited. 
These cells replace cells that need to be replaced on a regular basis in a living organism.  These cells has advantage of that they offer the opportunity to utilized small samples of adult tissues of a patient's own cells for expansion and subsequent implantation. This avoids the ethical issues of embryonic stem cells.

These tissue specific stem cells include:

Hematopeiotic stem cells

Mesenchymal stem cells

Gut stem cells

Liver stem cells

Bone and cartilage stem cells

Epidermal stem cells

Neuronal stem cells

Pancreatic stem cells

 



 

Stem cells can be classified according to the place of existence

Embryonic stem cells are the cells of blastocyst, isolated and cultured from inner cell mass to differentiate into different cell lines.

Fetal stem cells (FSCs), primitive cells found in the organs of fetuses and can be isolated in aborted fetuses. These cells includes: neural crest stem cells, fetal hematopoietic stem cells and pancreatic islet progenitors.

Embryonic germ cells are derived from fetal tissue. Specifically, they are isolated from the primordial germ cells of the gonadal ridge of the 5- to 10-week fetus. Later in development, the gonadal ridge develops into the testes or ovaries and the primordial germ cells give rise to eggs or sperm. Embryonic stem cells and embryonic germ cells are pluripotent, but they are not identical in their properties and characteristics.

Cloned embryonic stem cells by Somatic cell nuclear transfer (SCNT), The major problem facing ESCs use in cells therapies and organ replacement is their anticipated reaction by the patient's immune system. One way around this problem is to produce cloned embryonic stem cells by somatic cell nuclear transfer (SCNT) to produce cells and tissues matching one's self, so they would not be rejected when they are transplanted into the patient. SCNT is a technique by which the DNA from a cell from patient's body is removed and transferred into unfertilized oocyte that previously had its own DNA removed. The oocyte then is induced to divide to form a blastocyst (day 5-6 embryo) from which the cloned ESCs is derived. Most famous animal produced by this method was the Sheep Dolly, this method has shown that somatic cell nuclei can be reprogrammed when placed inside an enucleated oocyte. Embryonic stem cells derived from SCNT blastocyst can give rise to cells of all three germ layers. Furthermore, SCNT embryos are implanted in an animal uterus, development proceeds through the embryonic and fetal stages to produce viable animals.

Stem cells also can be obtained from the umbilical cord immediately after birth. Umbilical cord blood is rich source of hematopoietic stem cells which are less mature than stem cell of the bone marrow of adults, and have advantage that it is easily obtained and is abundant. Cord blood is recently collected and stored by cord blood banks, and emerged as an alternative source of hematopoietic stem cells for leukemia treatment and other blood disorders.

Adults stem cells as bone marrow.