Stem Cells:
Stem cells are the cells which have the ability to differentiate into different type of cells are known as stem cells. These cells are also referred to as "Internal Repair System". Each new cell formed by the division of stem cell has a potential to remain either as a stem cell or another type of cell such as red blood cell, nerve cell or muscle cell. A type of stem cell, known as embryonic stem cell was discovered from early mouse embryo by Scientists in 1981. This discovery helped researchers in developing a stem cell from human embryo. In 1998, scientists got success in discovering "Human embryotic stem cells" which has now helped in replacing damaged cells in adult human being.

Characteristics of stem cells:-

The main characteristics of stem cells include;

• Stem cells are unspecialized: these cells are not able to perform any specialized function. For example like a heart muscle cells it cannot pump blood with its neighbor. This is so because stem cell does not have any tissue specific structures.

• Stem cells can give rise to specialized cells: Stem cells are unspecialized cells but can give rise to specialized cells through the process of differentiation. Differentiation begins when stem cells are exposed to biochemical cues. These cues allow cells to grow into specific cell according to requirements.
• Stem cells can renew themselves after a long period of inactivity: stem cells renew through the process of cell division. For example muscle cell, blood cells and nerve cells cannot replicate on their own while stem cells have the ability to replicate several times.

Types of stem cells:-

Stem cells are mainly of three types:

• Embryonic stem cells:

These cells come from embryo and are found at the blastocyst stage of human development. Eventually these cells are undifferentiated but later these cells can be differentiated to form specialized cell.

• Embryonic germ cells:

These cells are derived from the human sex cells.

• Adult or somatic stem cells:

These cells are derived from bone marrow. Adult stem cells are mainly found in blood, brain, skin of fetus, child and adults. These cells are mainly responsible for repair and regeneration in the body.

Stem cells have the capability to differentiate into different cell types. According to their differentiation potential they are divided into following types:

• Totipotent cells:-
These cells have the capability to differentiate into any type of cell in human body. For example zygote and few cell of zygote formed at the time of division can be differentiated into any type of cell.

• Pluripotent cells:-
These cells also have the capability of differentiating into almost any different type of cell but they cannot be differentiated into placental tissue needed for uterus development. Cells derived from three germ layers are the example of Pluripotent cells.

• Multipotent cells:-
These cells have the capability to differentiate into certain cell types. Blood cells like red blood cells, white blood cells and platelet are the example of multipotent cells.

• Unipotent stem cells:-
These cell produce cell of their own type. Example adult muscle cells.

Among all these embryonic stem cells are most potent cells since they have the capability to differentiate almost into any type of cells.

Recently research on adult stem cells is in great focus. Scientists are trying to find the way to control differentiation of adult stem cells in the laboratory. If differentiation of these cells is controlled then they can be used to treat several neurodegenerative disorders like Parkinson's and Alzheimer's. If differentiation is controlled than any tissue or organ of the body can be created from a single stem cell. Isn't it fascinating?

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