Stem cells are unspecialized cells capable of dividing and renewing themselves for a number of cell cycle. When stem cells replicate many times, it is called as proliferation. A starting population of stem cells after many months of proliferation in the laboratory or in vitro condition can yield millions of cells. These stem cells have got the capacity to give rise to specialized cell types like muscle cell, nerve cell, kidney cell, bone cell or any type of specialized body cells.

Mainly there are two types of stem cells; they are adult stem cell and embryonic stem cells. Adult stem cells are undifferentiated cells which are present in an adult and they have got the capacity to differentiate into many types of specialized cells.

Embryonic stem cells are present in the embryos. They have got the capacity of differentiating into almost all types of specialized cells.

Stem cell culture:

1. Growing cells in the laboratory or in vitro is known as cell culture. Human embryonic stem cells are cultured in the laboratory by transferring the inner cell mass into nutrient broth or culture media containing plastic laboratory animal cell culture dish.

2. Human embryonic stem cells divide continuously and spread over the surface of culture dish. The inner layer of animal cell culture dish is most commonly coated with treated mouse embryonic skin cells, this treatment prevent these cells from dividing. This coated layer of cells is known as feeder layer. These treated mouse embryonic skin cells provide sticky surface to embryonic stem cells to attach and grow. Feeder cells (mouse embryonic skin cells) also release nutrients into the animal cell culture media.

3. But recently scientists and researchers have begun to discover methods of growing embryonic stem cells without mouse embryonic skin cells as feeder layer. This in turn decreases the risk of transmission of viruses, macromolecules or any other infectious agents from mouse cells to human cells.

4. After many days, the cells of the inner cell mass, that is human embryonic stem cells proliferate and begin to crowd the laboratory culture dish. At this stage, these human embryonic stem cells are gently removed and plated into many fresh animal cell culture dishes.

5. This process of replating the overcrowded human embryonic stem cells are done many times, for several months, and this process is called as subculturing.

6. Each cycle of subculturing the cells (any cells animal/microorganisms) is called as a passage. After six months or more, the original 30 cells of the inner cell mass or human embryonic stem cells yield around millions of embryonic stem cells.

7. Human embryonic stem cells that have proliferated in animal cell culture dish for more than six months without any differentiation are called as pluripotent embryonic stem cells, and cells which are genetically identical to the original cells are called as embryonic stem cell line.

8. Once these genetically stable embryonic stem cell lines are established, or even before, batches of these cells can be frozen and shipped to other laboratories for further culturing and also for the research purpose.

Identifying Adult Stem Cells:

1. Adult stem cells present in a living tissue are labeled with molecular markers and specialized cell types these cells produce /generate are determined.

2. Other method is these cells are removed from living animal, then they are labeled in cell culture, and after this procedure they are transplanted back into another animal to determine whether the original cells reproduce same type of cells as their tissue of origin.

3. Isolating the cells, growing them in the laboratory and then these cells can be manipulated by using growth factors or by introducing new genes, by this method we can determine what type of cells they can become.

4. A single adult stem cell has got the capacity to generate a line of genetically identical cells, and these cells are called as clone. These clones then give rise to all different types of cells and tissues.

5. Scientists often show and prove that a stem cell has got the capacity to give rise to a clone of cells in cell culture and also after transplanting a purified population of stem cells into an animal, they repopulate the tissue

6. Recently scientists have proven that each adult stem cell clones have got the capacity to repopulate injured tissue in a living animal.

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