Sources of Stem Cells
Stem cells can be harvested from various sources like the bone marrow, embryonic tissue, amniotic fluid or umbilical cord blood. Stem cells are classified according to the source from which they are obtained.

Somatic Stem Cells
Stem cells are found in small numbers in adult tissue such as brain, bone marrow, peripheral blood, blood vessels, skeletal muscle, skin, liver and umbilical cord blood. These cells are also called 'adult stem cells', even though the term 'adult' is incorrect as stem cells also occur in babies and children.

The primary role of stem cells in the body is to maintain and repair the tissues in which they are found, as they are multipotent in nature.

Of all adult tissue, the bone marrow is an extremely rich source of somatic stem cells. Drawn from the spongy tissue found in the center of bones, the main function of these stem cells is to make blood cells that circulate in our bodies and fight infection. In stem cell therapy, bone marrow was the earliest source of stem cells due to its rich supply.

The other rich source of stem cells is the blood left over in the umbilical cord and placenta of a newborn child. Till recently, this blood was (and continues to be) often discarded as medical waste. However, now that umbilical cord blood is known to be a rich source of stem cells, more people are choosing to bank these cells for its potential future use.

Embryonic Stem Cells (ESCs)
These cells are derived from embryos that develop from eggs that have been fertilized in vitro and are donated for research. The embryos from which these cells are derived are four or five days old and are a hollow microscopic ball of cells called the blastocyst. The cells are removed from the blastocyst and treated in a culture dish where they divide into more embryonic stem cells. Because these cells are pluripotent, they are attractive candidates for use in stem cell research and therapy.

Embryonic (or fetal) Germ Cells (EGCs)
These pluripotent stem cells are derived from primordial germ cells, which give rise to the gametes (sperm & eggs) in adults. They are found in a 5 to 9 week old embryo/fetus in the area that is destined to become either the testicles or the ovaries. Like the embryonic stem cells, primordial germ cells too are transferred into a specially treated culture dish to form germ cell colonies. Deriving stem cells through this process is controversial since the embryo or the fetuses from which the germ cells are obtained are destroyed. The embryo or the fetuses, even though not fully formed, are considered 'human' in many cultures and hence their destruction leads to ethical dilemmas. Such a dilemma exists even if the embryonic germ cells are derived from a fetus that is obtained as a result of a miscarriage or abortion.

Peripheral Blood Stem Cells
Stem cells, in limited quantities, can also be found in the peripheral blood circulation. Peripheral blood stem cells are easier to obtain than bone marrow as they can be drawn from blood. Some studies have shown that these stem cells engraft faster than bone marrow stem cells.

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