A cell that has the ability to continuously divide and differentiate (develop) into various other kind of cells or tissues is known as stem cell. They are unspecialized cells which are capable of dividing and renewing themselves for long period of time (proliferation and renewal). They also have the potential to give rise to specialized cell types (differentiation).

Types of stem cells

Embryonic stem cells: These cells are capable of forming any type of cell found in the human body, these cells come from five to six days old embryo.

Embryonic germ cells: These cells are derived from the part of a human embryo or foetus that will ultimately produce gametes (eggs or sperm).

Adult stem cells: These calls are undifferentiated cells found among specialized or differentiated cells in a tissue or organ after birth. They appear to have more restricted ability to produce different cell types and to self-renew.

Sources of stem cells

Stem cells come from several sources in the body. The names of the cells below indicate the sources from which they are derived.

Embryonic Stem Cells: These are derived from a four- or five-day-old human embryo that is in the blastocyst phase of development.

The blastocyst consists of an inner cell mass (embryoblast) and an outer cell mass (trophoblast). The outer cell mass becomes part of the placenta and the inner cell mass is the group of cells that will differentiate to become all the structures of an adult organism. This latter mass is the source of embryonic stem cells totipotent cells (cells with total potential to develop into any cell in the body). Embryonic cells can be grown in the laboratory and this is known as cell culture. Human embryonic stem cells are isolated by transferring the inner cell mass into a plastic laboratory culture dish that contains a nutrient broth known as culture medium. The cells divide and spread over the surface of the dish. The inner surface of the culture dish is typically coated with mouse embryonic skin cells that have been treated so they will not divide and this coating layer of cells is called a feeder layer which act as a sticky surface to which cells can attach. The feeder cells also release nutrients into the culture medium. Recently the devices are used to grow embryonic cells without feeder‘s layer so as to remove the risk of getting infected by virus or other macromolecule from mouse cells.

After several days, the cells of the inner cell mass proliferate and begin to crowd the culture dish. When this occurs, they are removed gently and plated into several fresh culture dishes. The process of replicating the cells is repeated many times and for many months and this is called sub-culturing. Each cycle of sub culturing the cells is referred to as a passage. After six months or more, the original 30 cells of the inner cell mass yield millions of embryonic stem cells. Embryonic stem cells that have proliferated in cell culture for six or more months without differentiating, are pluripotent and appear genetically normal are referred to as an embryonic stem cell line. Once cell lines are established, or even before that stage, batches of them can be frozen and shipped to other laboratories for further culture and experimentation.

Types of Stem Cells

Stem cells can be categorized into seven types based on their ability to differentiate or their potency. The stem cell types are as follows

Totipotent stem cells have the ability to differentiate into any type of cell in the body. Totipotent stem cells develop during sexual reproduction when male and female gametes fuse during fertilization to form a zygote. The zygote is totipotent because its cells can become any type of cell and they have limitless replicative abilities. As the zygote continues to divide and mature, its cells develop into more specialized cells called pluripotent stem cells.

Pluripotent stem cells have the ability to differentiate into several different types of cells. Specialization in pluripotent stem cells is minimal and therefore they can develop into almost any type of cell. Embryonic stem cells and fetal stem cells are two types of pluripotent cells.

Induced pluripotent stem cells (iPS cells) are genetically altered adult stem cells that are induced or prompted in a laboratory to take on the characteristics of embryonic stem cells. Although iPS cells behave like and express some of the same genes that are expressed normally in embryonic stem cells but they are not exact duplicates of embryonic stem cells.

Multipotent stem cells have the ability to differentiate into a limited number of specialized cell types. Multipotent stem cells typically develop into any cell of a particular group or type. For example, bone marrow stem cells can produce any type of blood cell. However, bone marrow cells do not produce heart cells. Adult stem cells and umbilical cord stem cells are examples of multipotent cells.

Mesenchymal stem cells are multipotent cells of bone marrow that have the ability to differentiate into several types of specialized cells related to, but not including blood cells. These stem cells give rise to cells that form specialized connective tissues, as well as cells that support the formation of blood.

Oligopotent stem cells have the ability to differentiate into just a few types of cells. A lymphoid stem cell is an example of anoligopotent stem cell. This type of stem cell cannot develop into any type of blood cell as bone marrow stem cells can. They only give rise to blood cells of the lymphatic system, such as T cells.

Unipotent stem cells have unlimited reproductive capabilities, but can only differentiate into a single type of cell or tissue. Unipotent stem cells are derived from multipotent stem cells and formed in adult tissue. Skin cells are one of the most prolific examples of unipotent stem cells. These cells must readily undergo cell division to replace damaged cells.