An explanation of stem cells and their potential to treat HD
What are the different kinds of stem cells?
There are three main types of stem cells under scientific study today:
Embryonic stem (ES) cells: ES cells are taken from the very early stages of embryo development and can give rise to all of the cells of the human body, except the placenta and other supportive tissues in the womb.
Embryonic germ (EG) cells: EG cells are taken from the later stages of embryo development and are slightly less “powerful” in their ability to divide.
Adult stem cells: Adult stem cells are found in the tissues of a fully developed child or adult and can only produce a limited number of cell types.
These three types of stem cells are easiest to understand in a discussion of human development. Human development begins when a sperm fertilizes an egg and creates a single cell, known as a zygote, which has the potential to form an entire organism. This single cell is said to be totipotent, meaning it has the “total” potential to give rise to all types of cells. About 24 hours after fertilization, the zygote divides into two identical totipotent cells, and is now known as an embryo. About five days after fertilization and after several cycles of cell division, these cells begin to specialize and form a hollow sphere, called a blastocyst. The blastocyst has an outer layer of cells that make up the shape of a sphere and a cluster of cells, known as the inner cell mass, inside the sphere. The outer layer of cells will eventually form the placenta. The inner cell mass will eventually form all the tissues of the human body. The inner cell mass cannot form an organism on its own, however, because it is unable to produce the placenta and the other supporting tissues necessary for development in a woman’s uterus. Therefore, the inner cell mass cells are said to be pluripotent, meaning they have the potential to give rise to most of the tissues required to produce an organism. In other words, they can give rise to all the cells of the human body, excluding the supportive tissues used in the womb. (See Figure 3).
Embryonic stem (ES) cells, which are also pluripotent, are isolated directly from the inner cell mass at this blastocyst stage. In 1998, researchers first isolated ES cells from human embryos that were obtained from in vitro fertilization clinics. Although these embryos were originally intended for reproduction, they were in “excess” and were headed for the trash. Instead of being disposed, however, they were donated to research.
Five to 10 weeks after fertilization, the growing embryo, now called a fetus, develops a region known as the gonadal ridge. The gonadal ridge contains the primordial germ cells, which will eventually develop into eggs or sperm.
Embryonic germ (EG) cells are isolated from these primordial germ cells of the 5- to 10- week old fetus. Like ES cells, EG cells are also pluripotent.
As the human fetus continues to develop, pluripotent stem cells specialize into stem cells that are geared for specific tissues. For example, they become blood stem cells (which produce blood cells) or skin stem cells (which produce skin cells). These specialized stem cells are said to be multipotent, meaning they can give rise to many, but not all, types of cells.
While all three types of stem cells discussed above (ES cells, EG cells, and multipotent stem cells) are found in the developing human, only multipotent stem cells are found in children and adults. Therefore, multipotent stem cells are often referred to as adult stem cells. Unlike other stem cells, adult stem cells are only found in specialized tissues and can only give rise to the specialized cell types that make up that tissue. Currently, adult stem cells have been found in the bone marrow, blood, blood vessels, skeletal muscle, skin, lining of the digestive track, dental pulp of the tooth, liver, pancreas, cornea and retina of the eye, and brain.
Last Modified: 7-5-03
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