Stem cells

Stem Cells

I. Introduction:

                                Stem cells are a class of undifferentiated cells that are able to differentiate into specialized cell types.  These cells have the remarkable property to get developed into many different cell types in the body during early life and growth. They may even show unlimited division resulting in internal repair and prevention from injury in some cases. As these cells are undifferentiated, each cell has a choice either to be an undifferentiated cell (stem cell) or get converted into other type of specialized cell having specific function like a nerve cell, a blood cell or any other cell for that matter.

                            Stem cells are different from other cells as they are the cells which are unspecialized but can renew themselves or can take up a special function under certain conditions. Their properties of differentiation also depend on type of organs or tissue they are present in. For instance in organs like bone marrow, Stem cells divide regularly too repair and replace the dead and worn out cells while in organs such as pancreas and the heart, stem cells only divide under special conditions.

 Commonly, stem cells come from two main sources:

•     Embryos formed during the blastocyst phase of embryological development (embryonic stem cells) and

• Adult tissue (somatic or adult stem cells).

Both types are generally characterized by their potential to differentiate into different cell types (such as skin, muscle, bone, etc.). Scientists discovered ways to derive embryonic stem cells from early mouse embryos more than 30 years ago, in 1981. The detailed study of the biology of human stem cells however came into existence as late as, in 1998. Embryos used in these studies were created for reproductive purposes. When they were no longer needed for that purpose, they were donated for research with the informed consent of the donor. Afterwards scientists have found many applications of these stem cells and their banking. One such application is to completely reprogram some specialized cells to get converted into stem cell like state, which can be a revolution. These stem cells as are induced, so named induced pluripotent stem cells.  

 In the 3- to 5-day-old embryo, called a blastocyst, the inner cells give rise to the entire body of the organism, including all of the many specialized cell types and organs such as the heart, lungs, skin, sperm, eggs and other tissues. In some adult tissues, such as bone marrow, muscle, and brain, discrete populations of adult stem cells generate replacements for cells that are lost through normal wear and tear, injury, or disease.

Due to these unique properties, stem cells have great futuristic aspects as they can be used to treat many diseases like Diabetes, thallesemia and much other deadly disease. Although these aspects still need to be researched heavily and yet to be tested properly but theoretically it is indeed a possibility.

Laboratory studies of stem cells enable scientists to learn about the cells’ essential properties and what makes them different from specialized cell types. Scientists are already using stem cells in the laboratory to screen new drugs and to develop model systems to study normal growth and identify the causes of birth defects.

Embryonic stem cells

Embryonic stem cells are derived from a four- or five-day-old human embryo that is in the blastocyst phase of development. Sexual reproduction begins when a male's sperm fertilizes a female's ovum (egg) to form a single cell called a zygote. The single zygote cell then undergo  a series of divisions, forming 2, 4, 8, 16 cells, etc. After four to six days - before implantation in the uterus - this mass of cells is called a blastocyst. 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).   In a normal pregnancy, the blastocyst stage continues until implantation of the embryo in the uterus, at which point the embryo is referred to as a fetus. This usually occurs by the end of the 10th week of gestation after all major organs of the body have been created.

 

Somatic  stem cells

                                   Adult or somatic stem cells exist throughout the body after embryonic development and are found inside of different types of tissue. These stem cells have been found in tissues such as the brain, bone marrow, blood, blood vessels, skeletal muscles, skin, and the liver. They remain in a quiescent or non-dividing state for years until activated by disease or tissue injury.

Adult stem cells can divide or self-renew indefinitely, enabling them to generate a range of cell types from the originating organ or even regenerate the entire original organ. It is generally thought that adult stem cells are limited in their ability to differentiate based on their tissue of origin, but there is some evidence to suggest that they can differentiate to become other cell types.

Stem cell cultures

  Stem cells are either extracted from adult tissue or from a dividing zygote in a culture dish. Once extracted, scientists place the cells in a controlled culture that prohibits them from further specializing or differentiating but usually allows them to divide and replicate. The process of growing large numbers of embryonic stem cells has been easier than growing large numbers of adult stem cells, but progress is being made for both cell types.

Stem cell lines

Once stem cells have been allowed to divide and propagate in a controlled culture, the collection of healthy, dividing, and undifferentiated cells is called a stem cell line. These stem cell lines are subsequently managed and shared among researchers. Once under control, the stem cells can be stimulated to specialize as directed by a researcher - a process known as directed differentiation. Embryonic stem cells are able to differentiate into more cell types than adult stem cells.

Potency

Stem cells are categorized by their potential to differentiate into other types of cells. Embryonic stem cells are the most potent since they must become every type of cell in the body. The full classification includes:

•   Totipotent - the ability to differentiate into all possible cell types. E.g. zygote formed at egg fertilization and the first few cells that result from the division of the zygote.

• Pluripotent - the ability to differentiate into almost all cell types.

                              E.g. embryonic stem cells and cells that are derived from the mesoderm, endoderm, and ectoderm germ layers that are formed in the beginning stages of embryonic stem cell differentiation.

•   Multipotent - the ability to differentiate into a number of closely related cells. e.g.  hematopoietic stem cells that can become red and white blood cells or platelets.

•   Oligopotent - the ability to differentiate into a few cells. e.g. include (adult) lymphoid or myeloid stem cells.

•   Unipotent - the ability to only produce cells of their own type.E.g. muscle stem cells.

Embryonic stem cells are considered pluripotent instead of totipotent because they do not have the ability to become part of the extra-embryonic membranes or the placenta.