What Are Stem Cells?
Stem cells are in a category of cells that have the ability during development to completely transform from their original generalized state into various other cell types that are specialized. The two different sources that these cells can be obtained from are as follows:
• • Embryos – Embryonic stem cells are formed when they are in a specific state of development that is referred to as the blastocyst phase.
• • Adult tissue – Adult stem cells exist in several different areas of the adult human body and within a variety of tissue types.
The characteristics that differentiate each type of stem cells involve their strength in potentially transforming into other types of cells. To name a few of these include muscle, skin and bone cells.
A Closer Look at Embryonic Stem Cells
This type of stem cells is obtained from human embryos that are about four to five days old and have reached the blastocyst development phase. They generally come from extra embryos that have formed when a test tube is used to fertilize several eggs. This is a highly specialized process that is performed in IVF (in vitro fertilization) clinics for the purpose of causing pregnancy by implanting one egg inside a woman.
When a female’s egg (ovum) is fertilized by sperm from a man, the process of sexual reproduction immediately begins and results in the growth of a cell known as a zygote. A sequence of divisions then begins in the zygote, first forming 2, then 4, then 8, then 16 cells and so forth. Before the actual implantation of the embryo into the uterus, which is about four to six days after the zygote forms, the cells mass is referred to as a blastocyst. It consists of an embryoblast which is an inner cell mass and a trophoblast which is the outer cell mass. The inner cell mass will continue to divide until it later grows into an adult organism. The outer cell mass connects to the placenta and actually becomes a part of it. Totipotent cells or embryonic stem cells that have the ability to transform into virtually any cell of the human body are obtained from the inner cell mass.
When the goal is to implant an embryo into a woman’s uterus, the blastocyst stage continues until the process of implantation takes place. For the purpose of extracting embryonic stem cells the inner cell mass is typically placed into a culture dish at the stage of blastocyst. Without the necessary stimulation for these cells to differentiate, they will start the process of dividing and replicating. During this time they maintain the ability to transform to any type of cell within the human body. When it is needed, stimulation of the cells can occur, which will promote the formation of specialized cells.
A Closer Look at Adult Stem Cells
The scientific term for adult stem cells is known as somatic stem cells. After embryonic development takes place, these cells can be found in many types of tissue that are contained within the human body. To name a few of the tissues these cells are located would include the following:
• • Bone marrow
• • Brain
• • Liver
• • Blood vessels
• • Skeletal muscles
• • Blood
• • Skin
Until these cells are activated by things such as injury of tissue or disease, they stay in what is known as a non-dividing or quiescent state. Adult stem cells have the ability to continue self-renewing or dividing forever. This enables these cells to produce types of cells of the originating tissue or organ and it can even completely redevelop to original organ as a whole.
The original thought in regards to adult stem cells was that they only had the ability to transform into the tissue of which they originated. However, evidence suggests they also have the ability to transform into many other types of cells as well and the implications are more than obvious.