WHAT IS A STEM CELL?

To precisely define the term stem cell has been a difficult task for researchers. A. G. Smith (2001) attempted this definition in an article in the Annual Rei’iew of Cell Develop,nent Biology: Stem cells are defined functionally as cells that have the capacity to self-renew as well as the ability to generate differentiated cells.” These cells have the amazing capacity to generate daughter cells that are identical to the mother cells, to

renew themselves, and to produce offspring that can differentiate into other cells. This is the reason scientists refer to these cells as stein cells: many cells can stem from them.

   Stem cells give rise to many different cell types that make up an organism. Thus, stem cells can develop into mature cells that have characterist ic shapes and specialized functions, such as heart, skin, or nerve cells. They have the ability to divide or self-replicate for long periods: this replic ation may continue throughout the life of the organism. Stem cells can form many different types of cells that make up an organism. The ability

to divide and form other cells is called differentiation. Another term that is used to describe this power to differentiate—this power even to change from one type of cell to another—is plasticity.

   All animals (and even plants) have stem cells. In fact, most of what we know about stern cells has come from the study of mice and other laboratory animals. Understanding what makes stem cells ‘stick” has been the result of animal research. Although a human may receive a heart transplanted from a pig, humans cannot use the stem cells of other animals. The DNA (deoxyribonucleic acid) of each species is programmed to

work differently. The possibility of repairing a severed human spinal cordor curing Parkinson’s disease depends on the use of human stern cells. Simply stated. stem cells are cells that have the ability to divide for

indefinitc periods in culture, and to give risc to specialized cells. The cells may be classified according to their origin as embryonic stem cells (ESCs), embryonic germ stem cells (EGSCs), and adult stern cells.

   As the name implies, embryonic stem cells come from embryos that have developed from eggs that have been fertilized in vitro, Latin for in a dish or test tube” in a laboratory environment. These embryos. used

with the consent of the donors, were leftovers from in ventro fertilization (IVF) clinics. Separated from the other parts of the early developing embryo. primitive cells can be grown in a culture medium to become

embryonic stern cells. These cells are not derived from eggs fertilized  within the body.

   Embryonic germ cells are similar to embryonic stem cells except they are collected from the fetus later in development. The cells come from a region known as the gonadal ridge, which will later develop into the sex

organs. Because the cells arc farther along in the developmental process. they are slightly limited in their ability to give rise to organs of the body.

   Adult stem cells originate in a mature organism and help maintain and repair the tissues in which they are found. These stem cells are responsible for replacing blood and tissues on a regular basis. For example. blood cells have only a 120-day lifespan; stem cells create new cells to replace the dying cells. One advantage in using adult stem cells is that small samples of tissues—or even the patient’s own cells—can be used for implantation. avoiding problems of tissue rejection, Furthermore, adult cells do not carry the baggage of ethical issues that accompany embryonic research. Some scientists use the term somatic stem cell rather than adult stem cell.