Stem Cells and Their Types

Definition

Stem cells are undifferentiated or ‘blank’ cells found in the human body that have the potential to develop into many different cell types that carry out different functions.

Introduction

Stem cells have the remarkable potential to develop into many different cell types in the body during early life and growth. In addition, in many tissues they serve as a sort of internal repair system, dividing essentially without limit to replenish other cells as long as the person or animal is still alive. When a stem cell divides, each new cell has the potential either to remain a stem cell or become another type of cell with a more specialized function, such as a muscle cell, a red blood cell, or a brain cell.

• Stem cells are distinguished from other cell types by two important characteristics:
•  First, they are unspecialized cells capable of renewing themselves through cell division, sometimes after long periods of inactivity.

Second, under certain physiological or experimental conditions, they can be induced to become tissue or organ specific cells with special functions. In some organs, such as the gut and bone marrow, stem cells regularly divide to repair and replace worn out or damaged tissues.

Cell Potency

Cell potency is a cell's ability to differentiate into other cell types. The more cell types a cell can differentiate into, the greater its potency.

Stem cells can be divided into four types on the basis of potential: Totipotent, Pluripotent, Multypotent and Unipotent stem cells.

1.  Totipotent Stem Cells: They have potential to develop into any cell type found in the body.They have ability to develop into a new organism. Totipotent stem cells can give rise to both the placenta and embryo.
2.  Pluripotent stem cells: They have potential to develop into any cell type found in the body. The difference between totipotent and pluripotent is that, totipotent stem cells can give rise to both placenta and embryo. And pluripotent stem cells don’t have ability to develop into a new organism.
3.  Multipotent stem cells: As the embryo grows, pluripotent cells develop into specialized cells known as multipotent cells. Multipotent stem cells have ability to develop into specific type of cells.For example a blood stem cell (multipotent) can develop into a red blood cell, white blood cell and platelets. There are multipotent stem cells for all the different types of tissues in the body.
4.  Unipotent stem cells: They have ability to develop into specific type of cell in the body. Skin cells are one of the most abundant type of unipotent stem cells.

Properties, Types of stem cells

•  Stem cells differ from other types of cells in the body. All stem cells regardless of their source have three general properties
•  Stem Cells are capeable for dividing and renewing themselves: Stem cells are capable of dividing and renewing themselves for long periods. Unlike muscle cells, blood cells, or nerve cells—which do not normally replicate themselves—stem cells may replicate many times, or proliferate. A starting population of stem cells that proliferates for many months in the laboratory can yield millions of cells. If the resulting cells continue to be unspecialized, like the parent stem cells, the cells are said to be capable of long-term self-renewal.
•  Stem Cells are Unspecialized: One of the fundamental properties of a stem cell is that it does not have any tissue-specific structures that allow it to perform specialized functions.For example, A stem cell cannot work with its neighbors to pump blood through the body (like a heart muscle cell), and it cannot carry oxygen molecules through the bloodstream (like a red blood cell).
•  Stem Cells can give rise specialized cell: However, unspecialized stem cells can give rise to specialized cells, including heart muscle cells, blood cells, or nerve cells. When unspecialized stem cells give rise to specialized cells, the process is called differentiation. While differentiating, the cell usually goes through several stages, becoming more specialized at each step.

There are three types of Stem Cells:

1.  Embryonic Stem Cells
2.  Adult Stem Cells
3.  Induced Pluripotent Stem Cells

Embryonic Stem Cells

Embryonic stem cells are stem cells derived from the undifferentiated inner mass cells of a human embryo. Embryonic stem cells are pluripotent, meaning they are able to grow into all derivatives of the three primary germ layers: ectoderm, endoderm and mesoderm. Most embryonic stem cells are derived from embryos that develop from eggs that have been fertilized in vitro fertilization clinic and then donated for research purposes with informed consent of the donors. They are not derived from eggs fertilized in a woman’s body. The embryos from which human stem cells are derived are typiclly four or five days old and are a hollow ball. Once formed embryonic stem cells have the potential to produuce body cells of all types.

Culturing of Human Embryonic Stem Cells

Growing cells in the laboratory is known as cell culture. Human embryonic stem cells are generated by transferring cells from a preimplantation-stage embryo 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. This coating layer of cells is called a feeder layer.

The mouse cells in the bottom of the culture dish provide the cells a sticky surface to which they can attach. Also, the feeder cells release nutrients into the culture medium. In doing so,there is a risk of viruses and other macromolecules in the mouse cells may be transmitted to the human cells. To avoid this danger embryonicstem are also grown with out mouse feeder cells. Researchers have devised ways to grow embryonic stem cells without mouse feeder cells.

The process of generating an embryonic stem cell line is somewhat inefficient, so lines are not produced each time cells from the preimplantation-stage embryo are placed into a culture dish. However, if the plated cells survive, divide, and multiply enough to crowd the dish, they are removed gently and plated into several fresh culture dishes.

The process of re-plating or subculturing the cells is repeated many times and for many months. Each cycle of subculturing the cells is referred to as a passage. Once the cell line is established, the original cells 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. At any stage in the process, batches of cells can be frozen and shipped to other laboratories for further culture and experimentation.

Properties of Embryonic Stem Cells

- Origin: Embryonic Stem cells are derived from pre- implantation of embryo.

- Self-renewal: These cells can divide to made copies of themselves for a prolonged period of time without differentiation.

- Pluripotency: Embryonic stem cells can give rise to cells from all three embryonic germ layers even after grown in culture for

- Three Germ Layers:

Ectoderm: It give rise to brain, spinal cord, nerve cells, hair, skin, teeth, sensory cells of eyes, ears, nose , mouth and pigment cells.

Mesoderm: It give rise to musscles, blood, blood vessels, connective tissues, and the heart.

Endoderm: It give rise to the gut(pancreas, stomach, liver etc.), lungs, bladder, and germ cells.

Adult Stem Cells

An adult stem cell is thought to be an undifferentiated cell, found among differentiated cells in a tissue or organ that can renew itself and can differentiate to yield some or all of the major specialized cell types of the tissue or organ. The primary roles of adult stem cells in a living organism are to maintain and repair the tissue in which they are found.

Location of Adult Stem Cells

Adult stem cells are hidden deep within organs, surrounded by millions of ordinary cells, and may help replenish some of the body’s cells when needed. In fact, some adult stem cells are currently being used in therapies. They have been found in several organs that need a constant supply of cells, such as the blood, skin, and lining of the gut, and have also been found in surprising places like the brain, which is not known to readily replenish its cells. Unlike embryonic stem cells, adult stem cells are already somewhat specialized. For example, blood stem cells normally only give rise to the many types of blood cells, and nerve stem cells can only make the various types of brain cells.

History of Adult Stem Cells

The histery of research on stem cells beguns about 50 years ago. In the 1950s it was known that bone marrow contain at least two kinds of stem cells. One population, called hematopoietic stem cells, forms all types of blood cells in the body. A second population, called bone marrow stromal cells discovered a few years later. These stromal cells of the bone marrow can generate bone, cartilage, fat, cells that support for the formation of blood, and fibrous connective tissue. In 1960s in rats it was discovered two regions of brain contain dividing cells that ultimately become nerve cells. In 1990s it is discovered that adult brain contain stem cells that are able to generate the brain’s three maror cell types, astrocytes and oligodendrocytes (non neuronal cells) and neurons.

Differentiation of Adult Stem Cells

Adult stem cells are able to divide for a long period, when they are needed, and can give rise to mature cell types that characteristic structures and functions of a particular tissue.For example, Hematopoitic Stem Cells, they cab give rise to all type of blood cells. Mesenchymal Stem Cells, they can give rise to bone marrow cartilage etc. Neural Stem Cells, they can give rise to nerve cells, ostrocytes and oligodendrocytes.

Induced Pluripotent Stem Cells

Induced pluripotent stem cells are adult stem cells that have been genetically reprogrammed to an embryonic stem cells like state by being forced to express genes and factors important to maintaining the defining properties of embryonic stem cells. Viruses are currently used to introduce the reprogramming factors into adult stem cells, and this process must be carefully controlled and tested before technique can lead to useful treament for humans.

Induced pluripotent stem cells were first reported in Mouse in 2006, and in human Induced pluripotent cells were first reported in late 2007. Mouse induced pluripotent stem cells demonstrate important characteristics of pluripotent stem cells, including expressing stem cell markers, forming tumors containing cells from all three germ layers, and being able to contribute to many different tissues when injected into mouse embryos at a very early stage in development. Human induced pluripotent stem cells also express stem cell markers and are capable of generating cells characteristic of all three germ layers.

Uses of Stem Cells

There are many ways in which human stem cells can be used in research and the clinic. In research we use stem cells to see complex events occuring during human development.A primary goal for this is to see how undifferentiated cells becomes differentiated cells that form tissues and organs. Drug Testing: Cancer cell lines are used to screen potential anti-tumor drugs. The availability of pluripotent stem cells would allow drug testing in a wider range of cell types. However, to screen drugs effectively, the conditions must be identical when comparing different drugs. Therefore, it is needed to control the differentiation of stem cells into the specific cell type on which drugs will be tested.

Regeneration of Cells and Tissues

Perhaps the most important potential application of human stem cells is the generation of cells and tissues which are often used to replace ailing or destroyed tissue. Stem cells directed to differentiate into specific cell types, offer the possibility of a renewable source of replacement cells and tissues to treat diseases including Alzheimer’s disease, spinal cord injury, stroke, burns, heart disease, diabetes, osteoarthritis, etc. For example, it may become possible to generate healthy heart muscle cells in the laboratory and then transplant those cells into patients with chronic heart disease.

Cardiovascular Disease

Most cardiovascular disorders are characterized by ischemia and heart muscle injury that result in arrhythmias, hypertrophy and congestive heart failures. Cardiovascular disease can deprive heart tissue of oxygen, thereby killing cardiac muscle cells.The use of embryonic and adult-derived stem cells for cardiac repair is an active area of research. A number of stem cell types, including embryonic stem cells, cardiac stem cells that naturally reside within the heart muscle stem cells, adult bone marrow derived cells including mesenchymal cells, endothelial progenitor cells, and umbilical cord blood cells, have been investigated as possible sources for regenerating damaged heart tissue. All these have been explored in mouse models. Severals of these injected into injured heart tissueappeared to improve heart function.

Type 1 Diabetes

In people who suffer from type 1 diabetes, the cells of the pancreas that normally produce insulin are destroyed by the patient’s own immune system. New studies indicate that it may be possible to direct the differentiation of human embryonic stem cells in cell culture to form insulin-producing cells diabetes.

Spinal cord Injuries

Severe accidents, falls, and birth defects like spina bifida cause serious injury to spinal cord. In several cases, nerve fiber bundles are severed, leading to paralysis. Currently, clinical investigations aim to use adult stem cells to regenerate new nerve cells and trigger growth of severed nerve fibers. Colour Blindness: In colour blindness a person cannot distinguish between green certain colours, usually between green and red. Embryonic stem cells are used to improve vision of colour blinded people.

Sickle Cell Anemia

In sickle cell anemia bone marrow cells do not produce normal blood cells to circulate in blood stream.They become malfunctional. Thus totipotent stem cells are used to enhance their function. Totipotent stem cells proliferate further to form bone marrow cells which replace the faulty cells. These normal cells perform normal functions.