What is a stem cell?
Stem cells are the cells which have the remarkable potential to develop into different types of cells through mitotic cell division and differentiate into specialized cell types.
Why are they important?
Stem cells are important for many reasons. Embryonic stem cells can give rise to the entire body of the organism. It forms specialized cell types and organs like heart, lung, skin and other tissues. In some adult tissues like bone marrow, muscles and brain, stem cells replace cells that are lost through tear and wear, disease or injury. Due to their regenerative property stem cells have potential to treat diseases like heart disease and diabetes.
Properties of stem cells
Stem cells differ from other kinds of cells in body. They have three general properties: they are capable of dividing and renewing, they are not specialized cells, and they can give rise to specialized cell types.
Stem cells have ability to divide and renew themselves for long period. Cells such as muscle cells don’t normally replicate but stem cells replicate repeatedly. The initial population of stem cells can produce millions of cell within months in the laboratory.
Stem cells are unspecialized. Stem cell does not have certain structures that allow it to perform specialized function. For example, a heart muscle pumps blood and red blood cells carry oxygen. Stem cells cannot perform specialized functions but they can give rise to specialized cells like heart cells, nerve cells, and muscle cells.
Stem cells can give rise to specialized cells. The process by which stem cells give rise to specialized cells is known as differentiation. Cell’s genes, having long DNA, control the internal signals and carry coded instructions to all cellular structures and functions. External signals include chemicals secreted by other cells and physical contact with other cells that help in cell differentiation.
Types of stem cells
Stem cells are classified into three types based on their ability to differentiate.
Totipotent cells
These are the most versatile of the stem cell types. They have the potential to develop into any cell found in the human body or entire organism. In human development, egg cell and sperm cell fuse together to form single cell called zygote. Zygote is totipotent which means it gives rise to all human cells like brain cell, heart cell, liver cell. The first few divisions in embryonic development produce more totipotent cells. After four days of embryonic cell division, the cells begin to develop into specialized pluripotent stem cells.
Pluripotent cells
These are like totipotent cells that can give rise to all tissue types but cannot rise to an entire organism. After four days of fertilization, totipotent cells form a cluster of cells called blastocyst. Blastocyst has small group of cells called inner cell mass. The inner cell mass is pluripotent as it giving rise to all tissues in the human body. The pluripotent cells continue to divide and begin to specialize further.
Multipotent cells
Multipotent stem cells have the same basic features of all stem cells. A multipotent stem cell can give rise to other types of cells but it is limited in its ability to differentiate. These other types of cells are also limited in numbers. Examples of multipotent cells include haematopoietic cells which give rise to blood cells or those in brain that give rise to neural cells. Multipotent stem cells are found in the tissues of adult mammals. It is thought that they are in most body organs, where they replace diseased or aged cells. Thus, they function to replenish the body's cells throughout an individual's life.
Based on the source of stem cells, they are of following types.
Embryonic stem cells
Embryonic stem cells are derived from the inner cell mass of blastocyst, an early stage embryo. Human embryos reach the blastocyst stage in 4-5 days after fertilization. These are pluripotent which means they are able to develop into three primary germ layers: ectoderm, endoderm and mesoderm including over 220 types of cells in the human body.
Because of their plasticity and potentially unlimited capacity for self-renewal, ES cell therapies have been proposed for regenerative medicine and tissue replacement after injury or disease. Some of the many diseases that could be treated by transplanting cells harvested from human embryos are: Diabetes, Cardiovascular diseases, Cancer, Parkinson's disease, Spinal injuries.
Adult stem cells
Adult stem cells are undifferentiated cells found throughout the body after embryonic development. The primary roles of adult stem cells in living organisms are to maintain and repair the tissue in which they are found
Adult stem cells have been identified in many organs and tissues, including brain, bone marrow, peripheral blood, blood vessels, skeletal muscle, skin, teeth, heart, gut, liver, ovarian epithelium, and testis. They are thought to reside in a specific area of each tissue (called a "stem cell niche"). In many tissues, current evidence suggests that some types of stem cells are pericytes, cells that compose the outermost layer of small blood vessels. Stem cells may remain quiescent (non-dividing) for long periods of time until they are activated by a normal need for more cells to maintain tissues, or by disease or tissue injury.
In a living animal, adult stem cells are available to divide, when needed, and can give rise to mature cell types that have characteristic shapes and specialized structures and functions of a particular tissue. The following are examples of differentiation pathways of adult stem cells that have been demonstrated in vitro or in vivo.
- Hematopoietic stem cells give rise to all the types of blood cells: red blood cells, B lymphocytes, T lymphocytes, natural killer cells, neutrophils, basophils, eosinophils, monocytes, and macrophages.
- Mesenchymal stem cells give rise to a variety of cell types: bone cells (osteocytes), cartilage cells (chondrocytes), fat cells (adipocytes), and other kinds of connective tissue cells such as those in tendons.
- Neural stem cells in the brain give rise to its three major cell types: nerve cells (neurons) and two categories of non-neuronal cells—astrocytes and oligodendrocytes.
- Epithelial stem cells in the lining of the digestive tract occur in deep crypts and give rise to several cell types: absorptive cells, goblet cells, paneth cells, and enteroendocrine cells.
- Skin stem cells occur in the basal layer of the epidermis and at the base of hair follicles. The epidermal stem cells give rise to keratinocytes, which migrate to the surface of the skin and form a protective layer. The follicular stem cells can give rise to both the hair follicle and to the epidermis.
Studying stem cells will help us understand how they transform into the dazzling array of specialized cells that make us what we are. Some of the most serious medical conditions, such as cancer and birth defects, are due to problems that occur somewhere in this process. A better understanding of normal cell development will allow us to understand and perhaps correct the errors that cause these medical conditions.
Another potential application of stem cells is making cells and tissues for medical therapies. Today, donated organs and tissues are often used to replace those that are diseased or destroyed. Unfortunately, the number of people needing a transplant far exceeds the number of organs available for transplantation. Pluripotent stem cells offer the possibility of a renewable source of replacement cells and tissues to treat a myriad of diseases, conditions, and disabilities including Parkinson's disease, amyotrophic lateral sclerosis, spinal cord injury, burns, heart disease, diabetes, and arthritis.
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