In 1998, scientists isolated pluripotent stem cells from early human embryos and grew them in culture. In the few years since this discovery, evidence has emerged that these stem cells can become almost any of the 200 known specialized cells of the body and, thus, may generate replacement cells to repair or replace cells or tissues that are damaged or destroyed by diseases and disabilities.
Meanwhile, as the political process restricts research in many areas of stem cells and therapeutic cloning, a flurry of information suggests great potential for adult stem cells.
There is tremendous expectation for stem cell therapy; at this time, it is too soon to say just how or when stem cells from any source will be useful for the treatment of disease or trauma. More research for all types of stem cells is needed. What follows is a brief primer on stem cell terminology.
Primitive, unspecialized cells that have the capacity to divide for indefinite periods in culture. They have the potential to produce both new stem cells and differentiated, specialized cells. Embryonic stem cells are 'totipotent,' meaning that they can give rise to all the different cell types in the human body, while other types of stem cells are more restricted in their potential, giving rise to only a subset of tissue types (e.g., hematopoietic stem cells, which can differentiate into all the different blood cell types but usually not other types of tissues).
Pluripotent stem cells
Can give rise to the cells that develop from the embryonic germ layers, from which all the cells of the body arise. Pluripotent stem cells are derived from early human embryos or fetal tissue destined to be part of the gonads. Pluripotent cells can also be created by manipulating the genetic content of adult cells (see Induced Pluripotent Stem Cells)
Induced Pluripotent Stem Cells (iPS Cells)
A type of pluripotent stem cell derived from an adult cell such as a skin cell, by inducing an expression of certain genes that reprogram the cell. iPS cells are believed to be identical in many respects to embryonic pluripotent stem cells, including the ability to form all cells in the body and to reproduce themselves indefinitely; the full extent of their relation to embryonic stem cells is still being assessed. iPS cells were first produced in 2006 from mouse cells and in 2007 from human cells.
This is an important advancement in stem cell research; it may allow researchers to obtain pluripotent cell lines without the use of embryos. iPS cells might also lead to cell lines that are genetically customized for individual patients.
Embryonic stem cells
Very primitive cells that develop within days of ova fertilization with the potential to develop into all of the body's cell types.
Embryonic germ cells
Derived from fetal tissue, specifically from the primordial germ cells that develop in to the testes or ovaries. Embryonic stem cells and embryonic germ cells are pluripotent but they are not identical in their properties.
The process by which an unspecialized cell (such as a stem cell) specializes into one of the many cells that make up the body. During differentiation, certain genes become activated and others are inactivated in an intricately regulated fashion.
Adult stem cells
An undifferentiated (unspecialized) cell that occurs in a differentiated (specialized) tissue, renews itself, and becomes specialized for the cell types of the originating tissue. Adult stem cells are rare and difficult to identify, isolate, and purify but are capable of making identical copies of themselves for the lifetime of the organism. There are insufficient numbers of cells available for transplantation and adult stem cells do not replicate indefinitely in culture.
The ability of nerve circuitry to remodel itself.
A cell that has almost fully differentiated into its final specialized form (e.g., an erythroblast is the intermediate step between an erythrocytic progenitor cell and a fully developed red blood cell, so it is considered the precursor to the red blood cell).
A cell that has started to differentiate down a particular path but has not fully developed yet. Unlike stem cells, progenitors do not have the capacity to create indefinitely self-renewing copies of themselves; they are fated to differentiate into a final, specialized cell type (e.g., an erythrocytic progenitor cell will eventually give rise to fully differentiated red blood cells, but it cannot go back to being a blood stem cell).
Somatic cell nuclear transfer (therapeutic cloning)
SCNT is the transfer of DNA from the nucleus of a "somatic cell" (skin, heart, nerve or anything or any other non-egg or sperm cell) into an egg that has had its nucleus removed. "Cloning," means to copy, whether one is copying a molecule, a cell or a collection of cells that make up a human blastocyst.
SCNT for the purpose of producing molecules or cell lines provides us with unique opportunities to develop effective treatments for a wide variety of currently incurable diseases and disorders. It is also a powerful technology for modern forensic science, vaccine development and drug discovery and testing. SCNT is a vital research tool, allowing scientists to learn more about disease, disease development and how the body heals itself.