James A. Thomson and his colleagues derived the first human embryonic stem cell lines by isolating and culturing inner cell mass cells from human blastocyst-stage embryos. At approximately the same time, John P. Gearhart and his colleagues made the first human embryonic germ cell lines by isolating and culturing human primordial germ cells from aborted human fetuses.
Stem cells are common to all multicellular organisms and provide the means to regenerate damaged tissues. They can maintain their primal developmental state throughout their lifetime, which allows them to continuously divide and replace themselves. Additionally, they can generate many progeny cells that form the principal cell types of the tissues in which they are found. Embryonic stem cells are short-lived and found only in early embryos, but they divide rapidly and are pluripotent—that is, they can give rise to all the tissues found in the adult organism. Therefore, scientists have believed that the ability to manipulate stem cells would transform medicine.
Stem cell biology began with Leroy C. Stevens’s work on mouse teratomas. Teratomas are bizarre, benign tumors that contain a pastiche of tissues that range from folds of skin, teeth, bone, and tangles of hair to bits of muscle, gut, tubules, glandular material, and neurons. Stevens noticed that in addition to containing differentiated (specialized) tissues, teratomas contained cells that did not differentiate, but instead continually divided and formed new copies of themselves. Transplantation of one of these undifferentiated cells could induce the formation of an entire teratoma in a host mouse. Some of these transplanted teratomas formed unusual structures called embryoid bodies that looked like mouse embryos turned inside out. These stem cells that could perpetuate, and ultimately form, the teratoma were later known as embryonal carcinoma (EC) cells. Stevens showed that EC cells were derived from primordial germ cells (PGCs), the cells in the embryo that give rise to the eggs and sperm. Because PGCs greatly resembled cells from early embryos, Stevens transplanted cells from mouse embryos and showed that they could also cause teratomas. He called these cells from early embryos that could form teratomas “pluripotent embryonic stem cells,” but because particular tissues (such as liver, thymus, and kidney) were never seen in teratomas, the potential of EC cells to form all adult tissues was questioned.
In 1975, Beatrice Mintz
A colony of embryonic stem cells.
In 1981, two research groups successfully cultured mouse embryonic stem cells (ESCs). At Cambridge University, Martin J. Evans and Matthew H. Kaufman harvested whole mouse embryos and grew them in tissue culture on drug-treated feeder cells. After researchers purified these cultures so that only the cells derived from the inner cell mass (ICM, the cells that give rise to the embryo proper) were growing in culture, they established ESC lines from them. Later, Gail R. Martin at the University of California, San Francisco, derived ESC lines by culturing isolated ICM cells from mouse blastocyst-stage embryos on drug-treated feeder cells. Martin succeeded where others had failed because she used culture media that had formerly been used to grow EC cells. The EC cells secreted particular factors into the medium that allowed the embryonic cells to grow and prevented them from differentiating. Removal of these ESC lines from the feeder cells caused the cultures to differentiate into a vast array of different tissues. Injection of ESCs under the skin of a host mouse led to the formation of teratomas. Because embryonic ICM cells divide rapidly and quickly commit themselves to particular developmental fates, their existence is somewhat evanescent. The ability of cultured ICM cells to grow indefinitely is a consequence of their removal from the embryo and maintenance in tissue culture. These groundbreaking experiments laid the foundation for the isolation and culture of human ESCs.
Human ESCs from human blastocyst-stage embryos were first made in 1998 in the laboratory of James A. Thomson, who had extensive experience establishing embryonic stem cell lines from nonhuman primates. The fertilized embryos used by Thomson’s group were donated, cleavage-stage embryos, left over from in vitro fertilization clinics. Thomson’s group began with twenty blastocyst-stage embryos and thirty-six fertilized embryos. They isolated and cultured ICM cells from fourteen of these embryos and grew them on layers of irradiated mouse feeder cells. Five of these cultures successfully grew to form human embryonic cell lines. When grown without the mouse feeder cells, each cell line differentiated into a mosaic of different tissues. Injection of cells from all five cell lines into immunodeficient mice caused the production of teratomas. These cells also retained normal chromosomes throughout their extended growth periods, and one particular culture, H9, remained chromosomally normal after eight months of culture growth(thirty-two passages). This experiment showed that these cells not only shared many properties with embryonic stem cells from mice and nonhuman primates but also were clearly pluripotent human ESCs, capable of forming all adult tissues.
In the same month of 1998, John P. Gearhart and colleagues reported the derivation of human embryonic germ (EG) cell cultures from PGCs. Researchers cultured disaggregated gonadal ridges (the developing gonads) and mesenteries (the membranes that hold the intestines in place) from five- to nine-week-old fetuses, obtained after elective abortions, on irradiated mouse feeder cells with a cocktail of growth factors. Outgrowing PGCs formed large, many-celled colonies over a period of one to three weeks that resembled those formed by mouse ESCs. EG cells expressed molecules common to early embryonic cells and, like Thomson’s ES cells, were found to be chromosomally normal and stable after multiple passages. In culture EG cells spontaneously formed embryoid bodies that displayed a wide variety of differentiated cell types. Thus human PGC-derived cultures have most of the properties of ESCs but cannot form teratomas when injected into immunodeficient mice.
Isolation of ESC lines represents one of the greatest biological discoveries of the late twentieth century. Mouse ESC cultures have completely revolutionized the genetic analysis of mammalian development. Clinically, differentiated ESCs can repair acute spinal cord injuries and ameliorate the symptoms of Parkinson’s disease in rodents and nonhuman primates. While many problems and questions remain, the potential to use embryonic stem cells in regenerative therapy for degenerative diseases and chronic injuries in humans remains great.
However, this same discovery opened a Pandora’s box of troubling questions that continued to beleaguer embryonic stem cell research. If one embryo can be adopted by a couple under the Snowflake Embryo Adoption Program and develop into a toddler, but another embryo is donated to an embryonic stem cell research lab and is destroyed, are we not arbitrarily determining who lives and who dies? Furthermore, the generation of therapeutically useful embryonic stem cells that match the tissue types of different people requires therapeutic cloning, whereby the genetic material of one individual is inserted into an unfertilized egg, which is induced to divide and grow into a blastocyst-stage embryo whose ICM cells are harvested and cultured to form patient-specific embryonic stem cell lines. If we laud therapeutic cloning, can we realistically prevent reproductive cloning, a practice that has been outlawed throughout the world?
Questions like these have caused most governments who allow embryonic stem cell research to heavily regulate it. However, researchers tend to oppose regulation and will typically fight to lessen it. How far should we allow this research to go? What dangers does it represent, if any? These are not simple questions, and embryonic stem cell research, despite its humanitarian benefits and clinical potential, presents difficult questions that must be considered before plunging headlong into it.
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Kiessling, Ann A., and Scott Anderson. Human Embryonic Stem Cells. Sudbury, Mass.: Jones & Bartlett, 2003. Accurate and readable but somewhat technical explanation of the biology, clinical potential, and legal status of ESC research. -
Lewis, Ricki. “A Stem Cell Legacy: Leroy Stevens.” The Scientist 14, no. 5 (2000): 19. Terse, informative summary of the history of early ESC research. -
Parson, Anne B. Proteus Effect: Stem Cells and Their Promise for Medicine. Washington, D.C.: Joseph Henry Press, 2006. Engaging and engrossing introduction to stem cell research, its history, the personalities within it, and its clinical possibilities. -
Scott, Christopher T. Stem Cell Now. New York: Plume Books, 2006. A very helpful insider’s overview of ESC research in nonspecialist language. -
Smith, Wesley J. Consumer’s Guide to a Brave New World. San Francisco: Encounter Books, 2004. Cautionary investigation of ESC research that raises some very pointed questions.
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