Embryonic stem cell research

Embryonic Stem Cell Research

In November 1998, two research groups independently announced that they had isolated human stem cells from embryonic tissues, had cultivated the cells, and shown these cells could develop into all three basic layers of cells in the human embryo (Lysaught 1999). Because these cells could potentially develop into nearly every type of human cell and tissue, they held great promise for applications in medicine and human development (Lysaught 1999).

Upon announcement of the news, Harold Varmus, director of the National Institutes of Health, was quoted as saying that human embryonic stem cell research "had the potential to revolutionize the practice of medicine and improve the quality and length of life" (Lysaught 1999). Concerns arose regarding the fact that the techniques by which stem cells were derived involved the destruction of human embryos, thus depriving them of their potential to develop into human beings (Lysaught 1999). Another concern was that some scientists might "tinker" with human life through some "grand" experiment in eugenics (Lysaught 1999).

Dating back to at least 1981, embryonic stem cells were first successfully cultured from mouse embryos, followed by the isolation of embryonic stem cells from various animals, including sheep, hamsters, pigs, cows, rabbits, mink, rhesus monkeys, and marmosets (Lysaught 1999). However, culturing human embryonic stem cells proved more difficult, for once isolated, they refused to stay undifferentiated, "seemingly driven to spontaneously differentiate and form primitive structures (Lysaught 1999). The first publicly acknowledged, yet unsuccessful, attempt to isolate and culture embryonic stem cells from human embryos in vitro was published in 1994 (Lysaught 1999). Then on November 6, 1998, James Thomson and his research team at the University of Wisconsin, Madison, reported in the publication "Science" that they had successfully maintained human embryonic stem cells in laboratory culture for a number of months (Lysaught 1999). The cells had been derived from 'spare,' week-old embryos produced by in vitro fertilization at a fertility clinic, after obtaining consent from the gamete contributors (Lysaught 1999). The research showed that the embryonic stem cells could differentiate into a "variety of tissue types, including the gut lining, muscle, cartilage, bone, and neural epithelium, representing derivatives of all three basic layers of the mammalian embryo" (Lysaught 1999). Four days later, John Gearhart and his group from Johns Hopkins University published his work in the "Proceedings of the National Academy of Culture" (Lysaught 1999). This work differed from Thomson's in two significant ways:

First, Gearhart's group isolated what are called primordial germ (PG) or embryonic germ (EG) cells, which are precursors of sperm and egg cells. Second, the cells were obtained not from embryos made in vitro but from aborted fetuses that were 5 -- 9 weeks old.

Tests showed that these cells had a number of characteristics typical of stem cells and could further develop into the three embryonic germ layers (Lysaught 1999).

These reports sparked much excitement because researchers could now design experiments to determine how human embryonic cells differentiate into various types of tissues, leading to the development of the human body, and moreover, because of the capability of these cells, a wide range of clinical applications were predicted (Lysaught 1999). Embryonic stem cells can be coaxed to differentiate into nerve or brain cells that could then be used to treat neurological disorders including Alzheimer's and Parkinson's diseases (Lysaught 1999). Moreover, if embryonic stem cells were transformed into heart muscle cells, they could be used to replace damaged tissue in the hearts of those who have suffered heart attacks or congestive heart failure (Lysaught 1999). Or they could be used to produce pancreatic islet cells in culture which could then be injected into the pancreas of a diabetic to manufacture insulin, thus reducing or eliminating the need for daily injections (Lysaught 1999). Furthermore, cell lines derived from embryonic stem cells could be used to grow entire organs for replacement, while other cell line could be used as human tissue banks against which pharmaceuticals and other chemicals could be tested for toxicity and effectiveness (Lysaught 1999). Some researchers believe that genetic alterations to embryonic stem cells could be used with the purpose of growing tissues with specific characteristics to compensate for defective tissues, and other envision genetic engineering of whole embryos (Lysaught 1999). Other possibilities include "the treatment of spinal cord injury, stroke, burns, arthritis, muscular dystrophy, kidney disease, liver disease, and macular degeneration" (Lysaught 1999).

Embryonic stem cells form at a very early stage in human development and remain in an undifferentiated state for a brief period (Wright 1999). They are first recognizable about five to seven days after fertilization, when a human embryo forms a blastocyst, a hollow, fluid-filled sphere, consisting of merely 140 cells, gives little indication that it could develop into a complete human being (Wright 1999). At this stage, there are two types of cells in the blastocyst:

Trophoblast cells, which form the "shell" of the sphere, will become the supporting tissues of the fetus such as the placenta.

Inner-cell-mass cells, which are located at one end within the blastocyst, are the undifferentiated cells that will divide and develop into the individual (Wright 1999).

When the inner cell mass begins to differentiate it forms the three fundamental germ layers of the embryo, the ectoderm, the mesoderm and the endoderm (Wright 1999). Each of these layers has a "specific destiny as a particular set of tissues in the mature individual, and all organs are ultimately derived from them," the ectoderm will form the skin, the eyes and the nervous system, the mesoderm will form bone, blood and muscle tissue, and the endoderm will develop into the lungs, liver, and lining of the gut (Wright 1999). These cells are considered to be totipotent because they rise to a complete individual, and when the inner cell mass cells are cultured in a dish, they are called embryonic stem cells (Wright 1999).

Up until the formation of the primitive streak, which will develop into the spinal cord, and cell differentiation, which occurs about fourteen days after fertilization, the developing embryo can cleave naturally or artificially, resulting in the production of identical siblings, therefore, embryonic cells that are still part of the inner cell mass are described as totipotent because they can give rise to new organisms (McCartney 2002). Another possibility during this stage is that two developing embryonic cell masses with different genotypes will fuse to form what is called a chimera, "an organism whose cells derive from two or more distinct zygote lineages" (McCartney 2002). After the cells have begun to differentiate and the primitive streak has formed, twinning and chimera formation are no longer possible (McCartney 2002).