Captured: Mother Of All Homo Sapiens Cells
Wisconsin Scientist Isolates Human Embryonic Stem Cells; Dazzling Therapies Now PossibleBy David N. Leff
When sperm meets egg, their unicellular union sets off a process that nine months later - in humans, that is - produces a living body comprising several trillions of cells.
It takes a lot of cell divisions, in geometric progression, to produce this numerical outcome, which begins when the nuclei of those male and female gametes fuse. But the name of that numbers game isn't just multiplication by division; it's differentiation.
Right from their get-go, at the instant of fertilization, those first two founder cells launch the cascade of differentiation that will turn generalist cells into specialists. That process takes off when the initial cluster of all-look-alike cells rounds up into the 140-cell blastocyst. This is a bubble-like sphere of cells one layer thick - the trophoblast - enclosing a cell cluster bunched in one place on the inner surface. Those two divergent elements represent the very first differentiation of embryogenesis.
That blastocyst cell mass is destined to become the embryo. From it will be derived a few embryonic stem cells that are straining, like racing greyhounds at the starting post, to begin differentiating into the future body's cells, tissues and organs. In less than a week, the blastocyst will snuggle into the lining of the uterus and rev up the already-begun differentiation process.
But while they're still free agents, those universal blastocyst cells are pluripotent. That is, their DNA carries all the messages lying in wait to create this, that or whatever other bodily component.
Holds 'Great Promise For Transplantation'
To a developmental biologist, the holiest of all grails is to wrap his hands around a human embryonic stem cell (hES). Now, one such scientist has done just that.
He is developmental biologist James Thomson, at the University of Wisconsin, Madison. Thomson is senior author of a paper in today's Science, dated Nov. 6, 1998, titled: "Embryonic stem cell lines derived from human blastocysts."
"ES cells may exist in the blastocyst's inner cell mass very briefly at that stage of the game," explained university spokesman Terry Devitt. "Their nature is quickly to grow up to be something else." He observed that Thomson is the first of many scientists in hot pursuit of the human embryonic stem cell to come up with the trophy.
As a veterinary pathologist in the university's Primate Research Center, he had previously perfected, and patented, the technology in rhesus monkeys and marmosets.
Thomson's hES acquisition began in January of this year. His lab is just a few doors down from the University's In Vitro Fertilization (IVF) clinic.
"Once you have fertilization in a dish," Devitt recounted, "you get to the blastocyst stage in about six days."
At that point, it is exposed to an antibody that reacts to the trophoblast layer. This is then lysed by complement, leaving the inner cell mass intact, without having to extract it physically from the entire blastocyst.
"This is done in the clinic itself," Devitt went on. "The inner cell mass is put in a dish, and handed to Thomson down the hall. And that's when his work begins."
That work, to derive the hES cells, uses a microscope and a micropipette. Once extracted from the inner cell mass manually, Thomson placed them in culture, on a layer of irradiated mouse feeder cells."
Those cell masses from the IVF clinic were donated with informed consent by the patient couples in Wisconsin, and at a cooperating IVF clinic in Israel.
From these cultures, Thomson and his co-authors isolated five viable hES lines. Three of them, with XY chromosomes, were potential males; two, with XX, females. Four of the five were cryopreserved after five to six months of nonstop, undifferentiated proliferation. The active fifth line has now undergone 32 passages in culture during more than eight months, and still counting.
Human embryos propagate all of their diverse organ systems from three cell layers derived directly from the hES cells in the blastocyst inner mass. To wit: The ectoderm layer gives rise to the nervous system (as well as hair); endoderm to lungs, liver and gut; mesoderm to blood, bone and muscle.
All five of the Wisconsin hES cells, the Science paper reported, "maintained the potential to form derivatives of all three embryonic germ layers," which denotes pluripotency.
"Those cells," said Thomson, "are different from all other human stem cells isolated to date. As the source of all cell types," he continued. "they hold great promise for use in transplantation medicine, drug discovery and development, and the study of human developmental biology." (See related story, p. 2.)
Thomson's allowed primate patent and a pending one covering the human ES are held by the not-for-profit Wisconsin Alumni Research Foundation (WARF). It has licensed them with worldwide exclusivity to the Geron Corp. of Menlo Park, Calif., which funds Thomson's work.
A close runner-up to Thomson in the quest for human embryonic stem cells is John Gearhart, in the department of gynecology and obstetrics at Johns Hopkins Medicine, in Baltimore, who is also supported by Geron. He wrote a commentary accompanying the Thomson paper titled "New potential for human embryonic stem cells."
That potential, as Gearhart spelled it out, foresees "a renewable source of cells for tissue transplantation, cell replacement and gene therapies [which could] eventually preclude the direct use of fetal tissue in transplantation therapies. Obvious clinical targets," he wrote, "include neurodegenerative disorders, diabetes, spinal cord injury and hematopoietic repopulation."
Drug Discovery Now; Therapies Later
Looking closer to the here and now, Geron's vice president of research and development, Thomas Okarma, told BioWorld Today: "The potential to supply unlimited quantities of normal human cells of virtually any tissue type could have a major impact on pharmaceutical research and development." He explained: "We would genetically engineer these cells to overexpress markers of a disease. So we can, for example, immortalize neurons that would overexpress markers for Alzheimer's disease, and basically create the cellular equivalent of the human disease in a test tube. Then, [we can] screen chemical libraries in search of compounds useful to treat Alzheimer's."
As quoted in a separate Science "News" article, Thomson concluded: "Right now we don't know how to direct [stem cells] to become any specific cells. But it's no longer in the realm of science fiction. I really believe that within my lifetime I will see diseases treated by these therapies." n