Word: celling
(lookup in dictionary)
(lookup stats)
Dates: during 1990-1999
Sort By: most recent first
(reverse)
Their specialist did hold out one faint hope. In the future, he said, for men with few or sluggish sperm that could not penetrate egg cells on their own, scientists might be able to help fertilization take place by injecting a single sperm cell into an egg. At the time, that sounded futuristic even to a medical man like Jim. But he and Sarah took their doctor's advice, stopped trying to conceive through in vitro and kept Jim's last remaining samples frozen at the sperm bank...
...technique their physician had predicted, is known as ICSI--for intracytoplasmic sperm injection--and had first been performed successfully in Belgium in 1992. Injecting a sperm cell into an egg may sound like a simple procedure, but attempts had failed until researchers figured out how to manipulate the sperm and egg without damaging them. U.S. clinics now do thousands of icsi procedures a year, with a success rate of about 24%. The technique can help men with low sperm counts or motility, and even those who cannot ejaculate or have no live sperm in their semen as a result...
Gerontologists and gene theorists generally embark from the premise that cell division holds the key to unlocking the mystery of aging. Laboratory experiments have shown that over the course of a human lifetime, healthy cells complete a specific number of divisions. When that process is disrupted, as commonly happens in old cells, problems like osteoporosis and immunological failure set in. Conversely, so-called immortal cells that continue to divide past their allotted count turn cancerous and can proliferate indefinitely. "We'd be in worse shape if cells didn't undergo senescence," says Smith, who studies the aging of cells...
...popular theory posits the existence of clock-type genes that in essence predetermine a human's life-span. Human cells appear to have some sort of counting mechanism and "remember" where they are in the sequence of divisions they must go through. Certain "longevity assurance" genes then make sure that the cell population keeps dividing until the clock winds down. A newer theory champions the role of stress-response genes, which regulate the body's maintenance and repair functions. As yet, researchers are unsure whether these genes merely affect susceptibility to disease or actually control the aging processes...
Starzl acknowledges that this insight had also occurred to other researchers. Scientists, going back as far as 1960 Nobel prizewinner Peter Medawar, had come to recognize that tolerance was possible. If bone marrow, for instance, would only accept an interloping cell, the larger system would follow suit. The trouble was, the only way to achieve that was to kill off the body's entire current bone-marrow supply and replace it with another--a technique oncologists use as a last-ditch weapon to try to cleanse patients of such systemic cancers as leukemia and breast cancer...