CH.YBW.CTS.C05.Final.q 6/21/03 1:06 PM Page 75
The Nucleus
(also known as late prophase), metaphase, anaphase, and telophase. Meiosis is a form of nuclear division that occurs in the gonads, which reduces the chromosome number of each daughter cell by half during the formation of gametes. A tumor is an abnormal cell mass that develops when controls for the cell cycle malfunction. Benign tumors tend to grow slowly, although in an unprogrammed way, and are generally not fatal. In contrast, malignant tumors grow and divide more rapidly, and their cells appear undifferentiated and are capable of metastasis.
addition to being completely undifferentiated, they grow in culture better than most adult cells. They also induce a less vigorous immune response than adult cells, making tissue rejection less likely. An ideal source of human stem cells would be those from a very young embryo, typically in the first few days after fertilization. The reason for this is that all the cells at this stage are still undifferentiated and alike. Currently, the only source of embryonic cells is from embryos created in excess of need by fertility clinics for in vitro fertilization. However, these are rare and only available to a handful of researchers. Alternatively, the very first tissue in a human fetus suffices for treatment of some diseases, and non-living fetuses are more common than several-day-old embryos as a consequence of legal abortions. Presently, over 100 patients afflicted with Parkinson’s disease have received fetal nerve cell transplants. Not surprisingly, the use of embryonic cells and fetal tissues is highly controversial. Patient advocacy groups recognize potential benefits for treating debilitating illnesses. On the other hand, some human rights groups object to harvesting or using human stem cells under any circumstances. What do you think?
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