CH.YBW.CTS.C05.Final.q 6/21/03 1:06 PM Page 74
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CELLS, TISSUES, AND SKIN
which has the instructions to produce virtually every protein in the body. Each nucleus contains one or more nucleoli, small, non-membranous, dense bodies where ribosomal subunits are assembled. Chromatin consists of loosely coiled fibers of protein and DNA, and contains information for protein synthesis in regions called genes. The cell cycle refers to the series of changes that a cell undergoes from the time it forms until it reproduces and includes interphase, mitosis, cytoplasmic division, and differentiation. Mitosis is divided into five phases: prophase, prometaphase
A QUESTION TO PONDER: Stem cells are undifferentiated cells. This property makes them
pluripotent, meaning they have the potential to differentiate into virtually any cell type found in the body. In fact, the name “stem cell” refers to the notion that all the various cells of the body originate (stem) from them. As you know, every human starts out as a single cell, which has the ability to form all the different cells of the body. As a zygote develops into an embryo, cells begin to differentiate, which depends on activation of some genes and repression of others. As a general rule, once a cell differentiates, it loses the ability to become another cell type. Some adult tissues, such as the dermis of the skin, bone marrow, and brain, contain stem cells. However, because adult stem cells are already somewhat specialized, they do not have the same developmental possibilities as embryonic ones do. Nonetheless, stem cells from adult skin have been successfully induced to form nerve cells. Embryonic stem cells have several properties that make them more ideal for medicinal use to treat spinal cord injuries or certain diseases, such as Parkinson’s disease (degeneration of certain brain cells), Alzheimer’s disease (a degenerative brain disorder), diabetes mellitus (a disease of the pancreas affecting blood sugar regulation), and leukemia (a blood cell cancer). In