|MadSci Network: Cell Biology|
The largest cell in the human body is the ovum, the female sex cell. The ovum is about 1000 micrometers, or one millimeter, in diameter. The ovum needs this size as nourishment before it implants itself in the uterus. It becomes this large by this means: In meiosis, there are two stages. In the first stage, the nucleus divides in a way that reduces the number of chromosomes to half in each nucleus generated. In the second stage, the sister chromosomes separate, largely resemble the meitosis. When the nucleus of the cell that will become the ovum divides the first time, it simply expells one nucleus. The second time, it does the same. This leaves one huge ovum and three little cells, called polar bodies, outside. (The expelled nucleus divides again, too.) The ovum does not release the second nucleus until it is fertelized. The tiny cells quickly die, because they have no organelles.
The smallest cell in the human body, in terms of volume, is the sperm cell. When the cell that preceeds the sperm divides, by meiosis, it divides evenly, creating four small sperm cells. Each sperm cell is little more than a nucleus propelled by mitochondria and a flagellum. Interestingly, when the sperm cell reaches the ovum, only the nucleus enters. Therefore, no male mitochondria move on to the next generation. (Other cells, such as neurons, may be have much smaller cell bodies, but they often have long processes so that their total volume is typically larger than that of a sperm cell.)
As you have probably guessed, the example as to how fast cells divide involves the newly fertilized egg cell. These embryonic cells can divide every fifteen to twenty minutes. There is a short interphase between divisions, so the cell must be able to divide in a shorter amount of time. Cells in the adult human may divide at the same speed, but they have an interphase of at least eight hours between divisions. Some cells, like brain cells, or neurons, never divide again after the person reaches adulthood.
Thanks for asking. Keep those questions coming!
Matthew Barchok, Mad Scientist
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