There was a post about a woman who grew a “homonculus tumor”, with brain tissue, teeth, a spinal nerves and other organs. But how? Human bodies can’t grow those things after birth, AFAIK. For example, once your adult teeth are in, that’s it. So how are tumors able to do this? Are the cells in tumors different from those found normally in human body?
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Oh boy. I’m not seeing a good answer so far so I’ll weigh in. This is a special kind of tumor called a teratoma. They can form because we set aside a type of stem cell early in embryonic development especially to create sperm and eggs later on. These special cells are set aside *early* enough that they closely resemble the stem cells that give rise to…everything, at least in terms of their genetic expression programs and the marks on their DNA that allow or disallow certain gene programs to activate.
What also makes these cells interesting is that they have to migrate quite far to get to the future ovary or teste. Many get lost along the way during embryonic development and kind of die, but sometimes, and very rarely, they don’t die. These lost migratory cells can become a tumor if they also acquire mutations that let them grow out of control. Likewise, some of these develop from the same cells that do make it to the future testes or ovaries but don’t quite get integrated properly into either their cellular homes or don’t activate the right programs to restrict their properties down to just a future sperm or egg producing cell.
Since these cells don’t have the program restrictions to tell them to be one or a few types of cells, they have a tendency to just activate all of them, chaotically and in patches. Teeth, hair and nerves are a pretty good default program that many go down the path of. This is also why stem cells in a dish, if neglected, have the same tendency to become the cells that are precursors to skin and nerves. They are simply a good “if no signal otherwise, just make this” type of category of cells. However under the right circumstances the conditions are right for even more complicated cells types to arise from the lost and very capable cells that make up teratomas.
All in all, it’s quite a lot of hurdles and jumps to get through for all of this to happen. Even if they do, most are so small and benign that you’d never encounter any reason to be aware of their existence in your lifetime. It’s only the marvels that make headlines.
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