Greatest hits: What is a teratoma?

Now and then we will revisit some of our most popular posts and/or posts with the most comments.

One of the best: The Monster Tumor: What is a teratoma really?

You can go to the link above to read the original post and comments but I’ve pasted the post itself below.

Teratoma. Creative Commons image.

In stem cell research there is a lot of talk about a very unusual type of tumor called “teratoma” and researchers often do “teratoma assays”.  The name teratoma literally means something like monster tumor. The reason becomes obvious if you’ve ever seen pictures of one. They have mixtures of all kinds of tissues including teeth, hair, eyes, sweat glands, bones. Human can spontaneously get teratoma, but they can also be induced experimentally in immunodeficient murine models from human or mouse highly pluripotent stem cells.

But what is a teratoma really? I have been trying to learn as much as I can about these tumors and found some wrong assumptions and oversimplifications.
Surprisingly (to me at least and to many people I think), teratoma are not necessarily benign tumors. They can also be immature/malignant. The general assumption is that all teratoma are benign, but they aren’t.  Therefore, when one discusses teratoma in relation to potential side effects of stem cell based regenerative medicine, the practice should be to include the word in front of it, choosing between “benign” or “malignant”. This is the case in the clinical setting. So if one sees in a paper the word “teratoma” it could be benign or malignant.
Also, even benign teratoma often contain some fraction of immature, proliferative cells that have malignant potential, but are contained within a capsule around the tumor. So teratoma can be technically benign, but paradoxically contain malignant cells.
Teratoma are also extremely heterogeneous tumors making every one of them totally unique. While to some extent this is true of all tumors, teratoma are at least an order of magnitude more complex than any other type.
I’m still not clear on what’s the difference between a malignant teratoma and a teratocarcinoma? Anyone know?
I think there’s a lot more that we in the stem cell field need to know about teratomas as they are the main enemy of regenerative medicine. When papers in the field report on teratoma it would be helpful if they showed more pictures, low (showing the whole tumor) and high magnification, and told us if they are malignant or not.
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