I see your perspective, but I believe in general the research community seems to have a more specific definition on "immortalized cell lines". The immortalized cell lines that I use were established that way by various means (E6 E7 oncogenes hTERT, SV40). https://www.abmgood.com/marketing/knowledge_base/cell_culture_introduction.php#cellcycle

https://www.creative-bioarray.com/support/immortalized-cell-culture-guide.htm

Vad Pérez Stem cells are generally divided into two (very broad) groups, pluripotent stem cells and all other stem cells (such as neural progenitors, muscle progenitors and other progenitor cell types). Pluripotent stem cells can be either embryonic stem cells obtained from embryos ("ES cells") or induced ("iPS cells).

The growth rate of stem cells varies widely depending on the cell type. The same thing can be said about cancer cells. For example, I worked with more than 20 different cancer lines, of which about 8 were neuroblastoma cell lines. Neuroblastoma in particular is very hard to grow, has extremely high cell death rate, is prone to differentiating, and many lines actually grow slowly. Not all cancer cell lines grow rapidly. However, it is true that all of them are immortal (otherwise it would not be called a cell line).

Stem cells, in turn, are very different depending on the cell line, too. For example, now I am working with two human embryonic stem cell lines, KhES-1 and KhES-3. They have very different growth rate, and one grows much better than the other. To illustrate, my KhES-1 is still "passage 4", while my KhES-3 line is already "passage 10", even though I started them and split them always at the same time (because I work on both at the same time). The speed of their growth is quite different. ES and iPS cells can definitely divide just as fast as HeLa or faster. The speed of division does not directly correlate with immortality.

Now, about immortality, yes, all pluripotent stem cells (that is, ES cells and iPS cells) can divide indefinitely. However, whether the mechanism of this immortality is same as in cancer, is not clear. Pluripotent stem cells do activate telomerase, and extension of telomeres happens during reprogramming. They are very similar to cancer cells. The question "what is different between cancer cells and pluripotent stem cells" (i.e. why cancer is so aggressive while pluripotent cells are not, even though both are immortal) has not been answered yet. Whoever answers it, is going to publish pretty high.

Yulia Panina so, a question: if NSCs (ES/iPS) divide at the same rate that CSCs do — it should mean they will also form tumors, isn't?

I am not interested in finding differences between CSCs and iPSs)) my interest is much more narrow (although from your explanation it appears that in NCSs in the organism some mechanism exists decreasing proliferation rates of these cells, and this mechanism is broken in CSCs and their progeny)

Vad Pérez Hmmm, if we are talking about "in the organism", then normal stem cells DO form tumors. In stem cell biology, THE ultimate test that says "your cells are TRULY pluripotent", is the formation of tumors in NOD-SCID mice.

It's like, they define pluripotency by the ability of cells to form tumors. If they can = they are pluripotent, true stem cells.

The difference with cancer is that cancer forms metastases (pluripotent cells don't). Only that.

Wow, you just gave me a cool idea!!!! I never realized it but yeah... Actually, the only difference between stem cells and cancer is metastases; moreover, 99% of cancer deaths is attributed to metastases, not primary tumors............... hmmmmm......

Anyway. About proliferation rate. I think you need to be more careful with this because you need to distinguish between so-called "cancer stem cell" population and "cancer bulk cell" population. The pink (or red) Molecular Biology of the Cell textbook says, I remember, that "only 1% of cancer cells are actually able to form a tumor if placed into a mouse", and this 1% is called "cancer stem cells" (CSCs). CSCs divide slower. But they are responsible for maintenance of cancer indefinitely. If you place "bulk stem cells" into a mouse, no matter how much, they will die. Only CSCs can survive.

So I wouldn't say that "if the cell divides faster - it's stem", no. I think, possibly it's the opposite. Even in cancer, stem cells divide more slowly.