Discussion of all aspects of cellular structure, physiology and communication.
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No - metastases are all mutated forms of the cells of the original tissue from which they derived. Cancer is a single cell lineage. (Unless you're really unlucky and get a second lot of critical, transforming mutations in a second somatic cell!)
A primary tumour, by definition, is located in the tissue of origin. Tumour cells lose their adhesion to others (because of increasing mutations within the proliferating primary tumour cells) and can then penetrate the capillaries supplying the primary tumour, and thence escape into the blood supply. They are swept around the body in the blood (and lymph) as CTCs (Circulating Tumour Cells), until they reach another organ that that particular primary tumour tends to migrate to.
(They can remain dormant for years - though I don't know whether that's (a) as CTCs (b) as small collections (with leucocytes as well, I believe) adhering to the walls of the capillaries at the secondary site or (c) non-proliferating micro-met colonies in the secondary site tissues - or all of those possibilities. Whatever the mechanism of dormancy, it's why cancer can recur so many years later - though, again, just what triggers metastatic proliferation is still unclear. One possibility is the involvement of the immune system.)
Just why some primary tumours typically metastaise to particular secondary sites is also still a bit of a mystery. There is a 'seed and soil' theory (quite old) that likens the met cells to 'seeds' that require the right 'soil' to make metastatic colonies in. There is also some evidence sometimes that the site for metastasis is determined by the architecture of the circulatory system - ie, it's a question of which suitable site is closest to the primary in terms of blood supply. The concept of 'sentinel' nodes is similar - it's the lymph node closest to the drainage area from the primary. Overall, the general rule is that metastases form in the tissues that receive the most blood supply, typically the lungs.
Metastatic tumour cells are more mutated than those in the primary, in the sense that they first of all have the ability to avoid cell-to-cell adhesion (cahderin etc - as I'm sure cytologists here will elaborate more on!), and that they tend to be further down the lineage from the original primary cells.
I believe that because advanced metastaic cells can sometimes be SO mutated from the primary such that in the strange case of CUP (Cancer of Unknown Primary), it can be very hard to tell from the metastases just what the cell type original was, and therefore where the cancer actually started. (Also, of course therefore, what the best therapeutic drugs are for it....)
Grimly, far, far too many patients get their diagnosis courtesy of their secondary tumours becoming symptomatic. Metastatic cancer is generally incurable, but therapies are increasingly being used as 'maintenence therapies' to stop the cancer becoming lethal.
I hope all this information is correct. I'm no expert, but it's just a subject I'm studying currently.
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