Plausibility of "multi-type transmissible animal cell tumor" to form more complex structures.
There's apparently a type of transmissible cancer that came from new world dogs thousands of years ago, the Canine transmissible venereal tumor. As I understand, this self replicating surviving cancer has DNA from new world dogs no longer found in the current canine population. It's its own "thing", not from a virus, radiation, or chemical phenomena.
I would like to postulate the possibility of more than just one type of cell metastasizing in a animal. Would it be possible for multiple types of "cancer" cells (or at least host foreign animal cells) to work in unison to create new structures? For example, extending a cardio vascular system, or nervous system through out the tumor, or, at the most far fetched side of things, a new appendage, like a cancerous "ear" formation (like a bunch of ears starting to grow in an area on the dog/animal due to this foreign cell "infection"?).
Even if this couldn't happen in general, would this be feasible with compatible immune systems, such as those found with inbred animals?
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For the cells to act exactly as described would require cooperation between different metastasising cell lines, both during infection and when growing structures in the new host. That implies a kind of control not usually found in a tumour.
Even a simple tumour tends to grow into an amorphous mass unless it's growing from stem cells, in which case it can form recognisable structures including multiple cell types. If using a single line involving stem cells is acceptable for an answer, then I can think of two ways it might work.
1. Take a stem cell and make it cancerous
In the comments, Jordan suggested a stem cell becoming cancerous and some of its daughter cells developing into limbs. There are tumours called teratomas that have been found containing skin, hair, eyes and teeth, so that part at least is possible. Teratomas are believed to originate from germ cells (which are the ultimate stem cells) and do not usually metastasise.
To become transmissible the stem cell would have to become more mobile than usual as well as becoming cancerous, and would have to wait until it had infected a new host before it started to differentiate. That is three changes that need to be made to the cell's behaviour, all without losing its ability to act like a stem cell - and "becoming more mobile" might involve activating multiple genes.
2. Take a cancerous cell and make it stem cell-like
There is evidence that some cancer cells can become partly de-differentiated, turning "back" into more potent stem cells. This raises the question of whether a transmissible tumour cell could become so dedifferentiated that it could give rise to an ear or whatever. A normal(ish) cell can be made into a fully 'pluripotent' stem cell in the lab, but it requires a set of changes that don't normally happen in an animal.
Dedifferentiation means losing most of the cell's differentiated characteristics, and my (non-expert) guess is that this would include the ability to infect a new host.
For the tumour to be infections and grow new structures, the same kind of dedifferentiation would have to happen again in each new host. That implies that either this tumour has cells that frequently dedifferentiate spontaneously, or they do it in response to certain conditions in the host.
Plausibility
Both of the above scenarios require several unusual changes to a cell. Tumour cells divide and mutate frequently so any one change is likely to happen from time to time, but getting a cell line to acquire them all while still being viable would be a bit like building a working radio from scratch - possible with a lot of work and the right technology, but unlikely by chance.
In this case the "right technology" might be available soon if it isn't already. Recent years have seen a lot of interest in stem cell generation, including its implications for tissue regeneration and cancer. Whether anyone would or should actually try to produce a transmissible cancer that can create complex structures is another matter.
If it did happen by chance, the new cancer would suddenly be growing new structures in its host that are likely to cause more problems than a normal tumour, so it might have trouble spreading through the population.
Immunity
Every cell type in the new tumour/organ/limb will carry a (slightly?) different set of antigens, so I'd expect that having more cell types involved means getting more immune rejection. Using inbred hosts would be the way to go.
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Would it be possible for multiple types of "cancer" cells (or at least host foreign animal cells) to work in unison to create new structures?
Normal cancer, inasmuch as there is such a thing, doesn't really seem to work like that... cancer cells are fundamentally broken things, because of the wide range of safeguards that healthy cells have to prevent them becoming cancerous. They carry on doing the things they used to do, to the best of their ability given their damaged state.
For example, extending a cardio vascular system,
Angiogenesis is very much a thing in many kinds of tumour, but it isn't the cancer cells themselves forming the blood vessels. Instead they make use of existing signalling mechanisms to induce healthy tissue to form new blood vessels to feed the tumour.
or nervous system through out the tumor
Whilst blood vessel (re)generation is a common trick, regrowing nerves (and especially regrowing nerves quickly) is rather more limited. It doesn't seem totally beyond the realms of possibility, but it does seem dubious, if you were expecting to have complex signalling through the tumour.
or, at the most far fetched side of things, a new appendage, like a cancerous "ear" formation
Stem cell cancers can produce differentiated tissue, but the structures seem to be fairly simple (see above, cancer cells are broken) and messy things.
In a species with the ability to regerate complex structures it might be possible for a cancer to hijack regeneration mechanisms in the same way that more conventional mammalian cancers can hijack angiogenesis. This wouldn't be an effect that could cross species though; the capability would have to be present in the "host" species infected by your transmissible cancer, rather than being something the cancer could bring itself.
As a potentially interesting aside, stem cells can be quite hazardous in themselves... there's at least one documented case of someone undergoing experimental stem cell therapy ending up with healthy snot-producing cells developing in their spine. Removing proliferating but healthy stem cells is much harder than dealing with cancer as the usual mechanisms of cancer treatment are much less effective on healthy cells (obviously).
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