Mentsch
A cell is not a davar chashuv nor is rennet. However even a non-davar chasuv, if it is a “maamid” Ie without it there is no final product (this may not be the best explanation I am open to others). Just like since without rennet you wouldn’t have cheese, that minute amount of rennet isnt batul, likely here too since without that original cell you wouldnt have your In vitro meat, that cell cannot be batul.
On the other hand, clearly rules of bittul dont apply to microscopic organisms, otherwise bacterium wouldnt be batul as they are beryeh a creature. Clearly on a microscopic level even a beryeh is batul perhaps a davar hamamid too (of course the rules for bitul are not necessarily equivalent and it is quite possible that while a LActobacillum acidophulus is batul althoug a beryeh a stem cell might not be batul) I’m not sure if the fact that it is deliberately added is necessarily relevant, since yogurts often advertise “live cultures” which are sometimes added and nonetheless certified kosher.
Furthermore hwen a cell divides its DNA is split between the 2 cells. So discussing which is hte “original stem cell” doesnt really make sense when there are 2 cells the “original cell” is gone and half of the DNA of each new cell came from the original cell. so the idea of bitul might not make sense at all. (I’m not sure if this makes it better or worse)
Of course all this ignores a perhaps more pertinent issue of davar habah min hatamei.