As a teen I read a story about a scientist who had kept a piece of heart tissue alive - a piece of it was discarded into a pond and grew into a menace (the detective figured out that the 'blob' was causing the problems in the story).
"The Thing in the Pond", a short story by Paul Ernst; first published in Astounding Stories, June 1934, available at the Internet Archive. It has appeared in a number of anthologies; does any of these covers ring a bell?
It started with a bit of heart tissue from a chicken:
"Professor," said Sharpe, "it was about ten years ago that your discharged servant got back at you by dumping that laboratory stuff into the pool, wasn't it?"
"Yes," said the old man, his lips twitching.
"Among the stuff was a lot of sodium, potassium, and calcium salts, and probably a barrel of sugar," guessed Sharpe.
"Y-yes," admitted Weidbold.
"Now, while I was here ten years ago you cut off a bit of that chicken-heart muscle you've kept living and pulsing for sixteen years in a solution of potassium, calcium, sodium, and sugar. I remember that distinctly. You've cut off several bits; otherwise the stuff would outgrow the nourishment-capacity of the case. What happened to that fragment?"
"I-it got lost, or something."
"It's conceivable that it was among the stuff your servant dumped into the pond, isn't it?"
"Such a fantastic accident—" mumbled Weidbold.
"All accidents are fantastic," said Sharpe curtly. "That's why they call them accidents. It's conceivable—isn't it?"
"And in this warm, life-filled pond," Sharpe pursued relentlessly, "the tiny bit of muscle substance flourished. It absorbed the chemicals freakishly dumped in with it, and finally all the small life. Then it began to reach out for more food in its voracious growth."
"I tell you it's impossible!" almost shrieked the professor. "It could not live outside a laboratory! Ask any scientist—"
"I'd prefer to ask Raeburn's cow or your dog," Sharpe cut in dryly.