From Ancient Greek τέρατος (tératos, “monster”).
Aptly named, I would say.
"Pluripotent stem cells including human induced pluripotent stem cells have a unique property of being able to generate teratomas when injected in rodents in the research laboratory.[62] For this reason, the so-called "teratoma assay" is one of the gold-standard validation assays for pluripotent stem cells.[63] Because differentiated human pluripotent stem cells are being developed as the basis for numerous regenerative medicine therapies, there is concern that residual undifferentiated stem cells could lead to teratoma formation in injected patients, and researchers are working to develop methods to address this concern.[64]"
Our bodies differentiate cells into different lineages precisely when it is developmentally necessary. You don't want to turn the clock in reverse. That's a disease state and a hallmark of cancer.
https://en.wikipedia.org/wiki/Cell_lineage
On the subject of developmental biology, this is a really fun image if you aren't familiar:
Well, you actually do want to turn the clock in reverse. That's a hallmark of the highest so far methodology standard in science: if you want to position that you gained a full understanding of an object, you need to be able to reverse engineer it both in space and time. Many cancers contain senescence associated secretory phenotype (SASP) and senolytic drugs have been researched to be able to neutralize cancer cells as well as senescent cells.
You do want to turn the clock in reverse, only maybe not the whole way back to the embryonic state, but a couple of formal years or decades back to reset the epigenome, and this is exactly what David Sinclair, Vittorio Sebastiano and many others are undertaking now.
We can’t even do this with simple black box game playing AIs, so a new paradigm may be needed.
What would happen if the adult human donor took these embryonic-like cells and reinjected them? Would the person develop the vim and vigor of a young adult?