totipotent
/ toh-TIP-uh-tent /
Imagine a single seed that can grow not only the whole plant but also the soil-bag, pot, and support stake it needs — everything required, with nothing supplied from outside. Totipotent is the highest grade of cell potency: a totipotent cell can give rise to a complete, viable organism, including both the body and all the supporting tissues an embryo needs to develop, such as the placenta.
Precisely, totipotency means a cell can produce every cell type of the embryo proper and of the extraembryonic tissues (the placenta and related membranes). In mammals only a very small set of cells are truly totipotent: the fertilized egg (zygote) and, for a brief window, the first few cells it divides into. Within a few days these cells specialize and lose totipotency, dropping down to pluripotency, which can build the body but not the placenta.
Totipotency matters because it is the literal starting point of every animal — the one cell from which everything else is built. It is also why identical twins are possible: if those earliest totipotent cells separate, each can build a whole organism on its own. A common confusion is mixing up totipotent and pluripotent; the simple distinction is that only totipotent cells can also make the placenta and the other life-support tissues.
When a zygote divides into two cells and those two are separated experimentally in some mammals, each can still develop into a complete animal — direct evidence that the earliest cells are totipotent.
Totipotency: an early cell that can build an entire organism, placenta included.
Truly totipotent cells exist only at the very start of life and for a very short time; the famous lab-grown 'all-purpose' stem cells are pluripotent, not totipotent, because they cannot make the placenta.