Totipotency

Totipotency is the greatest developmental potential a cell can have: the ability of one cell, on its own, to give rise to an entire organism. In mammals the zygote, the fertilized egg, is totipotent, and so for a short time are the cells of the earliest cleavage stages. A totipotent cell can make not only every cell of the body but also the extraembryonic tissues, such as the trophoblast that forms the placenta, and organize them into a patterned whole.

Potential narrows as development goes on. By the blastocyst stage, the cells of the inner cell mass are pluripotent: they can form all three germ layers and therefore any body tissue, but not a complete organism with its placenta. Later, multipotent cells make only one related family of cell types. Some authors loosely call any cell that can make all cell types totipotent, but the strict test is whether one isolated cell can develop into a normal individual.

Plants show totipotency far more widely, since many cells from a piece of stem or leaf can be grown in culture into a complete new plant, the basis of plant tissue culture and cloning. In animals, cloning by nuclear transfer, as with Dolly the sheep, showed that an adult cell's nucleus can regain totipotency inside an egg.