totipotency

Totipotency refers to the developmental potential of a cell or group of cells to form all the cell types of a complete organism, including embryonic tissues and extraembryonic membranes such as the placenta. In animals, the zygote and the earliest blastomeres are typically regarded as totipotent.

Totipotency is distinct from pluripotency, where cells can form all embryonic tissues but not extraembryonic structures. More restricted states include multipotent and unipotent potentials that produce only a subset of cell types.

In mammalian development, totipotent capacity is associated with the zygote and the initial cleavage divisions. Early blastomeres can, in principle, develop into a full organism when isolated. As cleavage proceeds, cells begin to differentiate: the morula and early blastocyst begin to segregate into trophectoderm, which contributes to the placenta, and the inner cell mass, which forms the embryo proper, marking a shift from totipotency toward pluripotency.

In humans, totipotency is generally limited to the zygote and the very early cleavage-stage cells; by the blastocyst stage, cells are pluripotent. Laboratory work often uses pluripotent stem cells (embryonic or induced), and researchers have described totipotent-like states under specialized culture conditions, though these are not equivalent to true totipotency in vivo.

Totipotency informs cloning and developmental biology research and has implications for regenerative medicine. Ethical considerations accompany embryo manipulation and the creation or alteration of embryos.