What characteristic defines totipotent stem cells?

Totipotent stem cells are defined by their remarkable ability to develop into any type of cell in an organism. This includes not only the various cell types that make up the body tissues but also the extra-embryonic tissues necessary for early development, such as the placenta. This unique characteristic of totipotency distinguishes them from pluripotent stem cells, which can differentiate into nearly all cell types of the body but cannot form the extra-embryonic tissues.

The presence of totipotent stem cells is typically seen in the earliest stages of embryonic development, for example, in the zygote and the first few divisions that follow. As development progresses, cells begin to specialize, and their differentiation potential is restricted. This underpins the foundational principle that totipotent stem cells hold the most versatile capacity for differentiation compared to other types of stem cells.

In summary, the defining feature of totipotent stem cells is their unqualified capability to give rise to every cell type, including those necessary for complete organismal development.