Define totipotency.

Totipotency refers to the ability of a single undifferentiated cell to give rise to all the different cell types that make up an entire organism, as well as the extraembryonic tissues necessary for its development. In other words, a totipotent cell has the potential to develop into any cell type found in the organism's body, as well as the cells required for supporting structures like the placenta in mammals.

Totipotency is a characteristic mainly observed in the very early stages of development, such as during the first few divisions of a fertilized egg (zygote) or in the cells of the early embryo. As the organism progresses in development, cells start to specialize into different types through a process called differentiation. Cells become committed to specific lineages and lose their totipotent capabilities.

It's important to note that totipotency is distinct from pluripotency, which refers to the ability of cells to develop into almost all cell types in an organism but not the extraembryonic tissues. Pluripotent cells are found in later stages of development, such as in the inner cell mass of a blastocyst in mammals, and are used in various applications in regenerative medicine and research.