Totipotency is the ability of a single cell to divide and produce all the differentiated cells in an organism, including extraembryonic tissues. Totipotent cells formed during sexual and asexual reproduction include spores and zygotes. Zygotes are the products of the fusion of two gametes (fertilization). In some organisms, cells can dedifferentiate and regain totipotency. For example, a plant cutting or callus can be used to grow an entire plant.
Human development begins when a sperm fertilizes an egg and creates a single totipotent cell (zygote). In the first hours after fertilization, this cell divides into identical totipotent cells. Approximately four days after fertilization and after several cycles of cell division, these totipotent cells begin to specialize.
Totipotent cells have total potential. They specialize into pluripotent cells that can give rise to most, but not all, of the tissues necessary for fetal development. Pluripotent cells undergo further specialization into multipotent cells that are committed to give rise to cells that have a particular function. For example, multipotent blood stem cells give rise to the red cells, white cells and platelets in the blood.
Importantly, totipotent cells must be able to differentiate not only into any cell in the organism, but also into the extraembryonic tissue associated with that organism. For example, human stem cells are considered totipotent only if they can develop into any cell in the body, or into placental cells that do not become part of the developing fetus. This is an important aspect of the stem cell controversy.
Basis of totipotency
The molecular mechanisms controlling totipotency are not well understood and are a subject for current research. In particular, a February 2006 report in Science suggests that in the model organism Caenorhabditis elegans, multiple mechanisms including RNA regulation maintain totipotency at different stages of development.