Totipotent stem cells are the most developmentally capable cells in biology, defined by their unique ability to differentiate into every cell type required for an organism. This broad potential includes the cells that form the body itself and the necessary extraembryonic tissues, such as the placenta and the umbilical cord. This expansive capacity makes these cells the foundation of life, as they hold the complete genetic and developmental blueprint to form a viable individual. The window in which totipotent cells naturally exist is extremely brief and localized.
The Earliest Stage of Development
The search for naturally occurring totipotent cells begins at the start of life with the single-celled zygote, the fertilized egg. The zygote is the canonical example of totipotency, possessing the unrestricted capacity to develop into an entire organism. Following fertilization, the zygote rapidly undergoes a series of cell divisions known as cleavage, remaining within the protective layer called the zona pellucida.
The cells resulting from these first few divisions are called blastomeres, and they retain the zygote’s full developmental power. In mammals, this totipotent state typically lasts up to the four-cell stage, and sometimes the eight-cell stage, a structure often called the morula. At this point, any one of these individual blastomeres could theoretically develop into a complete embryo, including the necessary supporting structures.
This developmental stage is characterized by the earliest activation of the embryonic genome, switching control from maternal components to the embryo’s own DNA. The transient nature of this cell state makes it difficult to study, as it represents the single moment when a cell is completely unrestricted in its fate. The cells at this stage express a unique set of genes and mobile DNA elements, such as the mouse-specific MERVL retrotransposons, that are hallmarks of this potential.
Transition to Pluripotency
The totipotent state is quickly lost as cells commit to their first major fate decisions, leading to the formation of the blastocyst structure. This marks the transition from totipotency to a more limited, but still highly capable, state known as pluripotency. This transition occurs around four days after fertilization in humans, as the morula’s cells begin to organize into two distinct populations.
The outer layer of cells differentiates into the trophectoderm, destined to form extraembryonic tissues like the placenta and chorion. The inner cluster of cells forms the inner cell mass (ICM), which will give rise to the embryo proper. Once this segregation is complete, the ICM cells are considered pluripotent. They can generate all cell types of the body, but they can no longer form the extraembryonic tissues.
This commitment represents an irrevocable split in developmental destiny, where the cells lose the capability to form a complete organism independently. The formation of the trophectoderm closes the brief window of totipotency, restricting the developmental potential of the remaining embryonic cells. This event explains why stem cells derived from the blastocyst, such as embryonic stem cells, are classified as pluripotent rather than totipotent.
Totipotency Beyond Mammals
While totipotency is a fleeting phase in mammalian development, it is a more common and sustained capability in other kingdoms of life. Plant cells are an excellent example, as nearly every differentiated plant cell retains the inherent potential to regenerate an entire organism. This phenomenon, called cellular totipotency, forms the basis for vegetative propagation and tissue culture techniques.
Specialized regions in plants, such as the meristems located at the tips of roots and shoots, house actively dividing cells that function as stem cell populations. Mature plant cells, such as those from a tobacco stem or a carrot root, can undergo dedifferentiation. When cultured with the correct balance of growth hormones like auxins and cytokinins, these mature cells revert to an embryonic state, forming a callus from which a whole new plant can be generated.
In the animal kingdom, totipotency is found in certain lower organisms, particularly those with remarkable regenerative abilities. Planarian flatworms, for instance, possess adult stem cells called neoblasts that are responsible for regenerating a complete, properly proportioned body from tiny fragments. The neoblasts are considered truly totipotent stem cells, as a single transplanted neoblast can restore full regenerative capacity to an irradiated, non-regenerating worm.
Creating Totipotency in the Lab
Because of their immense developmental potential, scientists are actively working to induce totipotent-like cells in a controlled laboratory setting. Researchers have observed that a small, transient population of cells spontaneously emerges within cultures of mouse pluripotent stem cells. These rare cells are termed 2-cell-like cells (2CLCs) because they express the same molecular markers and genetic profiles as the naturally occurring two-cell stage embryo.
These induced cells are a major focus of research because they offer a path to studying the earliest stages of life without relying on natural embryos. Scientists can chemically reprogram pluripotent stem cells or fully differentiated somatic cells to adopt this totipotent-like state. Certain chemical cocktails have been shown to induce features resembling the totipotent eight-cell embryo stage in human pluripotent stem cells.
Laboratory induction often involves manipulating epigenetic regulators that control the accessibility of the cell’s DNA. For example, blocking the function of a protein called CAF1 can loosen the compact structure of the DNA in pluripotent cells, allowing activation of the genes necessary for the totipotent state. Generating these induced totipotent stem cells provides a powerful context for studying the molecular switch that governs fundamental potential in developmental biology.