What Are Totipotent Stem Cells?

Stem cells are characterized according to their degree of potency, which refers to their varying ability to differentiate into different cell types. Totipotent cells are the most potent of all stem cells, and defining them is important for research and the field of regenerative medicine.

Definition of totipotent stem cells

There are two definitions of totipotent stem cells, which reflects the inherent technical difficulty that lies in characterizing them:¹

1. A totipotent cell is a single cell that can give rise to a new organism, given appropriate maternal support (most stringent definition)
2. A totipotent cell is one that can give rise to all extraembryonic tissues, plus all tissues of the body and the germline (less stringent definition)

The original test of totipotency was performed in mice by Tarkowski (1959), who isolated a single blastomere (cells created by divisions of the zygote, consisting of 2–16 cells), placed it into an empty zona pellucida, and monitored its development into live born young.²

This approach is not bulletproof; the failure of blastomeres to support chimera development may indicate limitations related to the reconstructed embryo, rather than restricted development potential of the cell in question.

It is for this reason that the second, less stringent definition of totipotency is also widely used.

Having two definitions inevitably creates confusion, particularly as the term “totipotency” is often used inappropriately in the literature. It has been argued that this creates unnecessary ethical controversy with practical and political implications.³ 

Sometimes the term “totipotent” is awarded to cells if they merely participate in an embryonic process – however, this doesn’t mean that they necessarily could give rise to an organism. Other common causes for misclassification include assuming the expression of early embryonic markers to mean totipotency, and taking partial or superficial resemblance to an embryo as evidence for totipotency.³

Totipotent stem cells differ from pluripotent cells, which can differentiate into cells from any of the three germ layers, and multipotent cells which are less potent. 

Why the fuss about totipotent stem cells?

Totipotent stem cells are unique as they have a greater developmental potential compared with other stem cells. Having the ability to isolate and culture totipotent stem cells creates many therapeutic and research possibilities related to:¹

• Studying zygotic genome activation (the point at which development becomes exclusively controlled by the zygotic genome, rather than the maternal genome)⁴
• Rewiring the epigenome (treating pathological conditions induced by epigenetic alterations)⁵
• Understanding early embryonic development in more detail
• Creating human-animal chimeras⁶ (theoretically, animals with human organs could aid disease modeling, drug development and transplantation)¹

Stem cells are increasingly being used as model systems in research. Differentiating between stem cell types relies on an understanding of the embryological roadmaps and the factors that define their pathways.

Source of totipotent stem cells

The diploid zygote cell is totipotent. Totipotent cells also exist in subsequent divisions of the zygote, before the rise of the trophectoderm lineage (which occurs approximately four days after fertilization, depending on the species).

To understand the origin of totipotent stem cells, it helps to be familiar with the early stages of embryonic development, and the related terminology.

The table below highlights the important stages of early embryonic development and demonstrates the defining point at which totipotent stem cell no longer exist, and pluripotent cells arise.

Major stages of early embryonic development

*To provide a unified developmental chronology of mammalian embryology, the Carnegie Institution for Science developed a set of 23 stages known as the “Carnegie stages” whereby stages are defined by morphological development, and not directly by age or size. 

Cell	Potency	Fate
Epiblast	Pluripotent	The epiblast gives rise to the three primary germ layers (ectoderm, endoderm, mesoderm) which will form all somatic lineages plus the germline.
Hypoblast	Not pluripotent, not totipotent	The hypoblast gives rise to the extraembryonic primitive endoderm, i.e. the yolk sac which provides nutrients to the embryo when primitive placental circulation has been established.
Trophoblast	Not pluripotent, not totipotent	The trophoblast gives rise to various extraembryonic structures which enable implantation into the uterine wall, secrete human chorionic gonadotropin to enable progesterone secretion from the corpus luteum, and form the chorion (fetal part of the placenta).