During development, cells gradually lose their ability to become “anything,” and take on more specialized jobs. This spectrum of potential is called cell potency, and it’s important in understanding how a single zygote gives rise to a fully formed organism.

Totipotent Cells

Totipotent cells are the most versatile of all. They can give rise to any cell type in the body, including the extraembryonic tissues like the placenta. In humans, this potency is seen in the zygote and early morula stage. Because they can generate both the embryo and the supporting structures, totipotent cells are capable of forming an entire organism on their own.

Pluripotent Cells

As development progresses to the blastocyst stage, cells in the inner cell mass become pluripotent. These cells can still become any cell within the embryo itself (they can form all the tissues of the body) but they cannot develop into placental or other extraembryonic tissues. Pluripotent cells are responsible for forming cells of all three germ layers: ectoderm, mesoderm, and endoderm.

Multipotent Cells

Later in development, and throughout adult life, we encounter multipotent cells. These are more restricted; they can still differentiate, but only within a certain lineage. For example, hematopoietic stem cells can give rise to various blood and immune cells, but they can’t become neurons or skin cells. Similarly, mesenchymal stem cells can become bone, muscle, or fat cells, but not beyond that. These cells are often involved in tissue repair and regeneration and are sometimes referred to as adult stem cells.

Determination vs. Differentiation

A common point of confusion is the distinction between determination and differentiation, which are related but not the same.

Determination is when a cell becomes committed to a specific fate. Even if it still looks like an undifferentiated cell, its molecular programming has been set, like a train locked onto a specific track. For example, a formerly pluripotent cell might become committed to becoming an ectodermal cell, even though it hasn’t yet taken on a specialized shape or function.

Differentiation comes after determination. This is when the cell actually becomes its final identity, like a neuron, a skin cell, a muscle fiber, and takes on the structural and functional traits of that cell type. Differentiation involves visible changes in gene expression, morphology, and behaviour.

Progenitor Cells

While often lumped in with stem cells, progenitor cells are slightly different. They are already on the path to becoming a certain cell type, so they’ve lost some of the flexibility seen in true stem cells. Most progenitor cells are unipotent or multipotent, and unlike stem cells, they generally have limited self-renewal capacity.

Examples:

• Hematopoietic progenitor cells, which are precursors to red and white blood cells and platelets

• Satellite cells, which help regenerate skeletal muscle after injury

In lab settings, some progenitor cells can be pushed beyond their usual boundaries using specific transcription factors or growth conditions, but this doesn’t necessarily reflect their normal behaviour in the body.