The body’s tissues are maintained by unspecialized cells called stem cells, which can develop into many different cell types. This capability exists on a spectrum, with some types having broader potential than others. The classification of a stem cell depends on its differentiation potential. A frequent question that arises is whether embryonic stem cells, one of the most widely discussed types, are totipotent.
Defining Cellular Potency
Cellular potency describes a stem cell’s ability to differentiate into specialized cell types. This potential exists on a spectrum, with the highest level known as totipotency. Totipotent cells can give rise to all cell types that make up a complete organism, including both the embryonic tissues that form the body and the extraembryonic tissues, such as the placenta and umbilical cord. The first few cells that form after a sperm fertilizes an egg are totipotent.
A step down from totipotency is pluripotency. Pluripotent cells are able to develop into any cell type that makes up the body, which derive from the three primary germ layers known as the ectoderm, endoderm, and mesoderm. However, they cannot create the extraembryonic support structures like the placenta. This distinction separates them from their totipotent counterparts.
A more limited category is multipotency. Multipotent stem cells are found in adult tissues and are more specialized. Their differentiation potential is restricted to a specific family of cells. For instance, hematopoietic stem cells found in bone marrow can generate various types of blood cells, such as red blood cells and platelets, but they cannot become nerve or skin cells. This contrasts with the broader capabilities of pluripotent and totipotent cells.
The Origin of Embryonic Stem Cells
The origin of embryonic stem cells (ESCs) is traced to the earliest moments of development. The process begins with fertilization, which creates a single cell called a zygote. This zygote is totipotent, able to form an entire organism and its supporting tissues. As the zygote divides over the next few days, it forms a solid ball of cells known as a morula, and the cells within this structure remain totipotent.
Approximately four to five days after fertilization, the morula develops into a blastocyst. The blastocyst is a hollow ball of cells with two distinct parts: an outer layer called the trophectoderm, which will go on to form the placenta, and an inner cluster of cells called the inner cell mass (ICM).
The cells that scientists harvest to create embryonic stem cell lines are taken from this inner cell mass. The process involves isolating the ICM from the blastocyst. Once placed in specific laboratory culture conditions, these isolated ICM cells can multiply indefinitely while remaining in their undifferentiated state. These cultured cells are what are formally known as embryonic stem cells.
The Verdict on Embryonic Stem Cell Potency
The embryonic stem cells derived from the inner cell mass of the blastocyst are pluripotent, not totipotent. This conclusion is based on the observation that while these cells can differentiate into any cell type that forms the body, they cannot generate the extraembryonic tissues like the placenta.
The common confusion arises because earlier-stage embryonic cells are totipotent. The zygote and the cells of the morula have the capacity to form a complete organism, support structures included. However, by the time the blastocyst forms, the first step of differentiation has already occurred. The cells have segregated into two distinct lineages: the trophectoderm, which becomes the placenta, and the inner cell mass, which becomes the fetus.
This specialization means the cells of the inner cell mass have already lost their totipotency. They can no longer generate the trophectoderm. Therefore, the ESCs harvested from the ICM inherit this same limitation. The act of deriving these cells does not change their intrinsic potential; it simply allows them to be grown and studied in a lab.
The Significance of Pluripotency
The pluripotency of embryonic stem cells makes them a subject of scientific interest. Their ability to become any cell type in the body, from a heart muscle cell to a neuron in the brain, opens up possibilities for medicine and research. This versatility is useful in regenerative medicine, where the goal is to repair or replace tissues damaged by injury or disease. For conditions like heart disease or spinal cord injuries, the ability to generate new, healthy cells is a primary objective.
Pluripotent stem cells also provide tools for studying human development and disease in the laboratory. Researchers can guide ESCs to differentiate into specific cell types to create disease-specific models. For instance, by creating nerve cells from ESCs, scientists can study the progression of neurodegenerative diseases like Parkinson’s and test the effects of new drugs on human cells.
This broad potential distinguishes them from multipotent adult stem cells. While adult stem cells are useful for repairing the specific tissues in which they reside, their applications are limited by their restricted differentiation capacity. The capacity of pluripotent cells to form all derivatives of the three primary germ layers gives them a far wider range of research and therapeutic applications.