Embryonic stem cells (ESCs) are defined by two specific characteristics. The first is pluripotency, the ability to develop into any of the more than 200 cell types that make up the human body. The second is the capacity for indefinite self-renewal, meaning they can divide numerous times while remaining undifferentiated. These cells originate from a structure within an early-stage embryo called a blastocyst. The blastocyst contains the inner cell mass, and it is this mass that gives rise to ESCs.
Sourcing and Establishing Cell Lines
The journey of an embryonic stem cell line begins with blastocysts, typically donated for research from in-vitro fertilization (IVF) clinics with informed consent from donors. These blastocysts are often those not used for clinical purposes. Researchers isolate the inner cell mass (ICM), a small cluster of cells inside the blastocyst, using techniques like immunosurgery or mechanical isolation to separate it from the outer layer. The isolated ICM is the source of the pluripotent cells.
Once isolated, these ICM cells are placed onto a culture dish to multiply without differentiating. This process establishes a “cell line,” which is a population of cells that can be grown and propagated continuously in a laboratory. A successful derivation results in a stable, self-renewing population of pluripotent cells, providing an expandable source of cells for study.
Conditions for Laboratory Growth
Maintaining embryonic stem cells in their pluripotent state requires a controlled laboratory environment. The cells are grown in a culture dish on a surface that supports their attachment and growth. Historically, this involved using a “feeder layer” of inactivated mouse or human fibroblast cells that condition the environment and provide support. This method helped maintain the pluripotency and proliferative abilities of the stem cells.
More modern techniques are shifting toward feeder-free cultures, which offer greater consistency. In these systems, the culture dishes are coated with specific proteins, like Matrigel or vitronectin, that mimic the supportive matrix provided by feeder cells. This approach allows for a more defined and controlled culture system for standardizing research and future clinical applications.
The cells are bathed in a nutrient-rich liquid called a culture medium, a broth containing sugars, vitamins, amino acids, and minerals. Specific proteins known as growth factors must be added to this medium. For human ESCs, a primary growth factor is fibroblast growth factor 2 (FGF2), which signals the cells to continue dividing without specializing. Daily changing of this medium and constant monitoring are necessary to prevent the cells from spontaneously differentiating.
Guiding Differentiation into Specialized Cells
To make embryonic stem cells useful for research, scientists guide their development into specialized cell types through a process called directed differentiation. This involves altering the culture conditions that were used to maintain pluripotency. The process begins by removing growth factors, like FGF2, that signal the cells to remain in their undifferentiated state. This removal allows the cells to begin the process of specialization.
Scientists then introduce a new set of signaling molecules and growth factors into the culture medium. These new factors are chosen to mimic the specific biochemical cues that cells would receive during natural embryonic development. By providing a sequence of these specific signals, researchers can direct the stem cells down a particular developmental pathway.
This process allows for the creation of a wide variety of cell types in the laboratory. For example, adding specific factors can coax the stem cells to develop into neurons. Introducing a different combination of molecules can guide them to become cardiac muscle cells, which can be observed contracting in the culture dish. This ability to generate specific human cell types on demand is a powerful tool for scientific investigation.
The Role of Cultured Stem Cells in Science
Specialized cells created from embryonic stem cells have numerous applications. One use is in disease modeling, where scientists use gene-editing to create cell lines with specific genetic defects. This allows them to grow tissues in a dish that replicate diseases like Parkinson’s, providing insight into how these diseases progress at a cellular level.
These lab-grown cells are also valuable for drug discovery and toxicology testing. Before a new medication is tested in humans, it can be applied to cultured cells to assess its effectiveness and potential toxicity. This helps identify harmful side effects early, making drug testing safer and more efficient.
Another goal is regenerative medicine, aiming to generate healthy tissues for transplantation to treat conditions like spinal cord injuries or heart disease. This involves replacing damaged cells with new ones grown from stem cells. The field faces regulatory hurdles and ongoing ethical discussions related to the use of embryos in research.