Cell culture is the practice of growing cells outside their natural environment using controlled laboratory conditions. This technique allows scientists to cultivate large populations of specific cell types for study and application. When applied to stem cells, this process involves nurturing these unique, unspecialized cells, encouraging them to multiply while either maintaining their flexibility or guiding them to become specific, mature cell types.
Sources of Stem Cells for Culture
To begin the culture process, scientists first need a source of stem cells. One source is embryonic stem cells (ESCs), which are derived from the inner cell mass of a blastocyst, an early-stage embryo. These cells are pluripotent, meaning they can develop into any cell type in the body. Their versatility makes them a powerful resource for research and potential therapies.
A second source is adult stem cells, also known as somatic stem cells. These are found in small numbers in tissues like bone marrow, fat, and skin. Unlike ESCs, adult stem cells are multipotent, so their ability to differentiate is limited to the cell types of their tissue of origin. For example, hematopoietic stem cells from bone marrow primarily give rise to different types of blood cells.
A modern source is induced pluripotent stem cells (iPSCs). This technology allows scientists to take a specialized adult cell, such as a skin cell, and reprogram it back to a pluripotent state. This is achieved by introducing specific genes that turn back the cell’s developmental clock. A primary advantage of iPSCs is that they can be created from a patient’s own cells, providing a personalized model for studying disease or developing therapies without immune rejection.
The Laboratory Culturing Process
Growing stem cells requires recreating the conditions of the human body in the lab. This starts with a sterile environment inside a laminar flow hood, which is a ventilated cabinet that filters the air to prevent contamination. All materials and liquids that come into contact with the cells must be sterilized to maintain this environment.
The cells are grown in specialized plastic flasks or dishes coated with proteins that mimic the body’s natural environment. This coating allows the cells to attach and spread, a requirement for the survival and growth of most stem cell types. Without a surface to adhere to, many cells will not proliferate.
These flasks are filled with a culture medium, a liquid that provides all the necessary nutrients for the cells to survive and multiply. The medium is a complex mixture containing salts, vitamins, amino acids, and glucose. It also contains specific proteins and growth factors that send signals to the stem cells, instructing them to keep dividing without differentiating.
The flasks are housed in an incubator that maintains a constant temperature of 37°C, high humidity, and a controlled atmosphere with 5% carbon dioxide (CO2) to keep the culture medium’s pH stable. This regulated environment ensures the cells can function and grow as they would inside the body.
As the cells multiply, they eventually cover the surface of the flask. To continue expanding the population, scientists perform passaging, or subculturing. This involves using an enzyme to gently detach the cells from the flask’s surface. The cell solution is then collected, diluted, and transferred into new flasks with fresh medium, giving them more space to grow.
Directing Stem Cell Fate
Once a stable population of stem cells is established, scientists can pursue one of two goals. The first is to maintain the cells in their undifferentiated, pluripotent state. This requires using a specialized culture medium formulated with growth factors and inhibitors that suppress differentiation pathways. By controlling these signals, researchers can expand the population of stem cells for future use.
Daily maintenance involves removing spent medium and replenishing it with fresh liquid to ensure a constant supply of nutrients and signaling factors. Scientists also visually inspect the colonies each day, manually removing any areas that show signs of spontaneous differentiation. This quality control ensures the purity and pluripotency of the stem cell line.
The second goal is to direct the stem cells to differentiate into a specific cell type. This process, known as directed differentiation, involves changing the culture medium to mimic the signals that cells receive during natural embryonic development. By adding a precise sequence of growth factors and signaling molecules, scientists can provide the cells with a roadmap, guiding them down a particular lineage.
For instance, to create neurons, researchers might first expose the stem cells to factors that encourage them to form ectoderm, one of the three primary germ layers. Subsequently, adding other molecules can further guide these precursor cells to become specific types of neurons. Similarly, different cocktails of growth factors can coax stem cells to become heart cells, liver cells, or pancreatic beta cells.
Applications of Cultured Stem Cells
The ability to grow and manipulate stem cells has opened up numerous applications in medicine and research. One application is disease modeling. By generating iPSCs from patients with a genetic disorder, such as Parkinson’s disease, scientists can create a “disease in a dish.” Differentiating these iPSCs into the affected cell type allows researchers to study the disease process in human cells and identify potential targets for new treatments.
Cultured stem cells are also used for drug discovery and toxicity testing. Before new medications are tested in humans, their safety and effectiveness must be evaluated. By differentiating stem cells into specific cell types like liver or heart cells, pharmaceutical companies can test new compounds on human cells in a dish. This approach can reveal potential toxicity early, reducing the reliance on animal models.
Finally, regenerative medicine uses cultured stem cells to repair or replace damaged tissues and organs. This application holds the potential to treat a wide range of conditions. For example, scientists are working on generating skin grafts for burn victims, creating pancreatic cells for patients with diabetes, and developing methods to replace neurons lost in spinal cord injuries. While many of these therapies are still experimental, they represent a future where cell-based treatments could restore function for patients.