Stem cells are biological cells with unique properties, fundamental to life and development. These unspecialized cells do not yet have a specific function, unlike a skin or nerve cell. They play a role in the formation, maintenance, and repair of tissues and organs throughout an organism’s life. Understanding their growth and behavior offers insights into biological processes and opens avenues for advancements.
What Are Stem Cells?
Stem cells are defined by two characteristics: self-renewal and potency. Self-renewal is their ability to divide repeatedly, producing more stem cells. This allows for continuous replenishment of the stem cell pool, unlike most other cell types with limited replication capacity. When a stem cell divides, it can produce two new stem cells, one stem cell and one specialized cell, or two specialized cells.
Potency describes a stem cell’s capacity to differentiate, or mature, into various specialized cell types. A single stem cell can give rise to many different kinds of cells, such as heart muscle, blood, or nerve cells, depending on the signals it receives. The degree of potency varies among different stem cell types, influencing the range of cells they can become.
How Stem Cells Grow
Stem cells proliferate and maintain their numbers through specific mechanisms, both within the body and in laboratory settings. In the human body, stem cells reside in specialized areas called niches within various tissues and organs. They remain in a quiescent, or non-dividing, state until activated by the body’s need to replace cells lost to wear, injury, or disease. This natural self-renewal ensures the body’s repair system is continuously supplied with new cells.
Outside the body, scientists grow stem cells in controlled laboratory environments, a process known as cell culture. This involves placing cells in petri dishes or specialized flasks containing a liquid culture medium. The medium is formulated with nutrients, salts, and growth factors that signal stem cells to proliferate while maintaining their undifferentiated state. To further support growth and prevent unwanted differentiation, some stem cells are grown on a layer of “feeder cells,” often inactivated mouse embryonic fibroblasts, which provide additional support.
Advanced laboratory techniques also employ feeder-free culture systems. Here, the feeder layer is replaced by an optimized extracellular matrix and a finely tuned culture medium. These systems balance factors that promote “stemness” (the ability to self-renew and remain undifferentiated) and inhibit spontaneous differentiation. Such controlled conditions are important for generating large quantities of stem cells for research and potential therapeutic applications, as maintaining their undifferentiated state during expansion is necessary for their use.
Types of Stem Cells and Their Growth Characteristics
Different categories of stem cells exist, each with distinct origins, potencies, and growth characteristics.
Embryonic Stem Cells (ESCs)
Embryonic stem cells (ESCs) originate from the inner cell mass of a blastocyst, an early-stage embryo. These cells are pluripotent, meaning they can differentiate into almost any cell type of the adult body, but cannot form an entire organism independently. When cultured in a lab, ESCs are capable of indefinite self-renewal while retaining their pluripotency.
Adult Stem Cells
Adult, or tissue-specific, stem cells are found in small numbers within various mature tissues throughout the body, such as bone marrow, fat, brain, and skin. These stem cells are multipotent, meaning they can differentiate into multiple specialized cell types within their specific tissue of origin. For example, hematopoietic stem cells in bone marrow can give rise to all types of blood cells. Adult stem cells exhibit a slower growth rate in culture compared to ESCs and have a limited capacity for proliferation outside the body.
Induced Pluripotent Stem Cells (iPSCs)
Induced pluripotent stem cells (iPSCs) are adult cells genetically reprogrammed in the laboratory to acquire characteristics similar to embryonic stem cells. This technique involves introducing specific genes into mature cells, such as skin fibroblasts, to revert them to a pluripotent state. The growth characteristics of iPSCs in culture are comparable to ESCs, allowing for indefinite self-renewal and the potential to differentiate into virtually any cell type. iPSCs avoid the ethical considerations associated with embryonic stem cells and reduce the risk of immune rejection in patient-specific therapies.
Harnessing Stem Cell Growth for Practical Uses
Understanding and controlling stem cell growth is important for advancements in medicine and research.
Regenerative Medicine
In regenerative medicine, growing stem cells provides a renewable source for replacing damaged or diseased tissues. Stem cells can be expanded in the lab and guided to differentiate into specific cell types, such as heart muscle or nerve cells. These can then be implanted into a patient to repair injured organs. This approach holds promise for treating conditions like heart disease or neurodegenerative disorders, where damaged cells cannot naturally regenerate.
Disease Modeling
Stem cells are also useful for disease modeling, allowing scientists to create human-relevant tissue models in a laboratory setting. By growing stem cells from patients with specific diseases, researchers can generate disease-specific tissues, like neurons from an Alzheimer’s patient. This allows study of how the disease progresses at a cellular level. These models provide a more accurate representation of human biology compared to traditional animal models or immortalized cell lines, offering insights into complex conditions and supporting the identification of therapeutic targets.
Drug Discovery and Testing
The controlled growth of stem cells is also used for drug discovery and testing. Pharmaceutical companies can grow large quantities of human cells or tissues derived from stem cells to screen new drug compounds for efficacy and potential toxicity. This allows for early assessment of how drugs interact with human cells before human trials, potentially reducing drug development time and cost. Using patient-specific induced pluripotent stem cells (iPSCs) for drug testing also enables personalized medicine approaches, where drugs can be evaluated on cells from individual patients to predict their response.