By Melissa Mayer | Updated Aug 30, 2022

Researchers worldwide are unlocking the secrets of stem cells—tiny, versatile cells that hold the promise of turning a single cell into organs and tissues. Understanding their structure and behavior could revolutionize treatment for a wide range of diseases.

What Are Stem Cells?

During early embryonic development, a fertilized egg (zygote) divides into a swarm of cells known as stem cells. These cells are both proliferative—they divide rapidly—and pluripotent, meaning they can differentiate into any specialized cell type.

As the embryo matures, it transitions from a simple sheet of stem cells into a structured embryo called a gastrula, comprising three germ layers—ectoderm, mesoderm, and endoderm—each giving rise to distinct tissues and organs.

Core Structure of a Stem Cell

• Cell membrane—a lipid bilayer that regulates the passage of molecules.
• Cytoplasm—the aqueous interior that houses organelles.
• Nucleus—contains the DNA that dictates cell function.

While these components are common to all cells, stem cells uniquely maintain an undifferentiated state until specific signals trigger specialization.

Embryonic Stem Cells (hESCs)

hESCs are derived from surplus embryos created via in‑vitro fertilization (IVF). Because they originate before implantation, they are a blank slate capable of giving rise to any cell type.

In culture, hESCs tend to form clusters called embryoid bodies and spontaneously differentiate unless kept in carefully controlled conditions. Maintaining an undifferentiated state over six months and ensuring genetic stability are prerequisites for creating an embryonic stem cell line suitable for research.

Adult (Somatic) Stem Cells

Many mature tissues retain resident stem cells that replenish cells during normal turnover or repair after injury. These cells are tissue‑specific, typically generating only the cell types found in their native environment.

• Hematopoietic stem cells in bone marrow produce blood and immune cells.
• Mesenchymal stem cells give rise to bone, cartilage, fat, and stromal cells.
• Epithelial stem cells in the gut line produce absorptive, goblet, enteroendocrine, and Paneth cells.
• Skin stem cells in the basal layer generate keratinocytes that form the protective epidermis.

Adult stem cells are scarce, limited in division potential, and challenging to culture, yet they offer the advantage of reduced immune rejection when harvested autologously.

Induced Pluripotent Stem Cells (iPSCs)

Introduced in 2006, iPSCs are reprogrammed adult cells that acquire pluripotency. While they share many features with hESCs, technical hurdles—such as viral delivery methods that raise safety concerns—must be addressed before clinical use.

Clinical Applications

Stem cell research underpins drug screening, disease modeling, and the development of cell‑based therapies. Promising areas include:

• Cardiovascular disease—differentiating heart muscle cells for transplantation.
• Type 1 diabetes—producing insulin‑secreting beta cells.
• Burns and macular degeneration—restoring damaged tissues.
• Osteoarthritis and rheumatoid arthritis—regenerating joint cartilage.
• Spinal cord injury and stroke—promoting neural repair.

Key Hurdles to Clinical Translation

• Scaling up stem cell production to generate functional tissues or organs.
• Precisely directing differentiation to the desired cell type.
• Ensuring survival, integration, and long‑term function of transplanted cells.
• Preventing adverse outcomes such as tumorigenesis.

Although the journey from bench to bedside is complex, the foundational knowledge of stem cell biology continues to accelerate advances in regenerative medicine.