What Are Stem Cells?

What is a stem cell?

Our bodies are made of many types of cells, each with a specific purpose. Red blood cells carry oxygen, nerve cells create electrical pulses, liver cells break down fat, etc.

Early on in development, however, cells haven’t been assigned a “purpose” yet. These are stem cells.

Stem cells are the building blocks of the body. They help construct our tissues, organs, blood, and immune system as we develop and later act as a repair system to replenish lost or dying cells.


What are the different types of stem cells?

There are two major types of stem cells:

1. Adult Stem Cells

Adult stem cells replenish the organs they inhabit. For example, adult stem cells in the liver can only become new liver cells and adult stem cells in the brain can only become new brain cells. This turnover in cells ensures that our organs stay healthy and functional. 

One important type of adult stem cell is cord blood stem cells. Blood from the umbilical cord is a rich source of stem cells and can be harvested and banked after childbirth. Cord blood stem cells can be used to form blood and bone marrow cells, but not skin or brain cells.

More information about cord blood banking is located at the bottom of this page.

2. Pluripotent Stem Cells

While an adult stem cell is limited in what it can become, a pluripotent stem cell can become any cell type in the body. There are two types of pluripotent stem cells:


  • Embryonic stem cells (ESCs)

ESCs are a type of pluripotent stem cell found in early-stage embryos called blastocysts. These blastocysts are extremely small—only the size of the tip of a pin.

Blastocysts used for research are obtained from leftover embryos at in vitro fertilization clinics (with donor consent), or they are created through a process called somatic cell nuclear transfer (SCNT). In SCNT, the nucleus of an egg cell is removed and replaced with the nucleus of an adult cell, like a skin or blood cell. The egg cell then divides until it forms a blastocyst.

Human ESCs (hESCs) were first isolated in 1998 by Dr. Jamie Thompson and his colleagues at the University of Wisconsin. Since this time, hESCs have been considered the “gold-standard” of stem cells.


  • Induced pluripotent stem cells (iPSCs)

We can take a small sample of skin or blood cells and then revert these cells back into pluripotent stem cells called iPSCs. iPSCs can then be turned into all the cell types in the human body.

Each time a stem cell divides, it can either become another stem cell or take on a more specialized function. When it takes on a new function, we say that the cell has “differentiated”. In the lab, we can control the differentiation process and choose which cell type the stem cell will become. In doing this, we can create reserves of the actual human cells affected by a disease and use them for research or engineer healthy cells for treatment.

iPSCs were first discovered in 2006 by Dr. Shinya Yamanaka and Dr. Kazutoshi Takahasi, 2012 NYSCF Robertson Stem Cell Prize recipient, and completely revolutionized the field of stem cell research, leading to a Nobel Prize awarded in 2012. This new discovery allowed scientists to make a personalized stem cell line from any individual in the world. 

What can we do with pluripotent stem cells?

We use iPSCs and ESCs for several purposes:

1. Studying diseases in the laboratory

It’s hard to study human brain cells because we can’t take them out of a living brain to examine in the laboratory. But with stem cells, we can generate brain cells from patients with neurodegenerative diseases and study how they behave. This gives us a “living window” into the disease, helping us understand what factors trigger its onset. We can do this for any cell type implicated in a disease (such as pancreatic beta cells for diabetes or blood-forming cells for leukemia).

2. Finding new drugs for patients

Once we generate the affected cells, we can use these cells to rapidly and efficiently test drug efficacy and potential toxicity, identifying the safest and most promising compounds to advance to clinical trials. Currently and without stem cells, scientists test drugs on animal cells or human cells that are not affected by the disease, often times not providing an accurate readout of the effect of the drug.

3. Replacing diseased or damaged cells

Many diseases result from a dysfunction or loss of cells in a certain part of the body. Using stem cells, we can generate the cell types implicated in a disease, correct for their dysfunction, and use these new cells to replace the damaged or diseased ones. And since the replacement cells can be generated from the patients themselves, they can easily integrate into the patient’s body, reducing the need for immunosuppressant drugs and the chances that they will be attacked by the immune system.


Additional Cord Blood Information

What is Cord Blood Banking?

Cord blood banking is a procedure in which cord blood — a rich source of stem cells — is taken from a newborn’s umbilical cord after delivery and used for research or preserved for possible use in a stem cell transplant. Cord blood is a biological product regulated by the Food and Drug Administration.

How is cord blood used for therapies?

Cord blood is approved only for use in “hematopoietic stem cell transplantation” procedures, which are done in patients with disorders affecting the hematopoietic (blood forming) system. Cord blood contains blood-forming stem cells that can be used in the treatment of patients with blood cancers such as leukemias and lymphomas, as well as certain disorders of the blood and immune systems, such as sickle cell disease and Wiskott-Aldrich syndrome.