Stem cells make new heart valves

Harvard researchers have coaxed adult stem cells
into forming artificial heart valves that could one day mean fewer
surgeries for children suffering from heart defects.

The scientists, at Harvard-affiliated Boston Children’s Hospital,
grew the valves from a type of stem cell that normally gives rise to
the inner lining of blood vessels. They used a biodegradable scaffold
to give the cells shape and a mix of proteins and growth factors to
stimulate the cells to grow into the proper tissue type.

Harvard Medical School Instructor in Medicine Virna Sales, a
researcher in Boston Children’s Department of Cardiac Surgery, said the
work, which used the tissue of laboratory animals, builds on research
conducted over the past decade by HMS Professor of Surgery John Mayer.

Mayer, also in Boston Children’s Department of Cardiac Surgery, was
the senior investigator of the study. His research over the past 10
years has focused on helping the 25,000 to 30,000 children born each
year with heart defects.

Today, Sales said, the defects are corrected using prosthetics made
from artificial materials and tissue from pigs. The problem with those
materials, she said, is that they don’t grow. Depending on the defect
type and the age of the child at the time of surgery, the child may
(and often does) need additional surgeries as his or her heart gets
larger.

Researchers hope to be able to screen the blood-vessel stem cells,
called endothelial progenitor cells, from a child’s blood and use them
to make valves out of a child’s own tissue. Since the valves would be
made from living tissue, they would grow, sparing the child additional
surgery.

The research, published in the journal Circulation, is still five to 10 years away from being used in humans, Sales said.

The body has two major types of stem cells. Embryonic stem cells are
present at the very early stages of life and have the ability to change
into any type of cell in the body. More limited are adult stem cells,
which give rise to different cells within a particular tissue type.
Blood stem cells, for example, are found in the bone marrow and are
constantly growing and differentiating, replacing cells in the
bloodstream as they are needed.

Prior research has sought both the right cells to start with and the
proper mix of proteins and growth factors to coax the cells to
differentiate into the right kind of tissue.

The body uses these growth factors and proteins to signal stem cells to divide, grow, and change into the type of tissue needed.

Though the natural process may seem straightforward, the mix of
factors, including dosage, and timing, are delicately balanced in
nature and provide researchers a significant challenge to reproduce.

As researchers learn to fine-tune the process, Sales said, they can
use multiple coatings on the scaffold to expose the tissues to
different factors at different times. That way, she said, it may be
possible to “customize” one’s heart valve.

Now that they’ve successfully grown the valves from animal tissue,
researchers are working to refine the technology. They’re testing the
mechanical properties of the engineered valves in a bioreactor, a
device designed to mimic the never-ending motion of the heart. They’re
also experimenting with a different mix of the growth factors and
proteins to see if they can get the valves to grow more naturally,
without the supporting scaffold.

“I’m going back to the basics,” Sales said, “[We’re] going to mimic mother nature.”