US scientists have successfully grown new, functioning blood vessels in mice by implanting them with progenitor cells extracted from human blood and bone marrow. They said this is an important step in developing new treatments for growing tissue.

The study was the work of researchers at the Vascular Biology Program and Department of Surgery, Children's Hospital Boston, Massachusetts, and Division of Cardiology, Beth Israel Deaconess Medical Center, Harvard Medical School, also in Boston, and is published in the online before print June 12th issue of the American Heart Association journal Circulation Research.

Senior author and associate professor at Harvard Medical School and Children's Hospital Boston, Dr Joyce Bischoff, said the significance of the study was that:

"We are using human cells that can be obtained from blood or bone marrow rather than removing and using fully developed blood vessels."

Progenitor cells are like stem cells in that they are "master cells", but they can only differentiate into a limited type, whereas stem cells can differentiate into almost any type of cell in the body.

The researchers extracted two types of progenitor cells: endothelial progenitor cells (EPCs), which turn into cells that line blood vessels, and mesenchymal progenitor cells (MPCs), which turn into cells that surround the lining and make it stable.

They first cultured them in dishes with nutrients and growth factors, then washed off the nutrients and implanted the cells in mice with weakened immune systems (so they would not reject them). The implanted cells grew and differentiated into a small ball of healthy blood vessels, said the researchers in a press statement.

Moreover, the researchers found that: The cells made a vigorous network of vessels that connected to each other and the host mouse vessels within 7 days of implantation.
The new vessels were still transporting blood after four weeks.
The combined progenitor cells differentiated and grew into healthy blood vessels with minimal intervention, eg no genetic manipulation to control growth (this is important because many growth promoting genes also trigger cancer).
A mixture of progenitor cells taken from adult blood and bone marrow was as effective as a mixture taken from umbilical cord blood and adult bone marrow in producing a dense formation of new blood vessels. The researchers concluded that:

"This rapid formation of long-lasting microvascular networks by postnatal progenitor cells obtained from noninvasive sources constitutes an important step forward in the development of clinical strategies for tissue vascularization [formation of blood vessels]."

Bischoff said that the ability to grow two-layered blood vessels without using embryonic or umbilical cord stem cells could bypass many of the ethical worries about using stem cells.

One application would be to treat medical conditions that results from ischemia, where blood carrying oxygen fails to reach an organ or tissue, such as in a heart attack, healing wounds and other injuries. As Bischoff explained:

"What we are most interested in right now is speeding up the vascularization."

"We see very good and extensive vasculature in seven days and we'd like to see that in 24 or 48 hours. If you have an ischemic tissue, it's dying tissue, so the faster you can establish blood flow the better," she added.

Another application migth be non-surgical cardiac bypasses that grow new blood vessels around those blocked or constricted by atherosclerosis.

Another field that needs rapid formation of new blood vessels is tissue engineering, the growing of new organs for transplant.

"Engineering Robust and Functional Vascular Networks In Vivo With Human Adult and Cord Blood-Derived Progenitor Cells."
Juan M. Melero-Martin, Maria E. De Obaldia, Soo-Young Kang, Zia A. Khan, Lei Yuan, Peter Oettgen, Joyce Bischoff
Circulation Research. 2008;103:194-202
Published online before print June 12, 2008
doi: 10.1161/CIRCRESAHA.108.178590

Click here for Abstract.

Sources: Journal abstract and AHA press statement.

: Catharine Paddock, PhD

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