On the surface, plants and animals couldn't be more different. However, as researchers at Worcester Polytechnic University (WPU) have discovered, they also share surprising similarities in their vascular network structures. So what if there was a way to take advantage of these similarities to grow human cardiac tissue using leaves? The WPU researchers have done just that.

For all the promise that 3D printing has shown in the field of tissue engineering, lab-made organs remain a breakthrough in theory due to the fact that they can't replicate the cellular transport system needed to support tissue growth. This is a problem as the demand for transplant organs and tissues far exceeds the supply.

As radical as it might seem, plants could offer a solution despite their fundamentally different approaches to transporting oxygen, nutrients, and essential molecules required for tissue growth. For starters, their tapered, branching network design resembles the one found in the human cardiovascular system and they also have tissue structures that enable varied functions.

Furthermore, cellulose, one of the main components of plant cell wall tissue, is not only biocompatible, but has also been used in a wide variety of regenerative medicine applications, such as cartilage tissue engineering, bone tissue engineering, and wound healing, say the study authors. Given these factors, why not investigate whether plants and could be used to establish a vascular system that delivers blood to lab-grown tissues?

"Our hope is that we'll be able to use the vascular system in the spinach leaf to provide the cells that are growing on the leaf with nutrients and oxygen," says study author Glenn Gaudette, a professor of biomedical engineering at WPU, adding that the end goal would be to use this in cardiac applications to help heart attack victims. "What we're hoping to do is grow cardiac muscle on these leaves, which can then be perfused with a blood source by the veins that are inside the leaf so we can, in theory, sew those veins into those native arteries in the heart and therefore produce a contractile muscle that can replace the dead tissue."

However, before this can be attempted, the WPU team had to find out whether cardiac cells could adhere to and contract on the leaves of the plant.

Owing to its abundance, vascular network pattern and density, as well as the wide diameter of its stalk, the researchers decided to use spinach leaves as the model species for their study. In order to grow living human heart cells on leaves, they had to first strip them off their plant cells in a process known as decellulerization. This was done by flowing, or "perfusing," a detergent solution through the leaves' veins.

"I had done decellularization work on human hearts before and when I looked at the spinach leaf its stem reminded me of an aorta," explains first author Joshua Gershlak. "So I thought, let's perfuse right through the stem. We weren't sure it would work, but it turned out to be pretty easy and replicable."

To make sure that the vascular network of the decellularized spinach leaf could support the flow of human blood cells, the researchers flowed fluids and microbeads similar in size to such cells through the spinach vasculature before seeding its veins with human cells that line blood vessels. The result: the human cells were able to attach themselves to the leaf and stay contractile for 21 days. During this time, they looked and acted just like cardiac cells, says Gershlak.

During their experiments, the researchers were also able to successfully decellularize other plants, which might pave the way for other specialized tissue regeneration studies.

"The spinach leaf might be better suited for a highly vascularized tissue, like cardiac tissue, whereas the cylindrical hollow structure of the stem of Impatiens capensis (jewelweed) might better suit an arterial graft," note the authors. "Conversely, the vascular columns of wood might be useful in bone engineering due to their relative strength and geometries."

That said, while these proof-of-concept experiments were a success, the researchers acknowledge that the results are "still a long ways away" from being clinically relevant. As they note in the study, it is as yet unclear how they would integrate the plant vasculature into the native human vasculature. And while decellularized cellulose is biocompatible and biodegradable, the body's response to whole decellularized plant tissue remains unknown. Additionally, while the cells were able to adhere themselves to the inner leaf vasculature after 24 hours, full working endothelialization (i.e. the formation of the one-cell thick lining found inside blood vessels and the heart chamber) will need to be demonstrated.

Nevertheless, this proof-of-concept study is promising on several fronts, not least economically and environmentally.

"By exploiting the benign chemistry of plant tissue scaffolds we could address the many limitations and high costs of synthetic, complex composite materials," the researchers say."Plants can be easily grown using good agricultural practices and under controlled environments. By combining environmentally friendly plant tissue with perfusion-based decellularization, we have shown that there can be a sustainable solution for pre-vascularized tissue engineering scaffolds."

The study was conducted in collaboration with the University of Wisconsin-Madison, and Arkansas State University-Jonesboro and is published in Biomaterials.

Gaudette and Gershlak discuss their work in the video below.

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