tissue engineering
Capturing the Extracellular Matrix in 3D Color
Posted on by Dr. Francis Collins
For experienced and aspiring shutterbugs alike, sometimes the best photo in the bunch turns out to be a practice shot. That’s also occasionally true in the lab when imaging cells and tissues, and it’s the story behind this spectacular image showing the interface of skin and muscle during mammalian development.
Here you see an area of the mouse forelimb located near a bone called the humerus. This particular sample was labeled for laminin, a protein found in the extracellular matrix (ECM) that undergirds cells and tissues to give them mechanical and biochemical support. Computer algorithms were used to convert the original 2D confocal scan into a 3D image, and colorization was added to bring the different layers of tissue into sharper relief.
Skin tissue (bright red and yellow) is located near the top of the image; blood vessels (paler red, orange, and yellow) are in the middle and branching downward; and muscle (green, blue, and purple) makes up the bottom layer.
The image was created by Sarah Lipp, a graduate student in the NIH-supported tissue engineering lab of Sarah Calve. The team focuses on tissue interfaces to better understand the ECM and help devise strategies to engineer musculoskeletal tissues, such as tendon and cartilage.
In February 2020, Lipp was playing around with some new software tools for tissue imaging. Before zeroing in on her main target—the mouse’s myotendinous junction, where muscle transfers its force to tendon, Lipp snapped this practice shot of skin meeting muscle. After processing the practice shot with a color-projecting macro in an image processing tool called Fiji, she immediately liked what she saw.
So, Lipp tweaked the color a bit more and entered the image in the 2020 BioArt Scientific Image & Video Competition, sponsored by the Federation of American Societies for Experimental Biology, Bethesda, MD. Last December, the grad student received the good news that her practice shot had snagged one of the prestigious contest’s top awards.
But she’s not stopping there. Lipp is continuing to pursue her research interests at the University of Colorado, Boulder, where the Calve lab recently moved from Purdue University, West Lafayette, IN. Here’s wishing her a career filled with more great images—and great science!
Links:
Muscle and Bone Diseases (National Institute of Arthritis and Musculoskeletal and Skin Diseases/NIH)
Musculoskeletal Extracellular Matrix Laboratory (University of Colorado, Boulder)
BioArt Scientific Image & Video Competition (Federation of American Societies for Experimental Biology, Bethesda, MD)
NIH Support: National Institute of Arthritis and Musculoskeletal and Skin Diseases
Understanding Neuronal Diversity in the Spinal Cord
Posted on by Dr. Francis Collins
The spinal cord, as a key part of our body’s central nervous system, contains millions of neurons that actively convey sensory and motor (movement) information to and from the brain. Scientists have long sorted these spinal neurons into what they call “cardinal” classes, a classification system based primarily on the developmental origin of each nerve cell. Now, by taking advantage of the power of single-cell genetic analysis, they’re finding that spinal neurons are more diverse than once thought.
This image helps to visualize the story. Each dot represents the nucleus of a spinal neuron in a mouse; humans have a very similar arrangement. Most of these neurons are involved in the regulation of motor control, but they also differ in important ways. Some are involved in local connections (green), such as those that signal outward to a limb and prompt us to pull away reflexively when we touch painful stimuli, such as a hot frying pan. Others are involved in long-range connections (magenta), relaying commands across spinal segments and even upward to the brain. These enable us, for example, to swing our arms while running to help maintain balance.
It turns out that these two types of spinal neurons also have distinctive genetic signatures. That’s why researchers could label them here in different colors and tell them apart. Being able to distinguish more precisely among spinal neurons will prove useful in identifying precisely which ones are affected by a spinal cord injury or neurodegenerative disease, key information in learning to engineer new tissue to heal the damage.
This image comes from a study, published recently in the journal Science, conducted by an NIH-supported team led by Samuel Pfaff, Salk Institute for Biological Studies, La Jolla, CA. Pfaff and his colleagues, including Peter Osseward and Marito Hayashi, realized that the various classes and subtypes of neurons in our spines arose over the course of evolutionary time. They reasoned that the most-primitive original neurons would have gradually evolved subtypes with more specialized and diverse capabilities. They thought they could infer this evolutionary history by looking for conserved and then distinct, specialized gene-expression signatures in the different neural subtypes.
The researchers turned to single-cell RNA sequencing technologies to look for important similarities and differences in the genes expressed in nearly 7,000 mouse spinal neurons. They then used this vast collection of genomic data to group the neurons into closely related clusters, in much the same way that scientists might group related organisms into an evolutionary family tree based on careful study of their DNA.
The first major gene expression pattern they saw divided the spinal neurons into two types: sensory-related and motor-related. This suggested to them that one of the first steps in spinal cord evolution may have been a division of labor of spinal neurons into those two fundamentally important roles.
Further analyses divided the sensory-related neurons into excitatory neurons, which make neurons more likely to fire; and inhibitory neurons, which dampen neural firing. Then, the researchers zoomed in on motor-related neurons and found something unexpected. They discovered the cells fell into two distinct molecular groups based on whether they had long-range or short-range connections in the body. Researches were even more surprised when further study showed that those distinct connectivity signatures were shared across cardinal classes.
All of this means that, while previously scientists had to use many different genetic tags to narrow in on a particular type of neuron, they can now do it with just two: a previously known tag for cardinal class and the newly discovered genetic tag for long-range vs. short-range connections.
Not only is this newfound ability a great boon to basic neuroscientists, it also could prove useful for translational and clinical researchers trying to determine which specific neurons are affected by a spinal injury or disease. Eventually, it may even point the way to strategies for regrowing just the right set of neurons to repair serious neurologic problems. It’s a vivid reminder that fundamental discoveries, such as this one, often can lead to unexpected and important breakthroughs with potential to make a real difference in people’s lives.
Reference:
[1] Conserved genetic signatures parcellate cardinal spinal neuron classes into local and projection subsets. Osseward PJ 2nd, Amin ND, Moore JD, Temple BA, Barriga BK, Bachmann LC, Beltran F Jr, Gullo M, Clark RC, Driscoll SP, Pfaff SL, Hayashi M. Science. 2021 Apr 23;372(6540):385-393.
Links:
What Are the Parts of the Nervous System? (Eunice Kennedy Shriver National Institute of Child Health and Human Development/NIH)
Spinal Cord Injury (National Institute of Neurological Disorders and Stroke/NIH)
Samuel Pfaff (Salk Institute, La Jolla, CA)
NIH Support: National Institute of Mental Health; National Institute of Neurological Disorders and Stroke; Eunice Kennedy Shriver National Institute of Child Health and Human Development
Tackling Fibrosis with Synthetic Materials
Posted on by Dr. Francis Collins
When injury strikes a limb or an organ, our bodies usually heal quickly and correctly. But for some people, the healing process doesn’t shut down properly, leading to excess fibrous tissue, scarring, and potentially life-threatening organ damage.
This permanent scarring, known as fibrosis, can occur in almost every tissue of the body, including the heart and lungs. With support from a 2019 NIH Director’s New Innovator Award, April Kloxin is applying her expertise in materials science and bioengineering to build sophisticated fibrosis-in-a-dish models for unraveling this complex process in her lab at the University of Delaware, Newark.
Though Kloxin is interested in all forms of fibrosis, she’s focusing first on the incurable and often-fatal lung condition called idiopathic pulmonary fibrosis (IPF). This condition, characterized by largely unexplained thickening and stiffening of lung tissue, is diagnosed in about 50,000 people each year in the United States.
IPF remains poorly understood, in part because it often is diagnosed when the disease is already well advanced. Kloxin hopes to turn back the clock and start to understand the disease at an earlier stage, when interventions might be more successful. The key is to develop a model that better recapitulates the complexity and irreversibility of the disease process in people.
Building that better model starts with simulating the meshwork of collagen and other proteins in the extracellular matrix (ECM) that undergird every tissue and organ in the body. The ECM’s interactions with our cells are essential in wound healing and, when things go wrong, also in causing fibrosis.
Kloxin will build three-dimensional hydrogels, crosslinked sponge-like networks of polymers, peptides, and proteins, with structures that more accurately capture the biological complexities of human tissues, including the ECMs within fibrous collagen-rich microenvironments. Her synthetic matrices can be triggered with light to lock in place and stiffen. The matrices also will make it possible to culture the lung’s epithelium, or outermost layer of cells, and connective tissue that surrounds it, to study cellular responses as the model shifts from a healthy and flexible to a stiffened, disease-like state.
Kloxin and her team will also integrate into their model system lung cells that have been engineered to fluoresce or light up under a microscope when the wound-healing program activates. Such fluorescent reporters will allow her team to watch for the first time how different cells and their nearby microenvironment respond as the composition of the ECM changes and stiffens. With this system, she’ll also be able to search for small molecules with the ability to turn off excessive wound healing.
The hope is that what’s learned with her New Innovator Award will lead to fresh insights and ultimately new treatments for this mysterious, hard-to-treat condition. But the benefits could be even more wide-ranging. Kloxin thinks that her findings will have implications for the prevention and treatment of other fibrotic diseases as well.
Links:
Idiopathic Pulmonary Fibrosis (National Heart, Lung, and Blood Institute/NIH)
April Kloxin Group (University of Delaware, Newark)
Kloxin Project Information (NIH RePORTER)
NIH Director’s New Innovator Award (Common Fund)
NIH Support: Common Fund; National Heart, Lung, and Blood Institute
3D Printing a Human Heart Valve
Posted on by Dr. Francis Collins
It is now possible to pull up the design of a guitar on a computer screen and print out its parts on a 3D printer equipped with special metal or plastic “inks.” The same technological ingenuity is also now being applied with bioinks—printable gels containing supportive biomaterials and/or cells—to print out tissue, bone, blood vessels, and, even perhaps one day, viable organs.
While there’s a long way to go until then, a team of researchers has reached an important milestone in bioprinting collagen and other extracellular matrix proteins that undergird every tissue and organ in the body. The researchers have become so adept at it that they now can print biomaterials that mimic the structural, mechanical, and biological properties of real human tissues.
Take a look at the video. It shows a life-size human heart valve that’s been printed with their improved collagen bioink. As fluid passes through the aortic valve in a lab test, its three leaf-like flaps open and close like the real thing. All the while, the soft, flexible valve withstands the intense fluid pressure, which mimics that of blood flowing in and out of a beating heart.
The researchers, led by NIH grantee Adam Feinberg, Carnegie Mellon University, Pittsburgh, PA, did it with their latest version of a 3D bioprinting technique featured on the blog a few years ago. It’s called: Freeform Reversible Embedding of Suspended Hydrogels v.2.0. Or, just FRESH v2.0.
The FRESH system uses a bioink that consists of collagen (or other soft biomaterials) embedded in a thick slurry of gelatin microparticles and water. While a number of technical improvements have been made to FRESH v. 2.0, the big one was getting better at bioprinting collagen.
The secret is to dissolve the collagen bioink in an acid solution. When extruded into a neutral support bath, the change in pH drives the rapid assembly of collagen. The ability to extrude miniscule amounts and move the needle anywhere in 3D space enables them to produce amazingly complex, high-resolution structures, layer by layer. The porous microstructure of the printed collagen also helps for incorporating human cells. When printing is complete, the support bath easily melts away by heating to body temperature.
As described in Science, in addition to the working heart valve, the researchers have printed a small model of a heart ventricle. By combining collagen with cardiac muscle cells, they found they could actually control the organization of muscle tissue within the model heart chamber. The 3D-printed ventricles also showed synchronized muscle contractions, just like you’d expect in a living, beating human heart!
That’s not all. Using MRI images of an adult human heart as a template, the researchers created a complete organ structure including internal valves, large veins, and arteries. Based on the vessels they could see in the MRI, they printed even tinier microvessels and showed that the structure could support blood-like fluid flow.
While the researchers have focused the potential of FRESH v.2.0 printing on a human heart, in principle the technology could be used for many other organ systems. But there are still many challenges to overcome. A major one is the need to generate and incorporate billions of human cells, as would be needed to produce a transplantable human heart or other organ.
Feinberg reports more immediate applications of the technology on the horizon, however. His team is working to apply FRESH v.2.0 for producing child-sized replacement tracheas and precisely printed scaffolds for healing wounded muscle tissue.
Meanwhile, the Feinberg lab generously shares its designs with the scientific community via the NIH 3D Print Exchange. This innovative program is helping to bring more 3D scientific models online and advance the field of bioprinting. So we can expect to read about many more exciting milestones like this one from the Feinberg lab.
Reference:
[1] 3D bioprinting of collagen to rebuild components of the human heart. Lee A, Hudson AR, Shiwarski DJ, Tashman JW, Hinton TJ, Yerneni S, Bliley JM, Campbell PG, Feinberg AW. Science. 2019 Aug 2;365(6452):482-487.
Links:
Tissue Engineering and Regenerative Medicine (National Institute of Biomedical Imaging and Bioengineering/NIH)
Regenerative Biomaterials and Therapeutics Group (Carnegie Mellon University, Pittsburgh, PA)
FluidForm (Acton, MA)
3D Bioprinting Open Source Workshops (Carnegie Mellon)
Video: Adam Feinberg on Tissue Engineering to Treat Human Disease (YouTube)
NIH Support: National Heart, Lung, and Blood Institute; Eunice Kennedy Shriver National Institute of Child Health and Human Development; Common Fund
Progress Toward 3D Printed Human Organs
Posted on by Dr. Francis Collins
There’s considerable excitement that 3D printing technology might one day allow scientists to produce fully functional replacement organs from one’s own cells. While there’s still a lot to learn, this video shows just some of the amazing progress that’s now being made.
The video comes from a bioengineering team at Rice University, Houston, that has learned to bioprint the small air sacs in the lungs. When hooked up to a machine that pulsed air in and out of the air sacs, the rhythmic movement helped to mix red blood cells traveling through an associated blood vessel network. Those red cells also took up oxygen in much the way that blood vessels do when surrounding the hundreds of millions of air sacs in our lungs.
As mentioned in the video, one of the biggest technical hurdles in growing fully functional replacement tissues and organs is to find a way to feed the growing tissues with a blood supply and to remove waste products. In this study recently published in Science [1], the NIH-supported team cleared this hurdle by creating an open-source bioprinting technology they call SLATE, which is short for “stereo-lithography apparatus for tissue engineering.”
The SLATE system “grows” soft hydrogel scaffolds one layer at a time. Each layer is printed using a liquid pre-hydrogel solution that solidifies when exposed to blue light. By also projecting light into the hydrogel as a pixelated 3D shape, it’s possible to print complex 3D structures within minutes.
When the researchers first started, their printouts lacked the high resolution, submillimeter-scale channels needed to generate intricate vascular networks. In other manufacturing arenas, light-absorbing chemicals have helped control the conversion from liquid to solid in a very fine polymer layer. But these industrial light-absorbing chemicals are highly toxic and therefore unsuitable for scaffolds that grow living tissues and organs.
The researchers, including Bagrat Grigoryan, Jordan Miller, and Kelly Stevens, wondered whether they could swap out those noxious ingredients with synthetic and natural food dyes widely used in the food industry. These dyes include curcumin, anthocyanin, and tartrazine (yellow dye #5). Their studies showed that those fully biocompatible dyes worked as effective light absorbers, allowing the scientists to recreate the complex architectures of human vasculature. Importantly, the living cells survived within the soft scaffold!
These models are already yielding intriguing new insights into the vascular structures found within our organs and how those architectures may influence function in ways that hadn’t been well understood. In the near term, tissues and organs grown on such scaffolds might also find use as sophisticated, 3D tissue “chips,” with potential for use in studies to predict whether drugs will be safe in humans.
In the long term, this technology may allow production of replacement organs from those needing them. More than 100,000 men, women, and children are on the national transplant waiting list in the United States alone and 20 people die each day waiting for a transplant [2]. Ultimately, with the aid of bioprinting advances like this one, perhaps one day we’ll have a ready supply of perfectly matched and fully functional organs.
References:
[1] Multivascular networks and functional intravascular topologies within biocompatible hydrogels. Grigoryan B, Paulsen SJ, Corbett DC, Sazer DW, Fortin CL, Zaita AJ, Greenfield PT, Calafat NJ, Gounley JP, Ta AH, Johansson F, Randles A, Rosenkrantz JE, Louis-Rosenberg JD, Galie PA, Stevens KR, Miller JS. Science. 2019 May 3;364(6439):458-464.
[2] Organ Donor Statistics, Health Resources & Services Administration, October 2018.
Links:
Tissue Engineering and Regenerative Medicine (National Institute of Biomedical Imaging and Bioengineering/NIH)
Tissue Chip for Drug Screening (National Center for Advancing Translational Sciences/NIH)
Miller Lab (Rice University, Houston)
NIH Support: National Heart, Lung, and Blood Institute; National Institute of Biomedical Imaging and Bioengineering; National Institute of General Medical Sciences; Common Fund
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