Using Silk to Grow Salivary Glands in the Laboratory

Colloquially, we use the word “spit” to describe saliva, which is secreted by our salivary glands. Saliva is a complex combination of water, salts, proteins, small molecules, and other components that lubricate our throats and mouths to facilitate swallowing, coat our gums and teeth to maintain good gum health and keep tooth decay at bay, and keep our breath fresh. Insufficient salivation production increases tooth decay rates, causes chronic halitosis (bad breath), and can swallowing difficult. Additionally, there are no treatments for poorly-functioning salivary glands, and these glands have poor regenerative capabilities.

Patients who suffer from head/neck cancers and have been treated with radiation suffer from “xerostoma” or dry mouth. Certain medications can also cause dry mouth as can old age. 50% of older Americans suffer from xerostoma.

If that isn’t bad enough, salivary glands are notoriously hard to grow in the laboratory.  So they slow down when we grow old, do not regenerate and grow poorly, in at all, in the laboratory.  Is there any good news about salivary glands?

Make that a yes!  A research team from the University of Texas Health Science Center, led by Chih-Ko Yeh has discovered a process that may lead to the growth of salivary glands in cell culture.  Yeh and his team used purified silk fibers that had many of their contaminants removed to grow salivary stem cells from rat salivary glands.  These cells grew in the laboratory and after several weeks in culture, generated a three-dimensional matrix that covered the silk scaffolds and shared many characteristics with the salivary glands that grow in the mouth.

Yeh underscored the importance of this discovery: “Salivary gland stem cells are some of the most difficult cells to grow in culture and retain their function.”  This work in Yeh’s laboratory have is the first time that salivary gland stem cells have been grown in cell culture while retaining their salivary gland properties.

Yeh continued, “The unique culture system has great potential for future salivary gland research and for the development of new cell-based therapeutics.”

Silk, contrary to what you might think, is an excellent choice for stem cell scaffolding because it is natural, biodegradable, flexible, porous material that provides cells easy access to oxygen and nutrition.  Silk also does not cause inflammation, which is a problem with other types of stem cell scaffolds.

Since there are so few salivary gland stem cells in the human mouth, Yeh and his group plan to continue using the rat model to refine their techniques.  Eventually, Yeh and others would like to use stem cells derived from bone marrow or umbilical cord blood to regenerate salivary glands in human patients.

In fact, Yeh and his coworkers have pioneered protocols for harvesting large numbers of bone marrow stem cells from bone marrow and human umbilical cord blood and growing them in culture.  These stem cells are abundant and can be differentiated into different cell types by means of tissue engineering technologies.

Yeh hopes that by the next decade, human salivary stem cells or tissue engineered artificial salivary gland will be used to initiate salivary gland regeneration in human patients.

This research was published in Tissue Engineering part A 2015; 21(9-10).

Silk and Cellulose as Scaffolds for Stem Cell-Mediated Cartilage Repair

When two bones come together, they grind each other into oblivion. This results in inflammation, joint swelling and pain, and scar tissue accumulation, which eventually results in the immobilization of the joint. To prevent this, bone are capped at their ends with a layer of hyaline cartilage that acts as a shock absorber. However, cartilage regenerates poorly and the wear and tear on cartilage, particularly at the knee, causes it to degenerate. The loss of the cartilage cap at the end of long bones causes osteoarthritis . The only way to mitigate the damage of osteoarthritis is to replace the knee with a prosthetic knee-joint.

Stem cells can make a significant contribution to the regeneration of lost cartilage. The Centeno/Schultz group near Denver, Colorado has been using bone marrow-derived mesenchymal stem cells to treat patients for over a decade with positive results. However, finding a way to grow large amounts of cartilage in culture that is the right shape for transplantation has proven difficult.

One way to mitigate this issue is the use of scaffolds for the cartilage-making cells that pushes them into a three-dimensional arrangement that forces them to make cartilage that mimics the cartilage found in a living organism. However a problem with scaffolds is finding the right material for the scaffold.

A recent publication has formed scaffolds from naturally occurring fibers such as cellulose and silk. By blending silk and cellulose fibers together, researchers at the University of Bristol have made a very inexpensive and easily manufactured scaffold for cartilage production.

Silk scaffold
Silk scaffold

When mixed with stem cells, cartilage and silk coax connective tissue-derived stem cells to differentiate into chondrocytes or cartilage-making cells. In the silk/cellulose scaffold, the chondrocytes secrete the extracellular matrix molecules characteristic of joint-specific cartilage.

Wael Kafienah, lead author of this work from the University of Bristol’s School of Cellular and Molecular Medicine, said, “The blend seems to provide complex chemical and mechanical cues that induce stem cell differentiation into preliminary form of chondrocytes without need for biochemical induction using expensive soluble differentiation factors. Kafienah continued: “This new blend can cut the cost for health providers and makes progress towards effective cell-based therapy for cartilage repair a step closer.”

To make the blended silk/cellulose scaffolds, Kafienah and his colleagues used ionic fluids, which effectively dissolve polymers like cellulose and silk, but are also much more environmentally benign in comparison to the organic solvents normally used to process silk and cellulose.

Presently, the U of Bristol team to trying to fabricate three-dimensional scaffolds that can be safely and easily implanted into patients for future clinical studies. Before human clinical studies are commenced, however, they must first be extensively tested in animals and also, the nature of the interactions between the scaffold and the stem cells that drive the cells to form cartilage must be better understood.