Salivary glands are one of many small organs that we give little thought to until they fail, and then it becomes difficult to think of anything else. Just like every other tissue in the aging body, that failure becomes more likely with each passing year, with the accumulation of molecular damage and its consequences. One of the potential approaches to this general category of gradual organ failure is the generation of new organs or new functional tissue for transplantation, building tissues in bioreactors from the starting point of cells. This can in principle fix damage that is internal to an organ by replacing that organ entirely, or augment function of a failing organ with the use of tissue patches. The aged environment and its harmful influence on organ function through signaling will remain a challenge, however, until more general rejuvenation therapies are widely deployed.
Japanese researchers have been working on the tissue engineering of functional salivary glands for some years now, and the paper noted below reports on their latest success. Like most groups in the field, they are focused on discovering the necessary signals and environment that can direct cells to build a specific tissue in the same way that occurs during embryonic development. This is quite different on a tissue by tissue basis, but nonetheless progress is being made. The researchers here can build organoids, small sections of functional salivary gland tissue that are limited in size because they lack a capillary network. An important demonstration of functionality is to implant organoids into an animal and show that they integrate and perform the tasks expected of the naturally grown organ. That rarely implies complete success, as the assessed function usually isn't exactly the same, but nonetheless, it may indicate that the research program has progressed far enough to start thinking about use in human medicine.
Salivary glands develop from an early structure called the oral ectoderm, but the actual process is not fully understood. It is known that organ development takes place through a complex process of chemical signaling and changes in gene expression, so the scientists began to unravel what the important changes were. They identified two transcription factors - Sox9 and Foxc1 - as being key to the differentiation of stem cells into salivary gland tissue, and also identified a pair of signaling chemicals - FGF7 and FGF10 - which induced cells expressing those transcription factors to differentiate into salivary gland tissue.
To create an organoid, researchers used a cocktail of chemicals that allowed the formation of the oral ectoderm. They used this cocktail to induce embryonic stem cells to form the ectoderm, and then used viral vectors to get the cells to express both Sox9 and Foxc1. Adding the two chemicals to the mix induced the cells to form tissue that genetic analysis revealed was very similar to actual developing salivary glands in the embryo.
The final step was to see if the organoid would actually function in a real animal. They implanted the organoids into actual mice without saliva glands and tested them by feeding them citric acid. When the organoids were transplanted along with mesenchymal tissue -another embryonic tissue that is important as it forms the connecting tissue that allows the glands to attach to other tissues - the implanted tissues were found to be properly connected to the nerve tissue, and in response to the stimulation secreted a substance that was remarkably similar to real saliva.
Organoids generated from pluripotent stem cells are used in the development of organ replacement regenerative therapy by recapitulating the process of organogenesis. These processes are strictly regulated by morphogen signalling and transcriptional networks. However, the precise transcription factors involved in the organogenesis of exocrine glands, including salivary glands, remain unknown. Here, we identify a specific combination of two transcription factors (Sox9 and Foxc1) responsible for the differentiation of mouse embryonic stem cell-derived oral ectoderm into the salivary gland rudiment in an organoid culture system.
Following orthotopic transplantation into mice whose salivary glands had been removed, the induced salivary gland rudiment not only showed a similar morphology and gene expression profile to those of the embryonic salivary gland rudiment of normal mice but also exhibited characteristics of mature salivary glands, including saliva secretion. This study suggests that exocrine glands can be induced from pluripotent stem cells for organ replacement regenerative therapy.