Pathophysiology of radiation damage
Post-radiotherapy xerostomia is the consequence of damage that selectively affects serous acinar cells, with loss of aquaporin 5 expression, depletion of the progenitor compartment and alteration of the niche that supports them. To this are added interstitial fibrosis, reduced vascularization and dysfunction of parasympathetic innervation. The clinical picture, which includes dysphagia, radiation caries, candidiasis and a marked decline in quality of life, responds only partially to pilocarpine, saliva substitutes and the gland-sparing techniques now adopted in intensity-modulated radiotherapy.
For this reason, regenerating the tissue, and not merely stimulating the functioning remnant, is the goal of greatest interest in translational research.
Radiation-induced glandular damage also has a well-known dose-dependent component. The parotid glands, whose secretion is predominantly serous, are more radiosensitive than the submandibular glands, and mean doses above roughly 25-30 Gy are associated with a functional loss that tends not to recover. This difference in sensitivity shapes the strategy: residual tissue can sometimes be stimulated, whereas where the loss is extensive the only option for genuine restoration is the replacement of the secretory compartment.
The role of the matrix in gland biology
The branching morphogenesis of the salivary glands depends substantially on the microenvironment, with FGF10 gradients, interactions with the basement membrane and an adequate substrate stiffness. Silk fibroin hydrogels crosslinked enzymatically, for example with horseradish peroxidase and hydrogen peroxide, make it possible to tune elastic modulus and porosity independently, parameters that directly influence acinus formation and apico-basal polarization. They are also compatible with functionalization with adhesion peptides and with the controlled release of trophic factors.
A further possibility is the use of electrospun scaffolds that mimic the alignment of the ducts, in combination with organoids derived from salivary stem cells. The main challenge is to preserve secretory function, beyond morphology, in a three-dimensional system that can also be innervated.
The choice of the cell to be delivered is a crucial issue. Salivary stem cells isolated as salispheres, c-Kit-positive progenitor cells, mesenchymal stem cells and pluripotent-derived cells have different differentiation and safety profiles. A fibroin matrix can host several populations in distinct compartments, with a stromal component that sustains paracrine signals and an epithelial one that organizes the acinar structure, partly reconstructing the architecture of the native gland.
Functional endpoints and evaluation
Histology alone is not enough. Meaningful assessments include measuring salivary flow stimulated by cholinergic agonists, amylase activity, the expression of AQP5 and junctional proteins, the presence of innervation, and comparison with untreated irradiated tissue. In murine models of local irradiation, functional recovery has been observed to be associated with preservation of the progenitor compartment, which suggests that a scaffold-based approach may be more effective when applied in early windows.
A frequently overlooked parameter is drainage capacity. A regenerated gland is useful only if the secretory products reach the oral cavity, and the construct must therefore provide for or favor connection with the residual ductal system. Models that combine tubular fibroin structures, such as conduits, with an acinar component offer a more complete approach than one that considers only the implantation of secreting cells, even though the constructive complexity is considerably greater.
