Epithelial-mesenchymal transition in the lacrimal gland morphogenesis, damage and repair.

Epithelial-mesenchymal transition (EMT) constitutes an important pathway in organ fibrosis seen in the lungs, liver, eye, and salivary glands. This review summarizes the EMT observed within the lacrimal gland during its development, tissue damage and repair along with possible translational implications. Existing animal and human studies have reported the increased expression of EMT regulators i.e., transcription factors like Snail, TGF-β1 within the lacrimal glands, and a possible role of reactive oxygen species, which might be initiating the cascade of EMT. In these studies, EMT is typically detected by reduced E-cadherin expression in the epithelial cells and increased Vimentin and Snail expression within the lacrimal glands' myoepithelial or ductal epithelial cells. Other than specific markers, electron microscopic evidence of disrupted basal lamina, increased collagen deposition, reorganised cytoskeleton of myoepithelial cells also indicated EMT. Very few studies have shown myoepithelial cells to be the cells transitioning into mesenchymal cells with increased extracellular matrix deposition within the lacrimal glands. EMT in animal models seemed reversible as glands got repaired after damage with IL-1α injection or duct ligation and transiently used the EMT as a means for tissue repair. The EMT cells also expressed nestin, a marker for progenitor cells in a rabbit duct ligation model. However, lacrimal glands of ocular graft versus host disease and IgG4 dacryoadenitis demonstrate irreversible acinar atrophy along with signs of EMT-fibrosis, reduced E-cadherin, and increased Vimentin and Snail expression. Future studies exploring the molecular mechanisms of EMT and thereby developing targeted therapies capable of transforming the mesenchymal cells into epithelial cells or blocking the EMT might help in the restoration of the lacrimal gland function.