Abstract
The uveal tract consists of the iris, the ciliary body and the choroid; these three distinct tissues form a continuous layer within the eye. Uveitis refers to inflammation of any region of the uveal tract. Despite being grouped together anatomically, the iris, ciliary body and choroid are distinct functionally, and inflammatory diseases may affect only one part and not the others. Cellular structure of tissues direct their function, and understanding the cellular basis of the immune environment of a tissue in health, the “steady state” on which the perturbations of disease are superimposed, is vital to understanding the pathogenesis of those diseases. A contemporary understanding of the immune system accepts that haematopoietic and yolk sac derived leukocytes, though vital, are not the only players of importance. An array of stromal cells, connective tissue cells such as fibroblasts and endothelial cells, may also have a role in the inflammatory reaction seen in several immune-mediated diseases. In this review we summarise what is known about the cellular composition of the uveal tract and the roles these disparate cell types have to play in immune homeostasis. We also discuss some unanswered questions surrounding the constituents of the resident leukocyte population of the different uveal tissues, and we look ahead to the new understanding that modern investigative techniques such as single cell transcriptomics, multi-omic data integration and highly-multiplexed imaging techniques may bring to the study of the uvea and uveitis, as they already have to other immune mediated inflammatory diseases.
Highlights
The eye has several functions that make it unique amongst organs
The vast majority of the iris tissue mass is made up of structural cell types, such as epithelial cells of the anterior and posterior epithelial layers, fibroblasts of the stroma and melanocytes, which are present in considerable numbers in the human iris and throughout the rest of the uveal tract
Little work has been published identifying whether these may be cells which are circulating within the blood or whether they are truly present within uveal tissues and are perhaps resident lymphocytes such as have been described in several other tissues [37]
Summary
The eye has several functions that make it unique amongst organs. It is required to be optically clear along the visual axis to allow light to pass through with minimal interruption, to focus light reflected or produced from sources at variable distances, and to transduce information carried by light into neural impulses to be transmitted to the brain for integration and spatial interpretation. The uveal tract is the middle vascular and pigmented layer of the eye and is comprised of the iris, ciliary body and choroid.
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