**2.3 Trabecular meshwork**

The trabecular meshwork is a tissue included between the cornea and the iris in the anterior region that has the role of draining the aqueous fluid. It is divided into three parts which have their characteristic ultrastructures: inner uveal meshwork, corneoscleral meshwork, and juxtacanalicular tissue. Intraocular pressure is determined by the correct balance between aqueous production and outflow; a malfunction in this mechanism is a possible risk factor for the development of glaucoma. Trabecular meshwork cells also are implied in the removal of debris in the circulating aqueous humor. Trabecular meshwork cellular markers are vimentin, non-muscle actin, aquaporin-1, acetylated and acetoacetylated alpha-2 adrenergic receptor, matrix GLA protein, and chitinase-3-like-1. Recently, the isolation, characterization, and specific markers of trabecular meshwork cells have been widely studied. These studies suggest that trabecular meshwork cellular population has properties similar to stem cells, expressing mesenchymal cell-associated markers such as CD73, CD90, and CD105, and they have also the ability to differentiate into adipocytes, osteocytes, and chondrocytes. Moreover, further studies showed that trabecular meshwork cells with mesenchymal phenotype are isolated as a side population or as clones expressing specific stem cell markers, not present in mature cells, such as Notch1, OCT-3/OCT-4, ABCG2, AnkG, and MUC1. These stem cells have the ability to differentiate into the trabecular meshwork lineage expressing CHI3L1, AQP1, and TIMP3 markers that underlies to a phagocytic function. Lowering the intraocular pressure is the aim of treatments for glaucoma. The idea for this came primarily from the observation that trabecular meshwork cell division increased after argon laser trabeculoplasty. Current first-line treatments are topical and oral drugs, argon laser trabeculoplasty, and some surgical approaches. Stem cells isolated from human trabecular meshwork and expanded in vitro showed evidence of the ability to home to mouse trabecular meshwork and differentiate into trabecular meshwork cells in vivo according to recent studies. The expanded trabecular meshwork stem cells expressed the stem cell markers Notch1, ABCG2, and MUC1 and were expressing also the trabecular meshwork marker protein CHI3L1. These trabecular meshwork cells were multipotent and had phagocytic properties. Some groups are working on transplanting trabecular

meshwork cells or trabecular meshwork progenitor cells combined with argon laser trabeculoplasty as a novel cell-based therapy for glaucoma.

## **2.4 Lens**

The lens is composed of the lens capsule, epithelium, and fibers and, like the cornea, is transparent. It is hypothesized that lens-specific stem cells reside in the lens capsule, although they have not yet been identified. The most confirmed hypothesis is that this cell pool comes from the ciliary body, which is anatomically close to the lens. It has been demonstrated that lens capsule regeneration occurs in lower vertebrates from cells residing in the ciliary body. According to this fact, the probability that lens stem cells might reside in the lens capsule is high. Lens progenitor cells have been derived from human ESCs as well as from induced pluripotent stem cells (iPSCs). Lens stem cells are presumed to have a significant role in the maintenance of the lens transparency and might be implied in cataractogenesis process or other lens abnormalities.

## **2.5 Iris**

The iris has the anatomical role of dividing the space between the cornea and lens into anterior and posterior halves. The microscopic structure consists of an anterior limiting layer that lines the anterior part of the iris stroma that contains muscles, nerves, and vessels and is posteriorly lined by a layer of pigmented and non-pigmented cells. The stroma and the vascular structure of the iris take embryological origin from the anterior region of the optic cup. Epithelial cells of the iris pigment have the ability to grow in spheres and express markers of neural stem/progenitor cells such as Msi, Nestin, and Pax6. It has been revealed by studies from the mouse iris that these cells can also differentiate both in neuronal and glial lineages and express markers such as Rho, Chx10, Otx2, and Olig2. The iris pigment epithelial cells have the potential to be used in cell-based therapy, but nevertheless not much work on validation and quality assessment has been done. Further studies are needed before iris pigment epithelial cells can be used clinically.
