**1. Introduction**

Regenerative medicine has been a major topic in the last few decades. The use of stem cells has opened up new perspectives in the therapeutic approach to many different diseases. Even in the ophthalmological field, the use of stem cells has allowed an improvement in the clinical outcome of different pathologies such as limbal stem cell deficiency, corneal scarring, retinitis pigmentosa, Stargardt's disease, agerelated macular degeneration, and retinal atrophy following various vascular conditions. Many of these pathologies have always been considered untreatable, leading to a progressive sight loss; the arrival of stem cell therapy has led to an entirely new clinical and therapeutic approach to these conditions. Different types of stem cells have been tested as a solution for tissue repair in the different ocular structures.

As for the anterior segment, the most used cells are limbal epithelial stem cells (LESCs), found at the level of the Vogt palisades. As regards the posterior segment, the stem cells used for the treatment of retinal degenerative diseases are embryonic stem cells (ESCs), induced pluripotent stem cells (IPSCs), and mesenchymal stem cells (MSCs).

LESCs: Also known as corneal epithelial stem cells, they are located in the basal epithelial layer of the corneal limbus. They form the border between the cornea and the sclera and are implied in the regular corneal renewal. They are also implied in corneal repair activity after severe damage of corneal surface.

ESCs: The first ESCs were obtained from mouse embryos and immediately showed their ability to express neural markers and to migrate into the retina when applied intravitreally. Also, they seemed to be able to integrate into the retinal layers and act as neuroprotective factors. Clinical trials conducted in the human eyes have demonstrated that the subretinal application of these cells shows no signs of rejection, ectopic tissue development, negative proliferation, or tumor formation in a 1-year follow-up.

IPSCs: These cells are obtained from reprogramming adult somatic fibroblasts through retroviruses or lentiviruses. Compared with ESCs, they show less risk of rejection and less need for immunosuppressive therapy. However, further studies have suggested that IPSCs can stimulate oncogenes/suppress tumor suppressor genes, resulting in gene mutations and malignant transformation. The many molecular passages required for their production also seem to act as a trigger for the genetic instability shown by these cells.

MSCs: These cells are derived from many different tissues (peripheral blood, bone marrow, adipose tissue, cord blood, teeth, central nervous system, and liver). Once acquired, MSCs can be expanded in cell cultures maintaining their stemness. They can differentiate into various cells (mesodermal, ectodermal, and endodermal cells), including neuron-like cells. Since they are capable of secreting neurotrophic factors, repairing neural connections, and stimulating the formation of synapses, MSCs are also appreciated for their "structural" function. Moreover, they have shown a strong immunosuppressive action inhibiting the release of pro-inflammatory cytokines; therefore they allow both autologous and allogenic transplantation. Finally, their use does not seem to be related to tumor formation. For these reasons, researchers look at stem cells as a promising therapeutic option for degenerative retinal diseases. Nevertheless, it must be said that various ocular complications related with the use of these cells have been described (see Section 3).

### **2. Regenerative medicine in the anterior segment of the eye**

Stem cells are unspecialized cells that have been a focal point of the field of regenerative medicine, frequently considered as the future of medicine. The first medical science branch which directly benefits from stem cells for regenerative treatment was ophthalmology. The triumph of regenerative medicine in ophthalmology can be attributed to its accessibility, ease of follow-up, and the eye being an immune-privileged organ. Two key characteristic attributes of stem cells are pluripotency, the capacity to differentiate into multiple lineages, and proliferation. These cells have the ability to replace damaged or diseased cells under certain specific circumstances. Stem cell-based therapy has now reached a state where ocular tissues damaged by disease or injury can be repaired and/or regenerated. The eye is an ideal organ for studying regenerative medicine thanks to the ease of access for the therapeutic procedure as well as its status of being an immune-privileged organ. Such therapy involves various techniques in which stem cells are injected into both

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**2.2 Cornea**

*Regenerative Medicine and Eye Diseases DOI: http://dx.doi.org/10.5772/intechopen.92749*

rapid expansion time.

**2.1 Conjunctiva**

the cellular and extracellular matrix microenvironments. Corneal epithelial stem cell transplantation has been the most employed stem cell-based therapy after bone marrow transplantation. Stem cell-based treatment in ophthalmology follows two possible ways: a cell replacement therapy strategy or a strategy involving trophic factor-based guidance cues. Throughout treatment, outcomes are related to different factors like our in-depth knowledge of the disease, the source of stem cells, the plausible mechanism driving the therapeutic outcome, and the mode of treatment. Considering specifically the anterior segment, we will analyze separately stem cell pools of the conjunctiva, the cornea, the trabecular meshwork, and the lens. In addition we will make few words about the iris stem cell pool. This pool also subspecializes in three types of cells with different capacities for multiplication: putative stem cells with high reproductive potential, the generally slow-cycling activity cells, and transit amplifying cells (TAC), which have a reduced reproductive potential but

The conjunctiva, apart from being a protection against pathogenic entry, is a connective tissue provided by a high vascularization that offers channels for proper flow of nutrients and fluids. From the anatomical point of view, the conjunctiva is an unkeratinized stratified squamous epithelium, in which goblet cells are also present, that covers the exposed scleral surface (bulbar conjunctiva) and the interior part of the eyelids (tarsal conjunctiva). Conjunctival cells undertake renewal similar to the corneal epithelium, but with a still elusive source of stem cells. Conjunctival stem cells undergo a differentiation pathway that can take them to become either mucin-producing goblet cells or epithelial cells. The dividing basal cell migration starts from the bulbar conjunctiva and takes it to the corneal surface before differentiation. Conjunctival epithelial cells are negative for CK3 and CK12 but positive for CK19. As shown in clonal culture assays, the stem cells located in the fornical niche can differentiate into epithelial cells as well as goblet cells. This provides important evidence that the stem cell pool supporting conjunctival renewal is located in the fornix region. Commitment to differentiate into goblet cells occurs relatively late; in fact goblet cells are generated by stem cell-derived transient amplifying cells. The decision of a conjunctival keratinocyte to differentiate into a goblet cell appears to be dependent upon an intrinsic "cell doubling clock." Ocular processes that affect the cornea also affect the conjunctiva; some examples are conjunctival scarring, cicatricial pemphigoid, thickening, dry eye, or mucin. In order to treat conjunctival stem cell deficiency and scarring, conjunctival autografts, oral mucous membrane grafts, nasal turbinate mucosa grafts, and amniotic membrane are often used. Conjunctival cells cultured on amniotic membrane have been used for cell transplantation in patients with limbal stem cell deficiency (LSCD). Recent patient follow-up reports have shown that transplantation of autologous conjunctival epithelial cells improved the clinical parameters of total LSCD with respect to vision acuity, impression cytology, and in vivo confocal analysis. These cells were cultivated ex vivo on amniotic membrane with the presence of epidermal growth factor, insulin, cholera toxin, and hydrocortisone to produce the corneal lineage; the cells were transplanted after 2 weeks of culture. Ultrathin polymembrane substrate has also been shown to support conjunctival epithelial cell proliferation.

The cornea is at the outermost surface of the eye, and its fundamental characteristic is transparency, which is crucial for vision. It is a clear lens that determines the

#### *Regenerative Medicine and Eye Diseases DOI: http://dx.doi.org/10.5772/intechopen.92749*

the cellular and extracellular matrix microenvironments. Corneal epithelial stem cell transplantation has been the most employed stem cell-based therapy after bone marrow transplantation. Stem cell-based treatment in ophthalmology follows two possible ways: a cell replacement therapy strategy or a strategy involving trophic factor-based guidance cues. Throughout treatment, outcomes are related to different factors like our in-depth knowledge of the disease, the source of stem cells, the plausible mechanism driving the therapeutic outcome, and the mode of treatment. Considering specifically the anterior segment, we will analyze separately stem cell pools of the conjunctiva, the cornea, the trabecular meshwork, and the lens. In addition we will make few words about the iris stem cell pool. This pool also subspecializes in three types of cells with different capacities for multiplication: putative stem cells with high reproductive potential, the generally slow-cycling activity cells, and transit amplifying cells (TAC), which have a reduced reproductive potential but rapid expansion time.
