Table 1 Ocular stem cells: locations, functions, markers and therapeutic development stage

Cornea- limbus

Junction between cornea and conjunctiva. Basal layer of the limbal region at corneal periphery, called the palisades of Vogt

Generates transient amplifying cells that are responsible for corneal epithelial cell renewal

Limbal epithelial stem cells

Positive: isoform of p63, ABCG2, Musashi-1

Negative: CK3, CK12

Limbal stem cell deficiency

Limbal epithelial stem cell transplantation successfully used in human ocular surface reconstruction

[8, 18, 20]

Cornea- stroma

Corneal stromal cells are located in the anterior stroma sub-adjacent to the basal side of the palisades of Vogt

Restore organization and transparency to the cornea

Corneal stromal stem cells

Positive: ABCG2, Bmi1, CD166, c-kit, Pax6, Six2 and Notch1

Corneal scar-like disruption

Still under study

[22]

Conjunctiva

Bulbar epithelium covering slack and highly vascularized connective tissue

Proper flow of nutrients, fluids and barrier for the entry of infectious pathogens

Goblet cells, non-goblet cells, epithelial cells

Positive: keratin 19

Negative: CK3, CK12

Conjunctival scarring, cicatricial pemphigoid, thickening, dry eye or mucin deficiency

Conjunctival epithelial stem cell transplantation successfully used in human ocular surface reconstruction

[18]

Iris

Present between the cornea and lens

Control the diameter and size of the pupils

Iris pigment epithelial cells

Positive: neuronal stem/progenitor markers - Nestin, Msi , Pax6

Iritis

Potential for use in cell-based therapy and animal model studies ongoing

[30]

Exudative age-related macular degeneration

Clinical application

[78, 79]

Ciliary body

Continuation of the choroid at ora serrata

Aqueous humor production, accommodation, production and maintenance of the lens zonules

Ciliary body stem cells

Positive: neuronal stem/progenitor markers - Nestin, Chx10, Pax6, Sox2, Lhx2, Dach1, Six 3

Ciliary body detachment

Still under study

[2, 31, 32, 34]

Trabecular meshwork

Tissue between the cornea and iris

Drainage of aqueous fluid

Endothelial cells, justacanalicular cells, mesenchymal stem cells

Positive: CD73, CD90, CD105, ABCG2, Notch1, OCT-3/4, AnkG, MUC1, AQP1, CHI3L1, TIMP3

Intra-ocular pressure

Still under study

[38, 42]

Lens

Lens capsule

Play a role in maintaining the lens transparency

Lens stem cells

Not yet found

Cataractogenesis or other lens abnormalities

Still under study

[46, 47]

Retina - retinal pigment epithelium

Light-sensitive layer of tissue, lining the inner surface of the eye

Visual activity

Retinal cells -RPE

Positive: Nestin, Notch 1, Chx2, Map-2, CRALBP, tyrosinase, tyrosine-related protein 1 and 2,

Retinitis pigmentosa and age-related retinal degeneration, Stargardt’s macular dystrophy and dry age-related macular degeneration

Clinical trials using transplantation of fetal retinal cells and RPE in age-related macular degeneration

[17, 53, 55]

Choroid

Lying between retina and sclera

Supplies oxygen and nourishment to the retina

Mesenchymal originated stem cells

Positive: Sca-1, CD90.2, CD44, CD105, CD73, ABCG2, Six2, Notch1, Pax6

Ocular neurodegenerative diseases

Still under study

[70]

Sclera

Continuation of cornea, outer layer of the eye

Maintain the shape of the eye, resistance to internal and external forces, and provides an attachment for muscles

Mesenchymal originated stem cells

Positive: ABCG2, Six2, Pax6, Notch1

Myopia

Still under study

[70]

Orbit

Scaffold for the eye

Eye protection

Orbital adipose stem cells

Positive: CD34, zonal-occludin-1, CK3, CK19

Orbital inflammatory disease, protrusion of eyeball, oribital volume deficiency

Still under study

[73–75, 80]