Functional Anatomy and Immunological Interactions of Ocular Surface and Adnexa
Anatomy
The ocular surface and its adnexa comprise the cornea, the conjunctiva with bulbar, fornical and palpebral parts, the main lacrimal gland, the glands of the eyelids, i.e. meibomian, Moll's, and accessory lacrimal glands and the naso¬lacrimal system with the upper and lower puncta, the paired lacrimal canaliculi, the lacrimal sac and nasolacrimal duct. The nasolacrimal ducts collect the tear fluid from the ocular surface and convey it into the nasal cavity whereas all other structures contribute to formation of the preocular tear film. The tear film serves to protect and lubricate the ocular surface, as well as to provide the major refractive surface for the visual system.
The preocular tear film contains water, protective antimicrobials, cytokines, lipids, and mucins and can be divided in three components: a lipid component, an aqueous component, and a mucus component. The lipid component is secreted by the meibomian glands in the eyelid and forms the superficial layer of the tear film. The aqueous component contains electrolytes, water, and a large variety of proteins, peptides and glycopeptides and is primarily secreted by the lacrimal gland as well as the accessory lacrimal glands (glands of Krause; glands of Wolfring) of the lids. The mucus component is the product of conjunctival goblet and epi¬thelial cells, corneal epithelial cells and acinar as well as excretory duct cells of the lacrimal gland, which have recently been shown to produce mucins .
Ocular Surface
The apical surface of the ocular surface epithelia, both corneal and con- junctival (fig. 2a, b), provide a specialized interface between the tear fluid and the epithelium that stabilizes the fluid layer. That interface includes the undulat¬ing membrane ridges on the apical cell's apical membrane, termed microplicae, and emanating from their apices, a layer termed the glycocalyx. Membrane- bound mucins (MUCs 1, 4 and 16) of corneal and conjunctival epithelial cells are present in the glycocalyx layer ; soluble mucins (MUC5AC) from conjunctival goblet cells (figs 1b, 2b) as well as MUCs 5B and 7 from lacrimal glands are in solution in the tear film. Both MUC5B and MUC7 have been shown to bind bacteria [for review, see 1] and contribute to innate immunity of the tear film. Beside MUC5AC, conjunctival goblet cells secrete the trefoil factor family (TFF) peptides TFF1 and TFF3 . TFF peptides are, together with mucins, typical constituents of mucus gels that influence the rheological properties of the tear film, promote migration of corneal epithelial cells, have antiapoptotic properties, and induce cell scattering . Conjunctival and corneal epithelial cells are able to react against pathogens by the production of inducible antimicrobial peptides . Moreover, in certain disease states the corneal cells are able to produce TFF3 .
Anatomy
The ocular surface and its adnexa comprise the cornea, the conjunctiva with bulbar, fornical and palpebral parts, the main lacrimal gland, the glands of the eyelids, i.e. meibomian, Moll's, and accessory lacrimal glands and the naso¬lacrimal system with the upper and lower puncta, the paired lacrimal canaliculi, the lacrimal sac and nasolacrimal duct. The nasolacrimal ducts collect the tear fluid from the ocular surface and convey it into the nasal cavity whereas all other structures contribute to formation of the preocular tear film. The tear film serves to protect and lubricate the ocular surface, as well as to provide the major refractive surface for the visual system.
The preocular tear film contains water, protective antimicrobials, cytokines, lipids, and mucins and can be divided in three components: a lipid component, an aqueous component, and a mucus component. The lipid component is secreted by the meibomian glands in the eyelid and forms the superficial layer of the tear film. The aqueous component contains electrolytes, water, and a large variety of proteins, peptides and glycopeptides and is primarily secreted by the lacrimal gland as well as the accessory lacrimal glands (glands of Krause; glands of Wolfring) of the lids. The mucus component is the product of conjunctival goblet and epi¬thelial cells, corneal epithelial cells and acinar as well as excretory duct cells of the lacrimal gland, which have recently been shown to produce mucins .
Ocular Surface
The apical surface of the ocular surface epithelia, both corneal and con- junctival (fig. 2a, b), provide a specialized interface between the tear fluid and the epithelium that stabilizes the fluid layer. That interface includes the undulat¬ing membrane ridges on the apical cell's apical membrane, termed microplicae, and emanating from their apices, a layer termed the glycocalyx. Membrane- bound mucins (MUCs 1, 4 and 16) of corneal and conjunctival epithelial cells are present in the glycocalyx layer ; soluble mucins (MUC5AC) from conjunctival goblet cells (figs 1b, 2b) as well as MUCs 5B and 7 from lacrimal glands are in solution in the tear film. Both MUC5B and MUC7 have been shown to bind bacteria [for review, see 1] and contribute to innate immunity of the tear film. Beside MUC5AC, conjunctival goblet cells secrete the trefoil factor family (TFF) peptides TFF1 and TFF3 . TFF peptides are, together with mucins, typical constituents of mucus gels that influence the rheological properties of the tear film, promote migration of corneal epithelial cells, have antiapoptotic properties, and induce cell scattering . Conjunctival and corneal epithelial cells are able to react against pathogens by the production of inducible antimicrobial peptides . Moreover, in certain disease states the corneal cells are able to produce TFF3 .
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