Ординатура / Офтальмология / Английские материалы / Basic Sciences in Ophthalmology_Velayutham_2009

The NPE cells are columnar and contain numerous mitochondria, smooth and rough endoplasmic reticulum. They are highly active forming aqueous humor. The cell membrane of NPE has numerous basal infolding s and multiple convoluted lateral interdigitations forming the ciliary channels. Various types of intercellular junctions join the PE and NPE cells like desmosomes, gap junctions, puncta adherentia, and tight junctions all useful for the fluid transport of aqueous humor. The ciliary epithelium is rich in antioxidant activity with high concentration of catalase, superoxide dismutase and glutathione peroxidase. This is to reduce H2O2 present in normal aqueous humor derived from the non-enzymatic interaction between reduced ascorbate and molecular O2.

The iris ciliary body contain adrengeric, cholinergic, peptidergic, prostaglandins, and serotonin receptors. Cytochrome P450 is present in NPE of ciliary body to detoxify many compounds, first by hydroxylation and then conjugation with glucuronide or glutathione.

Blood Aqueous Barrier

The tight junctions between the NPE cells together with non-fenestrated iris vessels (tight junctions between the vascular endothelial cells) containing the protein occludin and cingulin contribute to the blood aqueous barrier. It does not allow large molecules like proteins to pass through. It maintains IOP by allowing the surrounding tissues to remove waste products of metabolism.

Breakdown of blood aqueous barrier can occur due to various causes like mechanical trauma, inflammation, paracentesis, vascular disease, administration of hyperosmotic agents, after cyclodestructive procedures for glaucoma, or after argon laser trabeculoplasty etc.

Mechanism and Effects

In these conditions tight junction is fragmented, leading to leakage of plasma proteins into the aqueous seen as "flare" and so IOP increases.

In addition, cell membrane disruption releases arachidonic acid from the phospholipids of the membrane. Arachidonic acid gives rise to prostaglandins through cycloxygenase pathway. Prostaglandins have irritative effects on the eye like miosis, vasodilatation, release of protein into aqueous and increased IOP.

Prevention of these prostaglandins effects may be achieved through pretreatment with inhibitors of PG synthesis such as aspirin, indomethacin, and other NSAIDs.

Mechanism of action of inhibitors: These NSAIDs bind irreversibly to cycloxygenase enzyme and inhibit the pathway synthesizing PG e.g., topical NSAIDs used in the treatment of anterior segment inflammation, aphakic and pseudophakic cystoid macular edema, allergic conjunctivitis (Ketorolac tromethamine 0.05%).

Pain after refractive surgery - Diclofenac 0.1% (voveran).

Preoperative use of flurbiprofen and suprofen drops (1%) - prevent PG mediated papillary miosis during ocular surgery.

Leukotrienes formed from arachidonic acid by lipoxygenase pathway is not inhibited by NSAIDs. Only nordihydroguaiaretic acid (NDGA) inhibits lipoxygenase.

AQUEOUS HUMOR

It is an ultra filtrate of blood produced by the ciliary body. It enters the posterior chamber from the ciliary processes through various mechanisms such as diffusion, ultra filtration and active transport and carbonic anhydrase II activity. The rate of formation is 2 µl/ minute. It exerts a hydrostatic pressure (IOP) of 10 -20 mmHg which maintains the shape of the ocular globe and protects it from physical shock to some extent. IOP is maintained by steady formation and drainage of aqueous.

Aqueous nourishes the cells of posterior cornea (corneal endothelium, stromal keratocytes, most of the corneal epithelial cells), lens and iris. (Fig. 13.10)

Aqueous humor is the source of antoxidants (ascorbate) for lens and corneal endothelium and removes their metabolic waste products. It does not contain RBC, still it carries released O2 and nutrients to the cells, it serves. Due to the lack of cellular components, proteins and K+ content are low in aqueous. This

Fig 13.10

enhances the capacity of aqueous to transmit light but reduces the buffering capacity. Still, pH is maintained by HCO3¯and PO4¯retention in sufficient quantity.

Composition of Aqueous humor in comparison with plasma.

Constituents of aqueous humor:

Glucose: passes across the ciliary epithelium by facilitated diffusion through transporters, not dependent on insulin. In diabetes mellitus, concentration of glucose increases.

Concentration of inositol, important for phospholipids synthesis is 10 times higher that of plasma.

Lacatate concentration is higher than that of plasma due to the increased anaerobic glycolysis in intraocular tissues.

Glutathione concentration is high for the antioxidant activity. Urea enters by passive diffusion.

Enzymes: Hyaluronidase, carbonic anhydrase and lysozyme.

Growth modulatory factors such as fibroblast growth factor, ß transforming growth factor, insulin like growth factor (IGF _1), insulin like growth factor binding proteins, vascular endothelial growth factor.

Formation of Aqueous humor: from the ciliary processes.

1.Diffusion: Movement of ions like Na+ across the membrane towards the side with most negative potential and down a concentration gradient.

2.Ultrafiltration: is the non-enzymatic component of aqueous formation that is dependent on IOP, BP and blood osmotic pressure in ciliary body. It is the "bulk flow" of material across epithelium and increases by augmenting its hydrostatic driving force.

3.Carbonic anhydrase II activity (CA II): CA II is present in the pigmented and non-pigmented epithelium. The inhibitors cause a reduction in the rate of entry of Na+ and HCO3¯ in the posterior aqueous leading to reduction in the aqueous flow.

4.Active transport: This requires cellular energy (ATP) to secrete solute against a concentration gradient. This is the major mechanism of aqueous humor formation by the inner non-pigmented epithelium involving membrane associated Na+/K+ ATPase found in highest concentration along the lateral cellular interdigitations. A surplus of Na+ is pumped into the posterior aqueous chamber by Na+/K+ ATPase causing water flow into the chamber osmotically (Fig 13.11). The enzyme thus generates an IOP indirectly.

HCO3¯also contributes to this mechanism. Thus, aqueous humor is formed by the transport of water and electrolytes from the leaky fenestrated

capillaries of the ciliary process to the epithelial syncytium and hence, across the plasma membrane of the non-pigmented epithelium as shown in

figures 13.12 and 13.13.

Regulations of aqueous flow

1.Adrenergic receptors present in the ciliary epithelium regulate the IOP through adenylate cyclase system.

2.Cholinergic receptors are linked to phosphotadyl inositol second messenger

system.

Adrenergic receptors (α and ß receptors stimulation):

Stimulation of α 2 receptors - inhibition of adenylate cyclase through G protein - decreased production of aqueous --- reduced IOP.

Fig 13.11

Fig 13.12

Fig 13.13

Epinephrine, an α adrenergic agonist, stimulates prostaglandin synthesis, PGE2 and PGF2 which decrease IOP.

Stimulation of β receptors - activation of adenylate cyclase through G protein - increase the production of aqueous and secretion.

Outflow of aqueous from the eye is regulated at different levels like trabecular meshwork, uveoscleral system and episcleral vessels.

Trabecular Meshwork

a)Trabecular meshwork - The juxtacanalicular cribriform meshwork contributes significant resistance to the outflow. The trabecular meshwork is formed by Type I, III, IV collagen and other matrix proteins such as

vascular supply to posterior segment of the eye. The choroid is almost entirely composed of vessels embedded in a loose connective tissue matrix with a high content of type III collagen necessary for an expansile or spongy tissue. Blood vessels are highly fenestrated and leaky like ciliary vessels. 85 % of total ocular blood flow passes through the choroid. The vessels are sensitive to changes in partial pressure of O2 and CO2 and also to vasoconstrictors.

The Normal Constituents of the Lens

An increase in Ca2+ is the hallmark of degenerative changes in the lens such as normal aging, sclerosis and cataract formation. The nucleus of the lens contains most of the calcium. It is involved in the maintenance of normal cell membrane permeability.

LENS

Next to cornea, the second refracting unit of the eye is the lens. The transparency of the lens is a function of the highly ordered state of its cells and extracellular matrix. It transmits long wavelength light but filters the majority of short wave length light< 360 nm and is an absolute barrier to light below 300 nm.

The extracellular matrix of lens is the capsule. It is a typical basement membrane surrounding the lens. The epithelial cells form a syncytium with interlocking cellular processes.

Anterior capsule is formed by the underlying epithelium and contains fibronectin.

Posterior capsule is formed by the cortical fibres and has tenacin. The capsule is non-cellular and composed of glycoprotein associated Type IV collagen. The GAG, heparan sulfate comprises < 1 % of the lens capsule and is important in maintaining the clarity of the capsule. The dense fibrillar outer layer of capsule is the zonular lamella and zonules running from ciliary processes fuse into the outer layer.

Function

•Capsule is the source of anti-angiogenesis factors.

•It is a barrier to vitreous, bacteria, chemicals and growth factors. Thus, it helps in the prevention of postsurgical secondary open angle glaucoma, reduction in the incidence of endophthalmitis following cataract surgery, if remained intact, reduction in the incidence of anterior segment neovasularisation.

•It is impermeable to small molecular weight proteins < 50,000 KDa including low molecular weight crystallins.

Epithelium

It is a single cell layer of cuboidal epithelium. It does not scatter or reflect light. It is a highly active, mitotic layer with a ring of germinative zone around the anterior lens. Anterior lens epithelium contains α6β1 integrin and α5β1 integrin receptor for laminin is present in the equatorial and lens fibre cells. Both are migratory. The newly formed epithelial cells migrate equatorially to form the lens fibre cells. As the epithelial cells progress to the 'bow region', they change in morphology and synthetic activity. There is increase in cell size, increase in mass of cellular proteins and in membrane of each cell. It elongates at both ends of the cell anteroposteriorly with decrease in and or disappearance of other cellular organelles. This is terminal differentiation into lens fibre cells.

The new fibre cell has its centre at the equator and the ends of each fibre meet the ends of other fibre at the anterior and posterior regions of lens giving rise to a pattern called "suture line". The new cell is layered over the old one. The oldest fibre lies in the centre of the lens as "nucleus" surrounding which are cortical fibres that are formed recently. The epithelium maintains the fluid and electrolyte balance of the lens syncytium via ion pump mechanisms. Fig 13.14.

Fig 13.14

Transport Function in Lens: Controlled by Membrane Proteins

A specific internal ionic, osmotic environment is important is normal lens metabolism and this is maintained by the cellular communication between epithelium and fibres. The epithelial layer shows typical polarization potentials, but lack tight junctions at the lateral surface. But, they have gap junctions that permit rapid intercellular communications. They restrict the passage of high molecular weight solutes between the cells.

Na+/K+ ATPase pump in the epithelium actively exchanges Na+ for K+ to maintain the internal Na+ at 20 meq and K+ at 120 meq (where as in aqueous it is 150 and 5 meq respectively), from the back of the lens, Na+ passively diffuses down a concentration gradient in the vitreous, across the posterior lens capsule into the lens body and rapidly diffuses to anterior epithelium and pumped out into the aqueous. K+ moves passively in the reverse direction across the posterior capsule into the vitreous.

Lens Fibre Cell

Gap junctions between lens fibre cells allow rapid movement of metabolite by forming channels between the cells e.g., MIP26 (main intrinsic polypeptide of 26,000 KDa) which is also called Aquaporin for transport of water out of the lens to maintain transparency. There is abundance of Na+/K+ATPase in the lens fibre around the lens sutures, Ca++/Mg ++ ATPase in the lens cortex, specific transporter proteins for glucose, and amino acids in the plasma membrane of lens epithelium and fibre cells. Lens fibre cells are organized in a densely packed cellular arrangement with interdigitations, having the gap junction protein for intercellular communication.

During development of lens fibre cells, they become anucleate and specialized for the production of specific lens protein, the crystallins.

The plasma membrane of lens fibre is very stable and rigid due to the high content of saturated fatty acids, cholesterol and phospholilpid with high amount of sphingomyelin contributing to the tight packing and low fluidity of membrane. The structural frame work of lens cells formed by the cytoskeleton is a complex system of intracellular filaments, as detailed in the table below.