Q1 LONG ANSWER 10M ★ High-yield

Describe the anatomy, physiology and metabolism of the lens. Classify cataract.

Ophthalmology · 3rd Professional

The crystalline lens is a transparent, biconvex, avascular body suspended behind the iris by the zonules OF ZINN.
It has NO blood supply and NO nerve supply, and is nourished entirely by the aqueous humour.

Feature	Value and significance
Dimensions	Diameter 9 to 10 mm; thickness 3.5 mm at birth rising to 5 mm in old age, because the lens grows throughout life; weight 190 to 260 mg
Refractive power	+16 to +20 dioptres — about one-third of the total power of the eye, the cornea supplying the other two-thirds
Refractive index	1.39 overall; but higher in the nucleus (1.406) than in the cortex (1.386).
Radius of curvature	Anterior 10 mm, posterior 6 mm — so the posterior surface is more steeply curved
Capsule	The thickest basement membrane in the body, secreted by the epithelium. Thickest in the pre-equatorial region (14 micrometres) and thinnest at the posterior pole (2 to 4 micrometres) — which is precisely why the posterior capsule is the structure most likely to rupture during cataract surgery
Epithelium	A single layer of cuboidal cells beneath the anterior capsule only; there is NO posterior epithelium. The germinative zone at the equator produces new fibres lifelong. This anatomy explains after-cataract: the residual anterior and equatorial cells migrate onto the posterior capsule after surgery
Lens fibres and nucleus	New fibres are laid down at the periphery and older ones compressed centrally. Zones: embryonic, fetal, infantile and adult nucleus, surrounded by the cortex
Sutures	Fibres meet in a Y-shaped suture — an upright Y anteriorly and an inverted Y posteriorly — formed in the fetal nucleus, and visible in sutural cataract

Energy metabolism — the lens is almost entirely dependent on anaerobic glycolysis, which supplies about 80% of its ATP despite the low yield, because the tissue is avascular and hypoxic. The hexose monophosphate (pentose phosphate) shunt supplies 15% and generates NADPH for glutathione reduction; the Krebs cycle only 3%, being confined to the epithelium; and the sorbitol (polyol) pathway 5% — which is the pathway that matters in diabetes
The pump-leak mechanism — a Na-K ATPase in the anterior epithelium actively transports sodium out and potassium in, while ions leak passively in the opposite direction posteriorly. The lens therefore has a high potassium and low sodium content, and maintains its state of relative dehydration. Failure of the pump allows the lens to hydrate, and hydration is the first step of cataract formation
Antioxidant defence — the lens contains the highest concentration of glutathione of any tissue, together with ascorbate, superoxide dismutase and catalase. oxidative damage is the central mechanism of age-related cataract, and the fall in glutathione with age is what permits it

Basis	Types
By aetiology — congenital and developmental	Hereditary; intrauterine infection (rubella and the other torch agents); metabolic (galactosaemia, Lowe syndrome); chromosomal (Down syndrome); maternal malnutrition and drugs; and prematurity
By aetiology — acquired	Senile (age-related) — much the commonest; traumatic; complicated (secondary to other ocular disease); metabolic (diabetes, hypocalcaemia, Wilson disease, myotonic dystrophy); toxic and drug-induced (steroids, chlorpromazine, amiodarone, miotics); radiational; electrical; and dermatogenic
By morphology	Nuclear; cortical (cuneiform, cupuliform); posterior subcapsular; anterior subcapsular; capsular; polar (anterior and posterior); lamellar (zonular); sutural; coronary; blue-dot (cerulean); Christmas-tree; and membranous
By maturity	Incipient; immature; intumescent; mature; and hypermature (Morgagnian or sclerotic)
By age at onset	Congenital (present at birth); infantile (first year); juvenile (up to 13 years); presenile; and senile

Feature	Cornea	Lens
Refractive power	+43 to +45 D — two-thirds of the total	+16 to +20 D — one-third
Variable?	Fixed	Variable — the basis of accommodation
Blood supply	Avascular; nourished by aqueous, limbal vessels and tears (oxygen)	Avascular; nourished entirely by the aqueous
Nerve supply	The most densely innervated tissue in the body	None — which is why cataract is painless
Regeneration	Epithelium regenerates from limbal stem cells; endothelium does not	Grows throughout life; nothing is ever shed.
Basis of transparency	Regular collagen lamellae; deturgescence by the endothelial pump	Regular fibre packing; absence of nuclei and organelles; crystallins in short-range order; dehydration by the epithelial pump
Principal metabolic pathway	Aerobic, with oxygen from the tears	Anaerobic glycolysis (80%), because it is avascular and hypoxic
Can it be replaced?	By keratoplasty, with a risk of rejection	By an intraocular lens — an inert implant with NO rejection risk.

Cataract IS the leading cause OF blindness IN the world, and remains so in India, where it accounts for the majority of avoidable blindness. It is entirely curable by a single operation, which is why cataract surgery is among the most cost-effective interventions in all of medicine and why the Indian national programme is built around it
Risk factors for age-related cataract — age above all; ultraviolet-B exposure, which is high in India; diabetes; smoking; corticosteroid use; malnutrition and low antioxidant intake; severe dehydrating illness such as cholera and dysentery.
The lens has no nerve supply, which is why cataract is entirely painless — and a painful eye with a cataract has something else wrong with it

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