The Crystalline Lens and Cataracts

This set of vocabulary flashcards covers the anatomical structure, physiological mechanisms, molecular composition, and relevant pathologies of the human crystalline lens as described in the lecture.

Human Accommodation

The process of focusing on objects by the deformation of the lens.

Axial movement

The method of accommodation used by some species, involving the moving of the lens or retina along the visual axis rather than deforming the lens.

Crystalline Lens

A transparent, biconvex structure situated between the iris and the vitreous body.

Equator

The region of the lens encircled by ciliary processes, lying approximately $0.5\,mm$ from them.

Zonules of Zinn

Suspensory ligaments with a diameter of $1-2\,\mu m$, comprised of fibrillin, that keep the lens in position.

Fibrillin

A connective tissue with elastic properties that makes up the suspensory ligaments of the lens.

Accommodation for close objects

A process where ciliary muscles contract, relieving tension on the zonule fibers, allowing the lens to assume a more globular shape.

Sphincter pupillae

The muscle that contracts during accommodation to make the pupil smaller, ensuring light reaches the retina through the thickest, central part of the lens.

Gradient refractive index (GRIN)

An optical property caused by the change in protein density from the cortex to the nucleus, important for minimizing aberrations.

Lens Capsule

An elastic basal membrane that envelopes the entire lens, consisting of about $40$ lamellae of collagen fibrils embedded in a glycosaminoglycan matrix.

Lens Epithelium

A layer of cuboidal cells found only on the anterior surface of the lens beneath the capsule.

Lens Fibres

Cells that are hexagonal in cross-section, measuring $4 \times 7\,\mu m$ and up to $12\,mm$ in length, which are tightly packed and interlocked.

Cortex

The outer part of the lens consisting of younger, nucleated fibers with a protein concentration of $100$ to $150\,mg/ml$.

Lens Nucleus

The central part of the lens consisting of older fibers without nuclei or organelles, featuring a higher protein concentration of $200$ to $300\,mg/ml$.

Presbyopia

The progressive loss of accommodation caused by the lens becoming less pliable and more compact as it grows.

Crystallins

The soluble proteins (Alpha, Beta, and Gamma) responsible for the special properties and transparency of the lens.

Alpha-Crystallin

A small Heat-Shock protein ($sHSp$) that acts as a molecular chaperone to protect other proteins from thermal denaturation and oxidation.

Polydisperse

A property of alpha-crystallin where it forms globular aggregates of different sizes.

Short range order

The structural regularity of atoms or molecules regarding their first- or second-nearest neighbors, which is critical for lens transparency.

Light scattering

The effect produced when high weight amorphous protein aggregates reach a size comparable with the wavelength of light, leading to opacification.

UV-A

Ultraviolet radiation between $300$ and $400\,nm$ that is absorbed by the lens and can lead to protein denaturation and bond breaking.

Cataracts

Opacities in the lens that absorb and scatter light, resulting in reduced visual acuity and less contrast in the retinal image.

Cortical cataracts

The most common age-related cataracts (especially up to age $65$), where the initial opacity is confined to the outer lens cells.

Nuclear cataracts

Cataracts where the opacity appears first in the center of the lens, dominating in prevalence after the age of $65$.

Posterior subcapsular cataracts

Opacities that lie in the outermost layers of the cortex at the posterior pole.

Brunescent cataract

A condition where pigments (initially protective) accumulate and deepen in color, causing a pale yellowish to deep pigmentation of the lens.