Human Eye Anatomy and Functions Quiz

Structure-Function Relations in the Eye: Cornea and Sclera

• The eye is a complex organ involving physics, physiology, neuroscience, and psychology.
• There's an interesting adaptation of vision in the dark, and questions about how we perceive from the corner of our eye, how light and colors are processed, and how shapes are distinguished.
• The eye's ability to withstand a punch and the importance of mechanics for the eye are also considered.

Evolution of the Eye

• Scientists understand how the complex human eye evolved.
• The human eye acts like a camera to collect and focus light, converting it into electrical signals that the brain interprets as images.
• It utilizes a specialized retina with numerous neurons for light detection and signal processing.
• The complexity of the eye is a prime example of irreducible complexity for creationists and proponents of intelligent design.
• Darwin noted that numerous gradations from a perfect to an imperfect eye can be shown to exist. Variations useful to an animal under changing conditions allow natural selection, making seemingly unimaginable complexity possible.

Darwin's Explanation

• Charles Darwin (1859) explained how numerous gradations from a perfect to an imperfect eye can exist and be useful in various conditions.
• Any variation or modification in the eye organ can aid an animal in changing conditions, diminishing the difficulty of believing it formed by natural selection.

Evolutionary View of the Eye

• Nothing in biology makes sense except in the light of evolution.
• Understanding the evolution of rod and cone photoreceptors and the organization of the retina helps comprehend their morphology and molecular structure.
• This evolutionary perspective helps understand how and when eyes originated and why different vertebrates have eyes different from those of insects.

Structure of the Eye

• The eye is a complex optical system that collects light, the intensity of which is regulated by a diaphragm, focusing it through lenses to form an image.
• Electrical signals are transmitted via the optic nerve to the visual cortex and other brain areas.
• Most animal species have a complex optical system.
• The eye has a number of components, including the cornea, iris, lens, vitreous chamber, sclera, retina, choroid, optic nerve, fovea, etc…

Description of Components

• Cornea: The transparent, front surface of the eye.
• Conjunctiva: A mucous membrane lining the inner surface of the eyelids and the visible part of the eyeball.
• Pupil/Iris: The iris controls light entering the eye (like a diaphragm).
• Eye Lens: Focuses light onto the retina.
• Retina: The inner lining of the eye, where images are formed and transmitted to the brain. The macula is the most sensitive area, and the fovea is the center of the macula where detail perception is best.
• Vitreous Body: The gel-like substance behind the lens.
• Sclera: The white, outer layer of the eyeball.

Processing of Light

• Light travels through the neural structure to the photoreceptors (rods and cones).
• The retinal pigment epithelium (RPE) supports the metabolic needs of the photoreceptors.
• Rods are for dim light, and cones for bright light and color vision.

Light Transduction

• Light triggers a process called transduction in photoreceptors, converting light to an electrical signal.
• This results in a transient change in the photoreceptor's membrane potential, leading to a graded response (hyperpolarization).
• A single photon can trigger a small current change (about 1 pA lasting about 1 second), acting as a photomultiplier.

Molecular Mechanism of Light Transduction

• The molecular mechanism involves proteins like rhodopsin, transducin, and phosphodiesterase.
• Light activates rhodopsin, which triggers a cascade of events leading to a change in the concentration of cGMP, affecting ion channels and membrane potential.

Cell-to-Cell Communication

• Photoreceptors communicate with horizontal and bipolar cells (forming a triad), often inhibiting neighboring cells but amplifying contrast.
• Bipolar cells transmit signals electrotonically (hyperpolarization or depolarization) to ganglion cells.
• Amacrine cells modulate the signal transmission from bipolar to ganglion cells, based on past light levels.
• Signals from ganglion cells are transmitted to the brain as action potentials.

Automatic Mechanisms

• The iris adjusts its size to control light entry—constricting with increased light and widening with decreased light.
• The lens adjusts its shape to focus light on the retina—allowing for seeing objects at various distances.

Evolution of the Camera Eye

• Diagrams illustrate evolutionary progression of the eye from simple forms to a complex camera-style eye.
• Examples of this progression include Euglena, Nautilus, Lamprey, hagfish, and mammals.

Echoes of Evolution and Embryonic Development

• The eye's structure and embryonic development reflect its evolutionary history (e.g., in vertebrate development).
• The development and progression of the eye matches that seen among various species.
• Features of the eye (e.g., retina) are present in development that were present and important to the evolution of the structure.

The Eye is Not Perfect

• The eye, while complex, has flaws indicative of its evolutionary origin.
• These flaws, despite interfering with image quality, are not detrimental and are a result of evolutionary steps from earlier forms.
• The vertebrate eye differs from that of squid and octopuses in the arrangement of nerve fibers, leading to a blind spot in the eye.

Macroscopic Structure of the Cornea

• The cornea is the transparent front of the eye, forming 15% of the eyeball.
• Components include epithelium (surface layer), Bowman's membrane, stroma, Descemet's membrane, and endothelium (innermost layer). A tear film is present at the surface.

Main Functions of the Cornea and Sclera

• The cornea and sclera protect the eye from pathogens.
• The cornea is a tough connective tissue that can withstand stress.
• The cornea's smooth surface, coated with a tear film, allows for optical clarity.
• The cornea focuses incoming light.
• The sclera contributes to the shape of the eyeball.

Lamellar Structure in the Stroma

• Collagen fibrils form the lamellae, a plywood-like structure within the stroma.
• Lamellae have varying angles and run parallel to the cornea's surface.
• The stroma contains keratocytes, fibroblasts that maintain and repair the collagen structure.
• Keratocytes repair any injuries to the lamellae structure.

3-Dimensional Reconstruction

• Second-harmonic imaging was used to study the corneal structure.
• This allows looking transversely at the collagen structure (lamellar structure).

Nanoscopic Structure

• The stroma is comprised of uniform, narrow-diameter collagen fibrils.
• The lamellae confer radial strength to the cornea.
• Fibril diameters are tightly regulated (~31 nm).
• Fibril diameters increase with age.

Cross-linking

• Collagen fibrils, primarily type I, are stabilized through the various covalent crosslinks, including (but not limited to): deH-HLNL, deH-HHMD, and HHL.
• The distribution of fibrils is responsible for transparency,
• This organization of collagen is important for the mechanical integrity of the cornea.
• The physical arrangement of the fibrils minimizes light scattering, contributing to the structure's transparency.

Composite Fibril Structure

• A proposed model of Type I-Type V composite fibril suggests light transmitting through the hole in the fibril
• This hole and extension of Type V creates steric hinderance to fibril-fibril distance and limiting possible fibril diameter

Composition of the Stroma

• Collagen accounts for ~58% of the stroma.
• Other proteins, proteoglycans, cellular water, and matrix water make up the remaining stroma.
• Collagen, type V, and other proteins are believed to contribute to the mechanical properties of the tissue.

Corneal Shape

• Corneal curvature is greater than the sclera's curvature, generating interfaced stress which explains fibril changing diameter.
• The optical zone of the cornea occupies two-thirds from center due to the differences in corneal curvature.
• Intraocular pressure (IOP) is a measure of the force that the fluids inside the eye exert on the cornea.

X-rays Scattering to Determine Structure

• X-ray scattering patterns identify the organization of collagen fibrils in the cornea.
• The patterns of scattering arise from the spacing and orientation of the collagen fibrils, aiding in understanding the tissue order.

X-ray Scattering of the Cornea

• Wide-angle X-ray patterns from the center of the human cornea show intermolecular spacing.
• Maxima results from excess collagen in the vertical and horizontal directions.

The Orientation Map

• The preferred orientation of collagen fibrils is mapped across the corneal area and sclera.
• The central 8 mm optical zone shows orthogonal orientations.
• Changes in fibril diameters align with locations of changing curvature in the cornea.

Biomechanics of the Cornea

• Corneal mechanical behavior depends on stromal collagen architecture.
• Collagen fibrils are strongest axially.
• Corneal stiffness and its response to intraocular pressure are important for vision.
• Changes in the cornea's shape as a response to intraocular pressure likely depend on the fibril arrangement.

Biomechanical Measuring Apparatus

• A custom rig for measuring corneal deformation under simulated intraocular pressure was developed.
• The posterior cornea is pressurized, and the deformation of the anterior surface is measured, correlating with pressure values for testing
• This allows evaluating corneal stiffness by measuring the initial curvature and stiffness changes.

Pressure-Radius Curves

• RGE chicken corneas' pressure-apical rise data differs significantly to that of normal corneas.
• Further investigation into the RGE chicken cornea stiffness could lead to an understanding of its behavior under pressure.
• RGE cornea differs in pressure response and may indicate potential differences in tissue properties and how it responds to pressure changes.

Stress-Strain Curves and Stiffness

• RGE chicken corneas show higher material stiffness compared to normal corneas.

Structure Comparison (In-Plane SHG)

• In-plane collagen signals of normal and RGE corneas differ significantly in the posterior and anterior portions of the cornea.
• Normal corneas display a clear lattice structure, whereas RGE corneas show a more irregular, less organized structure, especially in the anterior stroma.

Aging Cornea

• Corneal Young's Modulus (E) is correlated to age, generally decreasing.
• A mathematical model predicts the relationship between age and corneal stiffness.

Why is the Cornea Transparent?

• The cornea's transparency results from a homogeneous nature of its constituents and the absence of opaque structures, such as blood vessels.
• Measurements of refractive indices are used to understand the role of different constituents (e.g., collagen, fluid) in light transmission; understanding the role of these indices can reveal their importance to transparency.
• The heterogeneous medium of fibers in the matrix of differing refractive indices plays a critical role in transparency.

Transmission of Visible Light Through the Cornea

• Measured values of light transmission through the cornea are presented as a function of wavelength for both normal and edematous corneas.
• These curves demonstrate the differences between the transmission characteristics of normal corneas and those affected by edema (swelling).

Light Scattering

• Light can be scattered by particles in a medium due to interactions, thus reducing transmission.
• Forward scattering (on-axis) has equal wavelet phases, whereas off-axis scattering has random wavelet phases due to the arrangement of scattering particles within the tissue.
• Spatial arrangement of particles and spacing between fibrils influences scattering and light transmission.

Maurice's Theory

• Maurice hypothesized that the arrangement of fibrils and their refractive index differences lead to light cancellation within the cornea's stroma, ensuring transparency.
• Irregular spacing and arrangement of fibrils, especially in Bowman's layer, suggests that a perfect lattice arrangement may not be strictly necessary for transparency.

Current Understanding of Transparency

• Current models suggest short-range ordering of fibrils in the cornea.
• This ordering maintains a regular shape and refractive index difference within acceptable limits from the wavelength of light.
• This short-range order minimizes fluctuations in refractive indices to achieve near-perfect transmission of light.

Direct Summation of Fields (DFS) Method

• This mathematical method predicts light transmission by considering the individual scattering contributions of different-sized fibrils in the cornea.
• This method accounts for interference effects for whole tissue, unlike previous methods, which did not.
• The DSF method proves helpful in understanding the influence of numerous factors—fibril density, size, and fibril-fibril spacing.

Experiment vs. Theory

• Comparing experimental light transmission data to theoretical predictions helps in evaluating the models used for predicting transmission characteristics.

Secondary Waves

• Examining secondary waves resulting from fibril arrangements helps in understanding light scattering and the conditions required to mitigate light scattering.
• The arrangement and positioning of the waves and fibrils are illustrated and represent scattering patterns from fibrils.

Corneal Ectasia

• Progressive thinning or stretching of the cornea can occur due to various factors, including loss of collagen and hydration disturbances.
• The thinning and stretching of the cornea can compromise its transparency and mechanical strength.

LASIK

• This is a popular laser refractive surgery for correcting nearsightedness.
• LASIK removes a layer of collagen to reshape the cornea, thus improving eyesight.
• This procedure can potentially affect the cornea's mechanical properties, which can lead to complications (e.g., ectasia).

Tissue Engineering the Cornea

• Creating synthetic cornea tissues with similar mechanical and optical properties presents a challenge, particularly in mimicking subtle conditions that result in transparency.
• Challenges include creating tissues that are sufficiently strong and robust.
• Approaches include stacking layers of oriented collagen fibrils, using appropriate gels for filling collagen voids and introducing appropriate cells (e.g., fibroblasts, epithelial, endothelial cells).

Cell-Based Engineering of Cornea

• Engineering a cornea using stem cells and ECM scaffolds is feasible (i.e., using human corneal cells and their different component cells outside the body/in a laboratory).