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Retina I

Retinal Landmarks and Layers, Optic Nerve & Photoreceptors


Learning Goals

  • Identify important retinal landmarks.

  • Anatomically describe the optic disc, macula, and fovea.

  • Differentiate between arteries and veins on the fundus.

  • Anatomically describe the retina, its layers, and neighboring structures.

  • Anatomically describe the Retinal Pigment Epithelium (RPE) and its main functions.

  • Explain the "blood-retina barrier."

  • Differentiate between the two types of photoreceptors found in the retina.

Introduction to the Retina

The Retina

  • Internal layer of the eye.

  • The optical image is formed here.

  • Nerve impulses are created at the retinal level and transmitted along the visual pathway for cortical (brain) processing.

  • This process is known as photo-chemical transduction.
       Reference: Snell & Lemp (1998) pg 176.

Characteristics of the Retina

  • Thin, transparent membrane.

  • Purple-red in color.

  • Thickest at the optic disc (approximately 0.56 mm) and thinner at the ora serrata (approximately 0.1 mm).

  • Thinnest at the fovea.

  • Extends from the optic nerve up to the ciliary body and iris on each side.

Structural Relations

  • Outer surface of the retina is in contact with Bruch’s membrane of the choroid.

  • Inner surface is in contact with the vitreous body.

  • Firmly attached at the margins of the optic disc and ora serrata.

  • Extends forward slightly more on the medial side (the side facing the nose).

Retinal Landmarks

  • The human retina appears distinct when viewed with ophthalmic instruments.

  • This view is referred to as the fundus.

Fundus Structure Orientation

  • Fundus structures are described as:   - Nasal   - Temporal   - Superior   - Inferior

  • When looking into the eye, the optic disc is positioned nasally, the macula is always temporal.

Eye Side Detection

  • If the optic disc is on the right-hand side, then it is the RIGHT eye; if it’s on the left side, then it’s the LEFT eye.

Optic Nerve

Optic Disc

  • Shape: Circular or oval structure.

  • Color: Pale pink, with the temporal side slightly lighter than the nasal side.

  • Size: Approximately 1.5 mm in diameter; used as a measurement unit against other structures.

  • Physiological Cup: A small depression in the center of the optic disc.

  • Disc Edges: Flat and well-defined; the temporal edge is more distinct than the nasal edge, evident in imaging.

Features of the Optic Disc

  • The retina may not always reach the optic disc margin, resulting in a small pigmented arc made up of choroidal pigment.

  • The central retinal artery appears on the surface of the optic disc, and divides into two branches:   - Superior   - Inferior

  • Optic nerve fibers usually myelinate at the lamina cribrosa (where they exit the eyeball). Early myelination may present as white feathery patches near the disc.

Retinal Vessels

Arteries and Veins

  • Arteries:   - Bright red in color.
      - Smaller than veins.
      - Have thicker walls, reflecting light in a shiny stripe.

  • Veins:   - Dark red in color.
      - Larger/thicker than arteries.

Variations of Retinal Vessels

  • There exists variability in the appearance of retinal vessels among individuals.

  • A common variation involves the presence of cilioretinal vessels, which are small arteries extending from the disc to the macula.

  • These vessels become significant when studying retinal vascular diseases.

Fundus Color

  • Fundus color depends on the amount of melanin in:   - Retinal Pigment Epithelium (RPE): Contains melanin, which:     - Absorbs excess light, reducing light scatter.     - Improves image clarity, providing sharper vision.   - Melanocytes in the choroid produce melanin, contributing to the pigmentation.   - Hemoglobin in the retinal and choroidal vasculature.

  • There is a correlation between fundus color and a person’s:   - Skin color.   - Hair color.   - Iris color.

Fundus Color by Ethnicity

  • North African: Dark hair, brown skin, brown eyes.

  • European: Blonde hair, fair skin, blue eyes.

  • Asian: Dark hair, olive skin, brown eyes.

Other Retinal Landmarks

Macula Lutea

  • Located at the center of the posterior part of the retina.

  • Always temporal to the disc.

  • Slightly darker in color compared to surrounding retina.

  • Responsible for detailed vision (e.g., reading, recognizing faces).

  • Contains a central depression known as fovea centralis.

The Macula’s Identification

  • The macula can be identified by the retinal vessels leading to it, marked by a yellowish reflection of the fovea.

Features Distinguishing the Macula and Fovea

  • The retina consists of two types of receptor cells: rods and cones.

  • The macula exclusively contains cones.

  • Cone density is greater in the macula than in any other region of the retina.

Foveal Anatomy

  • Inner retina neural elements are displaced to allow light to reach photopigment-containing outer segments of cones, creating the foveal pit.

  • The foveal pit maximizes visual acuity by:   - Reducing light scattering, allowing photoreceptors to receive light directly.   - Increasing cone density, leading to better resolution and color perception.

  • The pit shape aids in directing light toward densely packed cones, ensuring no blood vessels cross over it, thus not blocking incoming light.

Avascular Nature of the Fovea

  • The fovea is avascular, meaning no blood vessels are present around it.

  • It receives metabolic nourishment from the choroid, essential for optimal visual perception.

  • The fovea is structured to enable light to hit cone photoreceptors directly, ensuring the sharpest vision.

Vitreous (Revision)

Vitreous Body

  • Fills the eye between the lens and the retina, occupying approximately 80% of the eye’s volume.

  • Anteriorly features a saucer-shaped depression called hyaloid fossa for the lens accommodation.

  • It is a transparent gel composed of:   - 98% water.   - A dense cortex and liquid center.

Composition of the Vitreous Body

  • Colorless, transparent gel with a refractive index of 1.33 (similar to aqueous).

  • Contains:
      - Hyaluronic acid
      - Amino acids
      - Soluble proteins
      - Salts
      - Ascorbic acid

  • An organized network of collagen fibers; the cortex has more collagen than the central region.

Hyaloid Canal

  • A narrow channel from the disc to the rear of the lens that contained the hyaloid artery during fetal development, nourishing the lens.

  • The hyaloid artery disappears 6 weeks before birth and fills with liquid.

A Closer Look at the Retina

Complexity of the Retina

  • When cross-sectioned and observed under a microscope, the retina is found to be complex and contains various nerve cell types.

Division Into Layers

  • The retina can be divided into 10 layers (from outermost to innermost):   1. Retinal Pigment Epithelium (RPE)   2. Rods and Cones   3. Outer Limiting Membrane (OLM)   4. Outer Nuclear Layer (ONL)   5. Outer Plexiform (Molecular) Layer (OPL)   6. Inner Nuclear Layer (INL)   7. Inner Plexiform (Molecular) Layer (IPL)   8. Ganglion Cell Layer (GCL)   9. Nerve Fibre Layer (NFL)   10. Internal Limiting Membrane (ILM)

Retinal Pigment Epithelium (RPE)

  • Comprises a single layer of pigmented cells extending from the optic nerve to the ora serrata; located between photoreceptors and the choroid.

  • Bases of RPE cells rest on a basement membrane forming part of Bruch’s membrane of the choroid, thus forming the outermost layer of the retina.

Functions of the RPE

  • Functions include:   - Acts as a protective barrier maintaining tissue health.   - Absorbs light due to melanin presence, reducing light scatter and improving visual image clarity.   - Part of the Visual Cycle — formation of rhodopsin and iodopsin (photosensitive derivatives of Vitamin A).   - Involved in phagocytosis: the process by which the RPE removes waste from rods and cones by digesting large particles (e.g. bacteria, debris).   - Removes outer segments of the photoreceptors to maintain vision function.

RPE Microvilli

  • The ends of RPE cells feature brush-like microvilli that interdigitate with the rods.

  • RPE then phagocytoses debris, with lysosomes breaking down the debris completely.

Retinal Detachment

  • Possible issue where the RPE, developed from the outer layer of the optic cup, becomes detached from rods and cones (which developed from the inner layer of the optic cup), creating a space for separation.

The Blood-Retinal Barrier

  • The RPE cells are joined by tight junctions that encircle them, forming a barrier that limits ion flow and prevents large toxic molecules from diffusing from the choroidal capillaries.

  • This barrier is crucial for protecting the integrity and function of the neural retina.

Neural Retina

  • Formed embryologically from the inner layer of the optic cup and consists of three main groups of neurons:   - Photoreceptors   - Bipolar cells   - Ganglion cells
    Reference: Snell & Lemp (2008) pg 178.

Photoreceptors

Rods and Cones

  • Rods:
      - Function best in dim light, producing images in shades of black and white.
      - Count: 110-125 million; none are present at the fovea and increase in number from fovea to periphery.

  • Cones:
      - Function best in bright light, resolving fine detail and color.
      - Count: 6.3-6.8 million; most dense at the fovea and decrease in number towards the periphery.

Structural Characteristics

  • Both rods and cones are long, slender cells, named for their outer segment shapes.