The Retina and Vitreous

The Retina and Vitreous

CHAPTER 11.

THE RETINA AND THE VITREOUS

BY THE END OF THIS CHAPTER YOU WILL HAVE COVERED THE

FOLLOWING TOPICS:

• 1. DEFINITION
• 2. GROSS ANATOMY
• 3. HISTOLOGY
• 4. THE RETINAL BLOOD SUPPLY
• 5. RETINAL BIOCHEMISTRY AND PHYSIOLOGY
• 6. COLOUR VISION
• 7. CLINICAL CONSIDERATIONS
• 8. THE VITREOUS.
• 1. DEFINITION.
• The BEEN is the inner neural layer or the nervous coat of the eye.
• It contains receptors that sense the light from the outer world and transmit
• it to the brain for higher processing.
• It is a thin transparent membrane with a purplish red colour in a living
• subject. Its thickness varies from 0.6mm near the optic disc to 0.1mm at its
• peripheral termination called the ora serrata.
• The outer surface is in contact with the choroid and the inner surface with
• the vitreous.
• The retina is firmly attached to the optic disc margin and at its peripheral
• a
• Externally, the peripheral termination of the retina corresponds with the site
• of insertion of the medial and lateral recti muscles.
• 2. GROSS ANATOMY.
• There are several prominent structures in the retina that can be identified
• with the help of an instrument called the ophthalmoscope.
• The procedure is called ophthalmoscopy or funduscopy , and the part of the
• eye visible on funduscpoy is called fundus.
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• 2.1. The optic disc.
• This is most prominent structure of the retina and it is located nasally.
• It is a yellow-pink ,oval to round structure, also called papilla or optic nerve
• head .
• It represents the beginning of the optic nerve,( the second cranial nerve-
• the nerve that is responsible for vision ).
• The optic nerve head or the optic disc measures 1.75 mm vertically and
• 1.5mm horizontally in diameter.
• The centre of the disc has a circular depression that appears whiter than the
• rest of the disc.
• This is called the cup of the disc and is a location where the retinal arteries
• Fine vessels are seen to pass over the surface of the disc, which give it the
• pinkish appearance.
• The retina is firmly attached to the margins of the disc.
• There is no retinal tissue over the disc,thus it is insensitive to light and is
• referred to as the blind spot.
• 2.2. The area centralis.
• ‘Temporal to the disc is an area called the area centralis.
• This is the most posterior part of the globe and central part of the retina.
• Clinically, this area is called the posterior pole.
• It measures about 5-6 mm in diameter and is enclosed within the
• At the centre of the posterior pole is an area measuring about 1.5 mm called
• the macula lutea by the clinicians and fovea by the anatomists.
• It has a yellowish appearance when seen by an ophthalmoscope due a yellow
• pigment called the xanthophyll.
• The macula lutea or the fovea is located about 3mm temporal and 1mm
• inferior to the disc.
• The photoreceptor layer of the fovea contains only cones.
• The centre of the fovea is depressed and is called the fovea centralis by the
• clinicians and foveola by the anatomists.
• It measures about 0.35mm in diameter.
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• It is thus apparent that anatomists and clinicians differ in their terminologies
• as

follows:

ANATOMISTS CLINICIANS

• Area centralis Posterior pole
• Fovea Macula lutea (or just the macula)
• Foveola Fovea centralis ( or just the fovea)
• As clinicians, we will stick to the clinical terminologies henceforth to avoid
• confusion.
• The macula is responsible for central and colour vision. The sharpest central
• vision is achieved at the fovea
• 2.3. The peripheral retina.
• The remainder of the retina outside the posterior pole is termed peripheral
• retina, although further subdivision exists but that is beyond the scope of
• this work.
• The peripheral termination of the retina shows teeth like projections called
• the ora serrata.
• The retinal cells at the ora serrata continue over the ciliary body to form the
• non- pigmented epithelial layer of the pars plana.
• In the peripheral retina the predominant photoreceptors are the rods.
• The retina is divided into temporal and nasal halves by an imaginary line that
• runs vertically through the fovea. _
• The centre of the optic disc is used to divide the retina into 4 quadrants:
• e Supero- nasal;
• ° supero-temporal;
• e infero-nasal;
• e infero-temporal.
• This division helps the clinician locate and document the position of lesions
• on the surface of the retina.
• With this division in mind one can visualise the retinal vessel's distribution
• on the surface of the retina.
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• The retinal artery emerges as a single vessel called the Central Retinal Artery
• , which then divides in to 2 branches the superior and inferior retinal arteries
• Each of these then further divides into 2 branches, one for the temporal and
• the other for the nasal quadrants of the retina.
• The same applies with the distribution of the Central Retinal Vein.
• Fig. 11.1. Anatomical landmarks of the left eye fundus.
• A- anatomical macula / clinical posterior pole;
• B- anatomical fovea / clinical macula;
• C- anatomical foveola / clinical fovea;
• D- optic disc;
• E- optic cup.
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• GD ait with wes offic
• 3. HISTOLOGY.
• The retina, the layer which develops from the inner and outer layers of the
• embryological optic cup, is divided into two major portions:
• e The Retinal Pigment Epithelial layer ( RPE);
• e The Neurosensory layer.
• 3.1. The Retinal Pigment Epithelium (RPE ).
• a. Anatomical features.
• The RPE is a GifigléllayerOnicellS deriving embryologically from the outer —
• layer of the optic cup.
• It is located between the choroid and the neurosensory retina, the apices of
• the cells pointing towards the vitreous while the base rests on its basement
• membrane towards the choroid.
• The RPE is a continuous monolayer of cuboidal / columnar cells which
• extend from the optic disc margin to the ora serrata.
• From ora serrata it continues over the ciliary body as the pigmented
• epithelium of the pars plana and pars plicata.
• The apical surface of the RPE shows many microvilli. The retinal
• photoreceptors, the rods and cones, occupy the spaces between the
• microvilli.
• The adjacent cell membranes are strongly bound together .These tight
• junctions are very important in maintaining the isolation of the retina from
• the systemic circulation (outer blood-retina barrier)
• The nuclei of cells are large, the Golgi apparatus, mitochondria and
• endoplasmic reticulum are well developed. In other words, the RPE is well
• equipped for metabolic activities.
• The RPE cells also contain numerous melanin granules that extend into the
• microvilli.
• \Lysosomes are present in large number too, in the RPE cells. They play an
• important role in phagocytosis.
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• b. Functions of the RPE .
• The RPE has many important physical, optical, biochemical and metabolic
• functions that ensure t
• e Maintaining adhesion to the neuroretina ( maintains the subretinal space).
• A potential space (the remains of the cavity of the embryological optic cup)
• exists between t!
• Normally, this space is closed and the two layers are closely apposed to each
• other by the physiological properties of the RPE.
• The RPE has a high capacity for water transport , hence fluid does not
• accumulate in the subretinal space , under normal circumstances.
• In certain conditions fluid accumulates between these two layers and the
• neurosensory retina separates from the RPE layer. This is called retinal
• detachment.
• After successful retinal detachment surgery , the RPE pumps out the residual
• fluid and within a short time the retinal layers appose once again.
• e Acting as a barrier between the choroid and the retina (the outer blood
• retinal barrier ) .
• The tight junctions between the RPE cells allow only selected elements to
• pass through the choroidal circulation into the retina. This is the outer blood
• retinal barrier.
• e Phagocytosis.
• The outer portions of the retinal photoreceptors continually shed off and the
• RPE cells phagocytose the debris produced from this process.
• e Absorbtion of excess retinal light and heat.
• The RPE cells contain a lot of melanin granules . These darkly pigmented
• granules absorb excess light that enters the eye and thus reduce light
• scatter.
• This process improves image resolution and quality. It also absorbs the heat
• energy that accompanies light energy.
• e Vitamin A metabolism.
• The RPE cells are actively involved in storage, transport and metabolism of
• vitamins especially vitamin A.
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• 3.2. The Neurosensory Retina:
• The neurosensory retina is the inner layer which is in contact with the
• vitreous. It is thin and transparent and appears purplish red in a living
• subject.
• Embryologically derives from the neuroectoderm of the inner layer of the
• optic cup.
• In cross section, from outer to inner retina, its layers are:
• photoreceptor layer;
• external limiting membrane;
• outer nuclear layer;
• outer plexiform layer;
• inner nuclear layer;
• inner plexiform layer;
• ganglion cell layer;
• nerve fibre layer;
• internal limiting layer.
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• Internal limiting membrane
• Nerve fiber layer
• |___ Ganglion cell layer
• Fig. 11.2. The layers of the retina.
• A —amacrine cells;
• B – bipolar cells;
• C -cones;
• R -rods;
• G -ganglion cells;
• Inner plexiform layer
• }—— Inner nuclear layer
• Outer plexiform layer
• | Outer nuclear layer
• External limiting membrane
• Retinal pigment epithelium
• Bruch's layer
• M -Muller cells.
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• The neurosensory retina consists of several cell types but can be
• categorised into 2 groups:
• @ neuronal cells
• @ non-neuronal cells.
• 3.2.1. Neuronal cells.
• The main groups of neurones are :
• a. The photoreceptor cells ( The Rods and Cones) ;
• b. The bipolar cells;
• c. The ganglion cells.
• d. The horizontal cells.
• e. The amacrine cells.
• a. The photoreceptor cells.
• The photoreceptors are similar to sensory receptors elsewhere in the body.
• They get stimulated by alight impulse .
• There are two types of photoreceptors, the rods and the cones.
• e THE RODS
• The rods are mainly responsible for:
• Vision in dim light;
• Sensing contrast;
• Brightness ;
• Motion.
• Its estimated that there are approximately 110-125 million rods in the retina,
• with maximum density in the mid periphery.
• There are no rods at the fovea.
• The rods are ‘rod like’, i.e. long and narrow.
• The outer ends are embedded between the RPE microvilli and are called the
• outer segment.
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• The outer segment is the true photoreceptor of the cell, as it contains the
• photosensitive pigment the rhodopsin.
• The rhodopsin, a light sensitive molecule, is derived from vitamin A bound to
• aprotein known as opsin.
• Complex biochemical reactions take place here. Light energy is converted —
• into biochemical energy then into an electrical impulse that gets propagated —
• ‘in the optic nerve. This is the process of phototransduction.
• The inner segment contains the cellular organelles like the nucleus, the
• mitochondria and the endoplasmic reticulum.
• e THE CONES
• The cones are mainly responsible for :
• e Fine resolution;
• e Spatial resolution;
• ¢ Colour vision.
• Its estimated there are 6.5 million cones, with maximum density at the
• macula, the fovea being exclusively cone dominated.
• Cones are similar to rods except the outer segment of cones is more conical
• hence the name.
• Physiologically 3 types of cones are known:
• + the Red cones stimulated by red light;
• + the Green cones stimulated by green light;
• + the Blue cones that get stimulated by the blue light.
• b. The bipolar cells.
• The cells act as intermediary between the photoreceptors and the ganglion
• cells , i.e they connect the photoreceptors to the ganglion cells. _
• They also synapse with horizontal cells and amacrine cells.
• There are several types of bipolar cells with different synaptic characteristics
• but these will not be discussed here.
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• c. The ganglion cells.
• The ganglion cells synapse with the bipolar cells and thus get connected to
• the photoreceptors.
• The nerve fibres converge towards the optic disc , where they all meet and
• exit the eye in one bundle, the optic nerve.
• There are approximately 1.2million ganglion cells in the retina.
• d. The horizontal cells.
• These cells are found at the axonal end of the photoreceptors lying
• horizontally and parallel to the retinal surface.
• They are multipolar and havéjonellong/and/several/shortiprocesses)One
• horizontal cell connects to as many as 7-10 photoreceptor cells and the long
• Processes connect to distant photoreceptors as well as bipolar cells.
• The horizontal cells respond to théneurotransmitters released by the
• photoreceptors following excitation by light and they in turn release
• neurotransmitters that inhibit bipolar cells some distance away.
• e. The amacrine cells.
• These are large bodied cells with abundant cytoplasm, lobulated nuclei and
• long processes that radiate widely.
• They are located close to the ganglion cell bodies and synapse with one
• another, with the dendrites of the ganglion cells and with the axons of the
• bipolar cells.
• The bipolar cells stimulate the amacrine cells, which then stimulate the
• Ganglion’ cells) This route ensures that the distant ganglion cells are also
• excited.
• 3.2.2. Non-neuronal cells.
• Glial cells;
• Microglia;
• Vascular endothelial cells;
• Pericytes.
• aooD
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• Glial cells (supporting cells ) are cells that support the neural framework of
• the retina. Possibly, they also assist in nourishing the retinal neurones.
• Muller cells are the most prominent glial cells. These long, narrow cells
• ,extend through almost the whole thickness of the retina filling in most of
• the space not occupied by neurons.
• The vascular endothelial cells and the pericytes form the inner blood retinal
• barrier.
• These cells ensure that only selected substances ( small molecules ) cross
• from the intravascular compartments into the retinal layers.
• This is important clinically because diseases that affect the vascular
• endothelium and the pericytes eg diabetes mellitus, cause a break down of
• this barrier resulting into leakage of ‘unwanted ‘ substances into the retina
• from the blood.
• Such a leakage leads to retinal oedema, then deposits of substances like fat
• and proteins on the retina, and ultimately to impaired vision.
• 4. THE RETINAL BLOOD SUPPLY.
• The retinal arterial blood supply is derived from:
• a. The Central Retinal Artery (CRA), which supplies the inner 2/3% of the
• retina.
• The CRA is a branch of the ophthalmic artery.
• From the ophthalmic artery, the central retinal artery penetrates the optic
• nerve and comes out in the eye at the optic disc cup.
• The central retinal artery then divides into 2 main branches the superior and
• the inferior branches.
• These are the hemi branches that further divide into temporal and the nasal
• branches and each of these divide and subdivide within its quadrant into
• Thus ,the sequence would be: central retinal artery, superior and inferior
• hemi retinal artery, supero- temporal, supero- nasal, infero- temporal and
• infero- nasal retinal arterioles.
• The arterial branches run in the nerve fibre layer then divide into arterioles.
• The arterioles are distributed in different retinal layers. There are no
• anastomoses between the arterioles.
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• b. The Choriocapillaris supplies the outer 1/3" of the retina.
• The choroidal vessels originate also from the ophthalmic artery but reach the
• ‘eyeball via the posterior ciliary arteries. They are described in another
• chapter .
• The pattern of the veins is similar to that of the arteries, the arteries having
• the tendency to lie superficial to the veins.
• The Central Retinal Vein formed by tributaries that accompany the arteries,
• empties into the ophthalmic vein .The ophthalmic veins drain into the
• cavernous sinus.
• The central part of the macula, the fovea, is devoid of blood vessels.
• This is the Foveal Avascular Zone (FAZ) measuring approximately 0.5mm in
• diameter. It is also called the capillary free zone and it is an important
• clinical landmarkiinjlaser/photocoagulation®
• Ophthalmoscopically, the veins and venules appear darker, bluish red in
• colour and arteries and arterioles are paler, light red in colour.
• An important feature of the retinal vasculature is that the arterioles do not
• anastomose with each other and are therefore an end-artery system,
• meaning that each quadrant has its own independent arterio- venous system
• and any occlusion along this system will result in that quadrant being
• affected immediately.

5. RETINAL BIOCHEMISTRY AND PHYSIOLOGY.

(HOW DOES THE EYE SEE?)

• The main function of the posterior segment of the eye, including retina, is to
• However, brain does not have light receptors and thus light cannot be
• transmitted to the brain. Light has to be transformed into electrical impulses
• that are then conducted in the optic nerve to the occipital cortex, the visual —
• The process of transforming light energy into electrical energy through a
• series of biochemical reaction, is called phototransduction.
• This process takes place in the photoreceptors (the rods and the cones ), by
• series of very complex biochemical reactions involving Vitamin A and the
• visual pigments (i.e. rhodopsin).
• When light falls on the photoreceptors, the visual pigment gets activated or
• bleached.
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• This bleaching is coupled with the photoreceptor ce// membrane ion
• movements, causing generation of a nervous impulse.
• The nerve impulse generated from this reaction in the rods and cones, gets
• transmitted to the optic nerve via horizontal, bipolar and ganglion cells.
• The impulses are then processed along the visual pathway at various places
• and ultimately terminate in the occipital cortex where final processing (the
• sensation of vision )takes place.
• 6. COLOUR VISION.
• Seeing colours is one of the important functions of the eye.
• The human eye can recognise about 150 different colours in the visible
• spectrum.
• Cones are specialised to do this work. There are three main types of cones,
• red, green and blue cones.
• The three types of cones form the primary colour cells.
• Each one has its spectral sensitivity, meaning that certain wavelengths in the
• visible light spectrum stimulate one cell type more than the others, giving
• rise to the sensation of color in that particular wavelength.
• Thus, blue receptors have a spectral sensitivity that peaks at 440-450nm;
• green receptors at 535-555nm and the red ones at 570-590nm.
• 7. CLINICAL CONSIDERATIONS.
• 7.1. Common symptoms of retinal disease.
• a. Floaters.
• Floater is the perception of asmall black moving spot (frequently “ like a
• fly”), in the field of vision.
• This denotes posterior vitreous detachment i.e. the vitreous face has
• detached from the surface of the retina.
• b. Light flashes ( photopsias ).
• These are subjective sensations perceived as flashes of light. Are caused by a
• traction on the retina.
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• Should be differentiated from photopsias caused by migraine.
• c. Visual field defects.
• Are perceived as a sensation of something obstructing the vision in a
• particular quadrant, sometimes described by a patient as if ‘a black curtain
• is hanging down’. This is classical description of the retinal detachment.
• Can be also caused by a vessel occlusion.
• d.Night blindness.
• Characteristic of retinitis pigmentosa and vitamin A deficiency, it is also
• called
• nictalopia.
• 7.2. Common retinal disorders.
• a. Retinal detachment.
• This happens when fluid accumulates between the sensory retina and the
• RPE causing it to separate. The sensory retina is then lifted off towards the
• vitreous cavity.
• b. Retinal vessel occlusion.
• The central retinal artery (CRAO ) or vein (CRVO ) or one of the branches
• (BRAO and BRVO ) may get occluded, usually by atthrombus.
• c. Hypertensive and Diabetic Retinopathy.
• As the name suggests, affects hypertensives and diabetics.
• d. Retinitis Pigmentosa.
• Characteristic is the retinal dystrophy, whereby the rods are progressively
• destroyed, resulting in progressive visual deterioration.
• e. Age Related Macular degeneration (ARMD ).
• As the name suggests, this is an age related problem affecting the macula.
• The macula gets scarred resulting in blindness.
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• 8. THE VITREOUS.
• 8.1. Definition, characteristics.
• The vitreous is a clear gel-like substance that fills the intraocular cavity
• behind the lens.
• Its volume is 4 mls, occupying 4/5 of the volume of the eyeglobe.
• Contains: 99% water ,some salts, soluble proteins and hyaluronic acid. The
• high viscosity of the vitreous (twice that of water ) is due to the presence of
• hyaluronic acid.
• Collagen is the major structural protein component of the vitreous.
• 8.2. The functions of the vitreous are:
• e to transmit light ;
• e to act as a shock absorber;
• e to support the posterior part of the lens;
• to support the retina;
• e to maintain the spherical shape of the globe.
• 8.3. Pathological aspects.
• The vitreous is strongly attached to the retina at the ora serrata and the
• optic disc
• Elsewhere the attachment to the retina is weak , and in pathologic
• conditions the vitreous is easily detached.
• Embryological remnants are commonly seen, none of them visually
• significant:
• e Mittendorf dot —a white dot on the posterior lens capsule;
• ¢ Bergmeister ‘s papilla — a fibroglial tuft at the optic disc;
• ¢ Cloquet’s canal persistence — persistence of the entire hyaloid
• artery;
• Persistent Hyperplastic Primary Vitreous results from failure of the primary
• vitreous to regress.
• A white, vascularized, fibrous membrane is present behind the lens and
• leukocoria, (white pupil ) is often noted at birth.

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