The Normal Retina, Retinal Imaging and the Interpretation of Pathological Changes

Published on 08/03/2015 by admin

Filed under Opthalmology

Last modified 08/03/2015

Print this page

rate 1 star rate 2 star rate 3 star rate 4 star rate 5 star
Your rating: none, Average: 0 (0 votes)

This article have been viewed 1782 times

13 The Normal Retina, Retinal Imaging and the Interpretation of Pathological Changes

THE NORMAL RETINA

EMBRYOLOGY

The optic cup develops from the optic vesicle in the first 6–7 weeks of gestation and consists of two layers of neuroectoderm separated by a space. The outer layer of cells will eventually form the retinal pigment epithelium (RPE) and the inner layer the neurosensory retina; the potential space between them is re-established pathologically in later life in conditions such as rhegmatogenous retinal detachment or central serous retinopathy.

The inner layer of neuroectoderm (i.e. that adjacent to the vitreous gel) is initially about 10 cells deep. By 3 months of gestation it has differentiated into two layers, the inner and outer neuroblastic layers, separated by the transient layer of Chievitz. During the next 2 months of development the inner neuroblastic layer further differentiates: the presumptive ganglion cells appear first and migrate towards the inner retinal surface where they form the ganglion cell layer, the remaining cells migrate downwards to form the amacrine cells of the adult inner nuclear layer, the inner plexiform layer of nerve fibres and synapses forms in between. Müller cells are differentiated early on from the inner neuroblastic layer and then their nuclei migrate downwards to lie in the inner nuclear layer. The outer neuroblastic layer also contributes to the inner nuclear layer by supplying the horizontal and bipolar cells, their migration obliterates Chievitz’s layer. The photoreceptors are the final layer to be differentiated although at 13 weeks the inner segments of the cones can be seen in the macular area. Some authorities believe that the photoreceptors are derived from the neuroectoderm of the outer neuroblastic layer but it is more likely that they are adapted from ciliated cells derived from the ependymal layer lining the primitive neural tube and optic vesicle.

The overall adult arrangement of retinal layers is present by 5½ months’ gestation but retinal development is not uniform. For example, although photoreceptors first differentiate at the macula, this is soon overtaken by development in other retinal areas so that at birth the macula is the only area not to be developed fully—it is not completely developed until about 3–4 months after birth, when the baby starts to fixate.

The RPE starts to become pigmented at about 6 weeks of gestation and the process appears to be complete by 3 months when the epithelium is seen as a densely pigmented monocellular layer. Thus the RPE, derived from neuroectoderm, is fully pigmented before the process of choroidal pigmentation, derived from neural crest cells, has even begun.

ANATOMY OF THE RETINA

The photoreceptors detect light and the RPE cells provide their metabolic support. The inner retinal structure is supported by the Müller cells. These are glial cells which support the neuro retinal structure, they also have important metabolic functions. The remaining neuroretinal tissue integrates and processes visual information so that by the time the visual signal reaches the axons in the optic nerve there has already been a considerable processing of information.

The retina consists of just four layers of cells and two layers of neuronal interconnections. In histological sections of the retina, this simple six-layered structure appears lost and a far more complex multistratified appearance is seen arising from the juxtaposition of anatomically similar parts of adjacent cells. The two so-called ‘limiting membranes’ are formed by components of the Müller cells. Their nuclei are found in the inner nuclear layer, and the cell extends across the retinal layers between the limiting membranes. The outer limiting membrane, lying at the innermost aspect of the inner segments of the photoreceptors, is not a real membrane, but an alignment of junctional complexes between adjacent Müller cells and photoreceptor cells. In contrast, the inner limiting membrane on the retinal surface is a tough acellular membrane, laid down by the Müller cells and into which fibres from the hyaloid membrane of the vitreous cortex insert.

The retinal pigment epithelium

The RPE is a single layer of hexanocuboidal cells lying on Bruch’s membrane. The function of the RPE is to service and maintain the overlying photoreceptor cells and in order to do this it sustains five major processes. These are the absorption of stray light, active transport of metabolites in and out of the photoreceptor, the provision of a blood–retinal barrier, regeneration of visual pigments and phagocytosis of the photoreceptor outer segments.

The RPE cells contain fusiform granules of browny-black melanin pigment in melanosomes which absorbs light strongly between 400 and 800 nm and limits reflection or scattering within the eye, thereby protecting the photoreceptor cells from image degradation. With increasing age the melanin content of the RPE cells decreases and many of the pigment granules are degraded into phagosomes and melanolysomes.

In younger eyes the border of the RPE cell adjacent to Bruch’s membrane is extremely convoluted and many mitochondria reside within this portion of the cell. These convolutions increase the surface area of the cell membrane which is covered with specific biochemical binding sites. RPE cells actively accumulate and transport metabolites diffusing through Bruch’s membrane from the underlying choriocapillaris and actively excrete waste products to the choriocapillaris (the retinal artery circulation does not contribute to the metabolic needs of the photoreceptors). Each RPE cell services up to 45 photoreceptors held in close physiological contact by membranous extensions from the surface of the RPE, termed ‘receptor sheaths’. These sheaths extend up to 50 per cent of the height of outer segments and play a major role in metabolite exchange between the cells. The extracellular space between the photoreceptors is filled with a glycosaminoglycans ground substance called the interphotoreceptor matrix. This differs in structure and chemical composition around rods and cones. There is no anatomical bond between the RPE cell and photoreceptor so that these two layers can be easily separated pathologically (e.g. by retinal detachment). However, the cone matrix sheaths are more strongly adherent to the RPE and resistant to the cone outer segment being withdrawn; this may explain the increased resistance of the macula to retinal detachment.

The action of light on the visual pigments in the outer segments of the photoreceptor cells causes structural changes in the visual pigment so that the chromophore separates from the protein. In the rods this process is known as bleaching and results in the visual pigment rhodopsin being split into retinol and the protein opsin. The enzymes required for recombining the two into rhodopsin are situated in the RPE. The RPE, therefore, has a critical role in maintaining the visual cycle. Each RPE cell also functions as a static macrophage in that throughout life it phagocytoses the tips of the overlying rods and cones. The engulfed particles, known as phagosomes, are progressively degraded intracellularly by the action of lysosomes. The breakdown products are then recycled to be incorporated into the photoreceptor cells or voided from the RPE into the choriocapillaris. With increasing age this system becomes less efficient and breakdown products may accumulate in the RPE as lipofuscin. The highest concentration of photoreceptors to RPE cell occurs in the macula and throughout life the highest concentration of lipofuscin is found here. Lipofuscin exhibits a strong autofluoresence and high levels are thought to be a risk factor for age-related macular degeneration. The mechanism may involve absorption of blue light resulting in RPE damage from the generation of free radicals and apoptosis.

The photoreceptor cells

Although the photoreceptor cells are of two distinct types (rods and cones), both show the same basic structural organization. The cells are elongated and their cytoplasmic components arranged in such a way that different functions take place at specific positions along their length. The photoreceptor cell transduces light to neuronal signals. The action of light on the photoreceptor surprisingly results in the ‘switching off’ of the cell. The sequence starts with photons being absorbed by visual pigment in the outer segment that then undergoes conformational change. The molecular changes in the visual pigments cause the release of an internal transmitter from the discs that passes in turn to the boundary membrane of the cell inducing membrane alterations and hyperpolarization. The net flow of current around the photoreceptor is changed and switches off the release of neurotransmitter at the cell’s synapse. On returning to darkness the situation reverses and the neurotransmitter is switched on again.

image

Fig. 13.7 The structure of a photoreceptor consists of:

• An outer segment, which is the light-sensitive portion of the cell and consists of a stack of hollow coin-like discs whose membranes contain the visual pigment molecules. In each rod there are about 1000 discs that are separate from all the others and the boundary membrane of the cell. Rod outer segments can be thought of as analogous to a stack of coins in a tube. If the tube is broken and the stack disturbed individual coins will be lost. In cones the outer segments differ in that all the ‘discs’ are joined to the boundary membrane with an aperture; through this hole the discs are in contact with the extracellular space and, therefore, with each other. Consequently, it is not possible to isolate a single ‘disc’ from a cone outer segment.

• A constricted region called the cilium which resembles the structure of other cilia in that it contains a number of paired microtubules.

• An inner segment which is the manufacturing portion of the cell; in each case this is divided into two, an outer ellipsoid and an inner myoid. The ellipsoid contains mitochondria and provides energy for the transduction processes in the outer segment, and the myoid contains Golgi bodies and ribosomes for manufacture of cell components and membranes.

• An outer connecting fibre that runs from the inner segment to the nucleus. In cones this tends to be short as the nuclei are situated in the outer part of the outer nuclear layer close to the outer limiting membrane. For rods the length varies with nuclear position.

• A nucleus.

• An inner connecting fibre that runs from the nucleus to the synaptic region. In cones it is this structure that becomes elongated as the fibres run out of the foveal pit to make contact with the displaced intermediary neurones and form the fibre layer of Henlé.

• A synaptic region. In rods this is sometimes called the rod spheral and in cones the cone pedicle. In both cells the synaptic region contains vesicles and mediation of transsynaptic information is by chemical transmitters. Both cells exhibit so-called invaginated connections with components from intermediary neurones deep to their synapse surface. These invaginated synapses are called ‘triads’ because they contain three processes, usually one from a bipolar cell and two from horizontal cells. Rods have a single triad whereas cones may have up to 20.

The inner retina

The inner nuclear layer contains the cell bodies of bipolar cells, horizontal cells, amacrine cells and the Müller cells which are a specialized glial cell. The bipolar cell was once thought to be the only retinal neurone to connect the outer plexiform layer with the inner plexiform layer thus effectively providing a channel for information from the photoreceptor cells to the ganglion cells. In reality, the bipolar cells do not connect photoreceptors directly to ganglion cells because in the outer and inner plexiform layers respectively the horizontal and amacrine cell processes intervene. Thus, horizontal cells connect groups of photoreceptor cells together and modify their group output through the triads in the photoreceptor synapses before the signal is passed by the bipolar cell to the inner plexiform layer. In the inner plexiform layer the bipolar cells commonly synapse with amacrine cells which in turn modify the signal before it is passed to ganglion cells.

Müller cells are specialized glial cells that form the scaffolding of the retina. As they approach the vitreous cortex they expand to form a large footplate which can sometimes be seen clinically in the posterior pole as tiny reflecting spots known as Gunn’s dots. Müller cells produce the fibrous, acellular, inner limiting membrane, a true membrane that is extremely strong. The outer limiting membrane is not a true membrane but rather the close alignment of specialized junctions of the Müller cells around the outer connecting fibres of the photoreceptors. Apart from supporting the retinal structure Müller cells are associated with nutrition of the photoreceptor inner segments and the generation of neuronal impulses. They act as an ionic reservoir during hyperpolarization of the photoreceptor by light and this is reflected in their contribution to the b-wave of the electroretinogram. They also proliferate to form the major element of scar tissue or gliosis which is the retina’s characteristic response to cell death or disease.

The ganglion cell layer, together with its nerve fibre layer, is the innermost layer of the retina. By the time the neuronal signal is generated from these cells information has been coded to a considerable extent. The retina responds to changes rather than to a steady state so that changes in contrast or movement of edges are important stimuli. Each neuronal cell in either the inner nuclear layer or ganglion cell layer has its own receptive field (i.e. an ultimate connection with a group of photoreceptors) but receptive fields have been most studied at the ganglion cell level. Foveal cones are said to have a 1:1:1 relationship with a bipolar cell and ganglion cell, thereby producing a highly specific pathway. Outside the fovea photoreceptors are grouped into larger receptive fields. If taken on average, one ganglion cell would service 130 photoreceptors, but, because the macular area is served by over 50 per cent of the ganglion cells, receptive fields in this area are far smaller than those in the periphery.

Some ganglion cells are turned off by a bright light projected into the centre of their receptive field; others are turned on, in effect giving a response to white on black or black on white. Rods and cones can be integrated in receptive fields; some receptive fields respond to a different colour or cone population when stimulated centrally or peripherally. Some ganglion cells give a sustained discharge if light is projected on to their photoreceptors whereas others fire only transiently and still others respond to a moving edge. There is good physiological evidence to link midget ganglion cells to spatial discrimination and visual acuity in the parvocellular pathway, whereas parasol cells are concerned with detection of motion and direction in the magnocellular pathway. Recently it has been shown that some ganglion cells contain a light-sensitive pigment called melanopsin with an absorption peak of 470 nm. These cells are thought to be an important element in the control of various diurnal rhythms.

Topographical variation in the retina

Clinically the macula is the area within the temporal arcades subserving the central 20° of visual field, the fovea is the darkened area of light reflex and the foveola is the central pit with a population exclusively of cones. The fovea is darker than the surrounding retina, partly because of the presence of yellow xanthophil pigment in the neural retina and partly because the underlying RPE cells in this region are smaller and more densely pigmented. The foveola contains only about 2500 cones, which have long thin outer segments. Cones are found in greatest density (15000/mm2) in the fovea decreasing to about 4000–5000/mm2 in the macula. In contrast, rods achieve their greatest density at about 20° from fixation (the field isopter just peripheral to the optic disc). In total, the young adult retina contains about 120 million rods and 6 million cones.