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Anatomy and Physiology of the Retina 2 Göran Darius Hildebrand and Alistair R

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Anatomy and Physiology of the Retina 2 Göran Darius Hildebrand and Alistair R Anatomy and Physiology of the Retina 2 Göran Darius Hildebrand and Alistair R. Fielder 2.1 Introduction optic disc and is about 1.5 mm, or one disc size, in diameter [7]. The center of the macula lies just below the horizontal meridian [8], a relationship that is used The retina remains the best studied part of the human to study rotation during incyclo and excyclotorsion. brain. Embryologically part of the central nervous sys- The presence of xanthophyll, a yellow carotenoid pig- tem [1–5], but readily accessible to examination, it can ment, gives the region its name – the macula lutea. be investigated with relative ease by both scientists and The most central part of the macula, the fovea (ana- clinicians. Moreover, an estimated 80% of all sensory tomic foveola), is formed by a central, circa 0.35 mm- information in humans is thought to be of retinal origin wide depression and represents the retinal region of [6], indicating the importance of retinal function for the greatest visual acuity [9]. Clinically, it is recognized ability to interact with the outside world. In this chapter, by the foveal reflex, blunting or loss of which may we examine the retina’s unique cytoarchitecture and indicate early macular disease. The foveola is demar- how it is assembled to give rise to its sophisticated cated by a sloping wall, the clivus, which contributes neurocircuitry. Most of our knowledge is based on stud- to the annular light reflex that is seen in children and ies in primates and adult humans, but reference is made young adults. The foveola has the highest density of to the development of the retina wherever possible. cone photoreceptors (199,000/mm2), which are nar- rowed and elongated in this location to maximize light detection further [10]. The long axons of the foveal 2.2 Anatomy of the Retina cones form Henle’s layer as they radiate out of the cen- tral depression. The fovea develops by an opposing 2.2.1 Topographic Organization process of outward displacement of the cells of the inner nuclear and ganglion cell layers, while the cone of the Retina photoreceptors migrate toward the center [11–13]. Rod photoreceptors are excluded from the foveal outer ret- The adult posterior pole (anatomic macula or area cen- ina (“rod-free zone”). As a result, the foveola contains tralis) is about 4.5–6 mm in diameter, centered on the only cone photoreceptors and some Müller cells. The fovea, and located between the superior and inferior central 500 mm of the fovea contains no retinal capil- temporal arcades. The macula (anatomic fovea centra- laries (the foveal avascular zone [FAZ]), making the lis) is located approximately 3 mm temporal to the fovea dependent on blood supply from the choriocapil- laris. The exact extent of the FAZ can be delineated with accuracy only by fluorescein angiography. Retinal G.D. Hildebrand blood vessels from the temporal retina do not cross the Department of Pediatric Ophthalmology, Great Ormond Street Hospital for Children, London, UK central fovea but arc around it. The peripheral retina comprises the remaining retina A.R. Fielder (*) outside the temporal retinal arteries. Anatomically, the Department of Optometry and Visual Science, City University, Northampton Square, London EC1V 0HN, UK peripheral retina possesses only one layer of ganglion e-mail: [email protected] cells. The ampullae of the vortex veins lie just posterior J. Reynolds and S. Olitsky (eds.), Pediatric Retina, 39 DOI: 10.1007/978-3-642-12041-1_2, © Springer-Verlag Berlin Heidelberg 2011 40 G.D. Hildebrand and A.R. Fielder to the equator, while the long posterior ciliary arteries the RPE can proliferate in response to a variety of and nerves mark the horizontal meridian. The ora ser- pathological conditions. Melanin renders the RPE dark rata delineates the anterior termination of the sensory brown to black. It is synthesized from tyrosine via the retina and the beginning of the pars plana of the ciliary tyrosinase pathway [30]. Pigmentation of the RPE is a body. At this junction, the sensory retina is reduced to a rapid process that begins at about 35 days gestation single cell layer which, anteriorly, becomes the nonpig- and is complete within approximately 1 week [31]. mented ciliary epithelium whereas the retinal pigment Pigmentation of choroidal melanocytes, in contrast, epithelium (RPE) is replaced by pigmented ciliary epi- does not start before the fifth month of fetal life and thelium. Junctional complexes between the pigmented continues postnatally. Unlike choroidal melanocytes, and nonpigmented ciliary epithelia abolish the potential which are neural crest-derived, RPE cells show no or subretinal space that exists between the RPE and the little racial variation in melanin pigmentation. Another neuroretina, making the pars plana a relatively safe site pigment, lipofuscin, accumulates as an end-product of for surgical access to the posterior segment. outer photoreceptor segment degradation in the RPE and in Bruch’s membrane. Though lipofuscin is a pig- ment of aging, small amounts of it can already be detected in the RPE of children [28]. 2.2.2 Cellular Organization of the Retina The major cellular components of the retina are the 2.2.2.2 Photoreceptors RPE cell, the photoreceptor cells, the interneurons, the ganglion cells, and the glial cells. Vascular cells are The photoreceptors are the sensors of the visual system described in greater detail later. that convert the capture of photons into a nerve signal in a process called phototransduction [32]. The human retina contains approximately four to five million cones 2.2.2.1 Retinal Pigment Epithelium and 77–107 million rods [32–34]. Only cones are found in the foveola, whereas rods predominate outside the Like the sensory components of the neuroretina, the foveola in the remaining fovea and all of the peripheral RPE cell is of neuroectodermal embryonic origin [1–5, retina. Among the three cone photoreceptors, red cones 14, 15]. Each adult human retina contains about (63% or 2.9 million) are more common than green 3.5 million RPE cells [16] whose diameters vary four- (32% or 1.4 million) and blue cones (5% or 0.2 million) fold between 14 mm in the central retina and 60 mm in [9]. Each photoreceptor consists of an outer segment the peripheral retina [17]. The density of RPE cells is (photopigment), inner segment (mitochondria, endo- greater in the fovea (5,000 cells/mm2) than in the plasmatic reticulum), a nucleus, an inner fiber (analo- periphery (2,000 cells/mm2) [18]. In the central retina, gous to an axon), and the synaptic terminal [35]. The where RPE cells are most tightly packed, they take the outer segment contains the photon-capturing photopig- shape of regular hexagonal tiles that form a single ment. Opsin is a transmembranous protein that anchors layer of cuboidal epithelium. Tight junctions between the photopigment in the plasma membrane. In the outer adjacent RPE cells form the outer blood-retina barrier, segments, the plasma membrane is stacked into hun- an important physiologic barrier to the free flow of dreds of flat discs, thereby increasing the density of molecules between the leaky choriocapillaris and the retinal-opsin photopigment per photoreceptor cell. The photoreceptors of the neuroretina [19–25]. discs in cones are deep invaginations of the outer seg- Cellular polarity and the abundance of mitochon- ment membrane, while in rods, the discs are separate dria, endoplasmatic reticulum, and free ribosomes all from the outer segment (except at the base). Shed indicate a very high level of metabolic activity in the discs are phagocytosed by the RPE. A nonmotile cil- RPE cell [23, 24, 26]. Infoldings of the basal and api- ium connects the outer and inner segments. The inner cal surfaces greatly increase the RPE surface area, segment contains the cellular machinery necessary to facilitating active transport across its cell surface with meet the high metabolic requirements of the photore- both the choriocapillaris and the photoreceptor layer ceptor cells. Its outer portion (the ellipsoid) is packed [7, 18, 24, 27–29]. Though normally nondividing cells, with mitochondria that produce ATP by oxidative 2 Anatomy and Physiology of the Retina 41 phosphorylation, while the inner portion (the myoid) within the IPL in which they synapse. The dendritic contains smooth and rough endoplasmic reticulum for fields of amacrine cells vary between less than 100 mm synthetic activity as well as microtubules for intracel- (narrow-field) and greater than 500 mm (large-field) lular transport. The photoreceptor nucleus contains all [39]. Similarly, amacrine cells are uni-, bi-, or multi- nonmitochondrial DNA. The inner fiber is the axon of stratified, creating connections both within and between the photoreceptor cell and transmits the photoreceptor the different strata of the IPL. Examples of different cell signals to the outer plexiform layer (OPL) via its amacrine cell types are the narrow-field, multistratified synaptic terminals. Due to the absence of inner nuclear AII amacrine cell that is involved in scotopic vision, layer cells in the foveola, foveolar inner fibers have to and the wide-field starburst amacrine cell that is travel to the OPL in the surrounding macula to make involved in motion detection [34]. Another interneuron synaptic contact. The synaptic neurotransmitter of the in the inner nuclear layer – the interplexiform cell – has photoreceptor cell is glutamate, which is released in processes extending into the inner and outer plexifom response to depolarization. The photoreceptor is most layers. Thus, on their way to the ganglion cell, visual depolarized in darkness, whereas phototransduction signals are transmitted and modified by bipolar, hori- results in gradated hyperpolarization. The terminal zontal, amacrine, and interplexiform cells as part of the endings of the photoreceptors interact with neighbor- visual processing within the retina [38].
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