Anatomy of retina

Retina extends from optic disc to ora serrata

1. Introduction Retina, the innermost tunic of the eyeball. It is a thin delicate & transparent membrane. Its thickness: At the posterior pole in the peri -papillary region is approximately 0.56mm At the equator 0.18 to 0.2mm At the ora serrata approx. 0.1mm It is the most highly developed tissue of the eye. It appears purplish-red due to visual purple of the rods. After death, the retina appears white opaque.

2. Gross Anatomy Retina extends from the optic disc to the ora serrata & has a surface area of about 266sq.mm

3. Gross Anatomy Grossly on ophthalmoscopic examination it can be divided into 3 distinct regions: Optic disc Macula lutea Peripheral retina

4. Optic Disc It is a pale pink, well-defined circular area of about 1.5mm diameter. “At the optic disc, all the retinal layers terminate except the nerve fibre layer” , which pass through the lamina cribrosa to run into the optic nerve. In comparison to the rest of retina, the optic disc appears white d/t lamina cribrosa and medullated nerve fibres behind it & absence of vascular choroid.

5. Optic Disc In the centre, where the nerve fibres are thinnest, the white lamina shines more brightly. The grey spot in the lamina are d/t to the non- medullated nerve fibres reflecting less light than the white connective tissue fibres. The physiological cup of the optic disc is a depression seen in it.The central retinal vessels emerge through the centre of this cup. Increase in the size of the cup & difference in the size of cup of two eyes should be watched suspiciously to rule out glaucoma .

6. Macula lutea The macula lutea ( yellow spot ) is a comparatively dark area 5.5mm in diameter, situated at the posterior pole of the eyeball, temporal to the optic disc. Horizontally ellipsed area demarcated approx. by the upper and lower arcuate & temporal retinal vessels. It corresponds to approx. 15 degree of the visual field and that photopic vision and colour vision are primarily functions of this area.

7. Macula lutea It consists of the following parts: Fovea centralis (fovea) Foveola Umbo Foveal avascular zone (FAS) Parafoveal area Perifoveal area

8. Fovea centralis / Fovea Central depressed part of the macula. 1.85mm in diameter & 0.25mm in thickness It corresponds to 5 degree of visual field & is most sensitive part of the retina.

9. Foveola 0.35mm in diameter. Forms the central floor of the fovea. It is situated about 2 disc diameter ( 3mm ) away from the temporal edge of the optic disc & about 1mm below the horizontal meridian.

10. Umbo Tiny depression in the center of the foveola which corresponds to the ophthalmoscopically visible foveolar reflex , seen in most normal eyes. Loss of the foveolar reflex may be an early sign of damage.

11. Foveal Avascular Zone Located inside the fovea but outside the foveola . Its exact diameter is variable and its location can be determined with accuracy only by fluorescein angiography.

12. Parafoveal Area The parafovea is a belt that measures 0.5 mm in width and surrounds the foveal margin.

13. Perifoveal Area The perifovea surrounds the parafovea as a belt that measures 1.5 mm wide

14. Peripheral Retina It can be divided into 4 regions: Near periphery Mid periphery Far periphery Ora serrata

15. Peripheral Retina Near periphery: Refers to circumscribed region of about 1.5mm around the macula. Mid periphery: Occupies a 3mm wide zone around the near periphery. Far periphery: It extends from optic disc 9-10mm on the temporal side & 16mm on the nasal side in the horizontal meridian.

16. Peripheral Retina Ora serrata : It is the serrated peripheral margin where the retina ends & ciliary body starts. At the ora , the sensory retina is firmly attached both to the vitreous & pigmented epithelium . 2.1mm wide temporally & 0.7-0.8mm wide nasally Its distance from the limbus is 6.0mm nasally & 7.0mm temporally. It is located 6-8mm away from the equator & 25mm from the optic nerve on the nasal side.

17. Microscopic Structure Of The Retina Retina consist of three types of cells & their synapses arranged in the following layers (from outwards to inwards): Retinal pigment epithelium Layers of rods & cones External limiting membrane Outer nuclear layer Outer molecular ( plexiform ) layer Inner nuclear layer Inner molecular ( plexiform ) layer Ganglion cell layer Nerve fiber layer Internal limiting membrane

18. Retinal Pigment Epithelium (RPE) Outermost layer of retina. Consist of a single layer of hexagonal-shaped cells containing pigment. The RPE cells shows fine mottling d/t unequal pigmentation of the cell & this is responsible for granular appearance of the fundus. RPE is firmly adherent to the underlying BRUCH’S MEMBRANE (basal lamina of the choroid) & loosely attached to the layer of rods & cones.

19. Retinal Pigment Epithelium The potential space between RPE & the sensory retina is c/d SUBRETINAL SPACE . A separation of the RPE from the sensory retina is c/d RETINAL DETACHMENT ,& the fluid between the two layers is c/d SUB RETINAL FLUID (SRF) Splitting of the retina at the level of the outer plexiform layer is cld SENILE RETINOSCHISIS . Splitting of the retina in the nerve fibre layer with the development of nerve fibre layer break is c/d JUVENILE RETINOSCHISIS.

20. Retinal Pigment Epithelium Adjacent RPE cells are connected to each other by tight junctions ( zonulae occludens & zonulae adherens ) & constitute the outer BLOOD-RETINAL BARRIER. The RPE cells at the fovea are taller, thinner & contains more & large granules than elsewhere in the fundus, thereby giving a dark colour to this area. The optical part of RPE is formed by the microvilli which project b/w the rods & cones processes. Apical surface of human retinal pigment epithelium as seen through a scanning electron microscope. Fine microvilli cover the surface and reach up between the photoreceptor outer segments (which have been peeled away in this view).

21. Retinal Pigment Epithelium Functions: Play important role in photoreceptor renewal & recycling of vitamin-A Maintains integrity of subretinal space by forming outer blood-retinal barrier & actively pumping ions & water out of this. Transport of nutrients & metabolites. Provides mechanical support to the processes of photoreceptors & maintenance of retinal adhesion. They manufacture pigments which presumably has an optical function in absorbing scattered light. Synthesis of growth factors to modulate adjacent structures. Phagocytosis and digestion of photoreceptor wastes. Electrical homeostasis . Regeneration and repair after injury or surgery.

22. Mechanism of serous detachment. When the retinal pigment epithelium (RPE) is normal, no serous detachment occurs beyond a focal site of leakage. When the RPE is compromised by choroidal or RPE disease that impairs outward fluid transport, a serous detachment forms until absorption across the exposed RPE balances the inward leak.

23. Layer Of Rods & Cones Rods contains a photosensitive substance visual purple ( rhodopsin ) & subserve the peripheral vision ( scotopic vision ). Cones responsible for highly discriminatory central vision ( photopic vision ) & colour vision. Rods are about 120 million & cones are about 6.5 million in number. The highest density of cones is at fovea with an average of 199000 cones/ sqmm The number of cones fall off rapidly outside the fovea.

24. Layer Of Rods & Cones Cones density is 40-45% greater on the nasal than on the temporal aspect of the retina, & slightly lower in the superior than the inferior retina at the mid periphery. Rods are absent at the fovea in an area of 0.35mm ( Rod-free Zone ) which corresponds to 1.25 degree of the visual field. They are maximum below the optic disc (170,000/sq.mm) & their number reduces towards the periphery. The entire nasal retina has 20-25% more rods than does temporal retina, & the superior retina has 2% more than the inferior retina.

25. Structure Of The Photoreceptor Each photoreceptor consists of: A cell body & nucleus (which lies in the outer nuclear layer) A cell process that extends into outer plexiform layer Inner & outer segment (which forms the layer of rods & cones) .

26. The Rod Cell 40-60 micro meter long Outer segment of the rod is cylindrical, highly refractile & contains visual purple . It is composed of numerous lipid protein lamellar discs stacked one on the top of the other & surrounded by a cell membrane. The disc contain 90% of the visual pigment. The number of discs varies from 600-1000/rod . Outer segment

27. The Rod Cell The inner segment of the rod is thicker than the outer segment. It consist of 2 regions: Ellipsoid (the outer portion) is adjacent to the outer segment & contains abundant number of mitochondria. Myoid (the inner portion) contains the glycogen as well as the usual organelles. Inner segment

28. The Rod Cell An outer rod fibre arises from the inner end of rod, which passes through the external limiting membrane & swells into a densely staining nucleus - the rod granule (which lies in the outer nuclear layer) & then terminates as inner rod fibre which at its end has an end bulb c/d the rod spherule that is in contact with the cone foot.

29. The Cone Cell 40-80 micro meter long. Largest at the fovea (80micro meter) & shortest at the periphery (40micro meter). The outer segment is conical in shape, much shorter than that of rod & contains the iodopsin . There are about 1000-1200 disc/cone.

30. The Cone Cell The cone inner segment & cilium are similar to the rod structures, however the cone ellipsoid is very plump & contains a large number of mitochondria. Outer fibre is absent . A stout cone inner fiber runs from the nucleus which at the end is provided with lateral processes c/d cone foot or cone pedicle (which lies in the outer plexiform layer)

31. The Interphotoreceptor Matrix (IPM) & Interphotorecepter Retinoid Binding Protein (IRBP) The IPM occupies the space between the photoreceptor outer segments & the retinal pigment epithelium. It is a complex structure consisting of proteins, glycoproteins , GAGs & proteoglycans such as chondroitin sulphate. The IPM has diverse range of functions, including retinal attachment & adhesin molecular trafficking, facilitation of phagocytosis & probably photoreceptor outer segment alignment.

32. The Interphotoreceptor Matrix (IPM) & Interphotorecepter Retinoid Binding Protein (IRBP) IRBP accounts for 70% of the soluble protein in the IPM. IRBP is produced mainly by the cones. FUNTIONS OF IRBP: Transport of retinoid between the photoreceptor & the retinal pigment epithelium. Minimize fluctuations in retinoid availability . Protect the plasma membrane from the damaging effect of high retinoid concentration.

33. External Limiting Membrane In low magnification, it appear as a fenesterated membrane extending from the ora serrata to the edge of the optic disc; through which pass processes of the rods & cones. Actually it is formed by the junctions ( zonulae adherentes ) between the cell membrane of photoreceptors & Muller’s cells ( & thus it is not a basement membrane) .

34. Outer Nuclear Membrane Formed by the nuclei of rods & cones . Rod nuclei forms the bulk of this multilayered outer nuclear layer except in the cone dominated foveal region. The number of rows of nuclei & thickness of this layer varies from region to region are as follows: Nasal to the disc: 8-9 layers of nuclei & 45 micro meter thickness. Temporal to disc: 4 rows of nuclei & 22 micro meter thickness. Foveal region: 10 rows of nuclei & 50 micro meter thickness. Rest of the retina except ora serrata : one row of cone nuclei & 4 rows of rod nuclei with a thickness of 27 micro meter.

35. Outer Plexiform Layer This layer contains the synapses between the rod spherule & cone pedicles with the dendrites of the bipolar cells & processes of the horizontal cells. It marks the junction of the end organ of vision & first order neurons in the retina. It is thickest at the macula (51 micro meter) & consist predominately of oblique fibers that have deviated from the fovea & is also known as HENLE’S LAYER .

36. Inner Nuclear Layer It is thinner as compared to the outer nuclear layer. This layer disappears at fovea & in the rest of the retina consist of following: Bipolar cells Horizontal cells Amacrine cells The soma of the muller’s cells Capillaries of the central retinal vessels

37. Bipolar Cells (Neurons) These are the neurons of first order of vision. They are connected to the rods & cones by their dendrites & to the ganglion cells by their axons.. On the basis of morphology & synaptic relationship, nine types of bipolar cells are seen under light microscopy: Rod bipolar cell Invaginating midget bipolar cells Flat midget bipolar cells Invaginating diffuse bipolar cells. Flat diffuse bipolar cells. On-centre blue cone bipolar cells. Off – centre blue cone bipolar cells. Giant bistratified bipolar cells. Giant diffuse bipolar cells.

38. Horizontal Neurons These are flat cells & they connect rods & cones with bipolar cells. They are of two types: The type A horizontal cells have seven groups of dendrites, which have contact with triad of seven cone pedicles & their single axon has contact with distant cone triad. The type B horizontal cells dendrites have contact with rod receptors only & their axons with the distant rod cells.

39. Amacrine Cells Situated within the innermost part of inner nuclear layer. They connect the axons of the bipolar cells to the dendrites & soma of the ganglion cells, thus they are similar in function to the horizontal cells.

40. Muller’s Cell Their nuclei & cell bodies lie within the inner nuclear layer. Fibres from their outer ends extend upto external limiting membrane & those from their inner ends reach upto the internal limiting membrane. The muller’s cell provide structural support & contribute to the metabolism of the sensory retina. Fibres from the Muller’s cell & cell membrane of photoreceptors together constitute the external limiiting membrane. In outer & inner nuclear layer, Muller’s cell provide reticulum around the cell somata . In nerve fibre layer their processes interweave with axons of ganglion cells. Fibres of the Muller’s cell takes part in the formation of internal limiting membrane also.

41. Other Glial Cells In addition to Muller’s cell , the retina contains other glial cells: Astrocytes . Microglia. Oligodendrocytes . The astrocytes are most abundant & are located around the blood vessels.

42. Inner Plexiform Layer It consists of synapses between the axon of bipolar cells (first order neuron), dendrites of ganglion cells (second order neuron), & the processes of integrative amacrine cells. This layer is absent at the foveola .

43. Ganglion Cell Layer The cell bodies & the nuclei of the ganglion cells lie in this layer. It is absent at the foveola . Ganglion cells have been variously classified few classification are as follow: W,X,& Y ganglion cells P (P1 & P2 )& M GANGLION cells. OFF-centre & ON- centre ganglion cells. Monosynaptic & polysynaptic ganglion cells.

44. Nerve Fibre Layer (Stratum opticum ) It consists of the unmyelinated axons of the ganglion cells which converge at the optic nerve head , pass through lamina cribrosa & become ensheathed by myelin posterior to lamina. This layer also contains: Centrifugal nerve fibres. Processes of Muller’s cell. Neuroglial cells. Retinal vessels.

45. Features Of Nerve Fibres Non- myelinated . Thickness 0.5 to 2 micro meter. Cytoplasm of the axons contains microtubules, fine fibrils, mitochondria, & occasional vesicles.

46. Arrangement Of Nerve Fibres In The Retina Fibres from the nasal half of the retina come directly to the optic disc as superior & inferior radiating fibres ( srf & irf ). Fibres from the macular region pass straight in the temporal part of the disc as papillomacular bundle ( pmb ). Fibres from the temporal retina arch above & below the macular & papillomacular bundle as superior & inferior arcuate fibres ( saf & iaf ) with a horizontal raphe in between.

47. Arrangement OF Nerve Fibres Of The Optic Nerve Head Fibres from the peripheral part of retina lie deep in the retina, but occupy the most peripheral (superficial) part of the optic disc. While the fibres originating closer to the optic nerve head lie superficially in the retina & occupy a more central (deep) portion of the disc.

48. Thickness Of Nerve Fibre Layer At The Disc Thickness of the RNFL around the different quadrants of the optic disc margin progressively increases in the follwing order: Most temporal/lateral quadrant (thinnest). Upper temporal & lower temporal quadrant. Most medial quadrant . Upper nasal & lower nasal quadrant (thickest)

49. Clinical Significance Of Distribution & Thickness Of Nerve Fibres At The Optic Disc Margin Papilloedema appears first in the thickest quadrant (upper nasal & lower nasal) & last in the thinnest quadrant (most lateral ). Arcuate nerve fibre which occupy the superior temporal & inferior temporal quadrant of optic nerve head are most sensitive to glaucomatous damage . Macular fibre are most resistant to glaucomatous damage & explain the retention of the central vision till end.

50. Internal Limiting Membrane It consists of: A PAS (+) ve true basement membrane (unlike ELM) that forms the interface b/w retina & vitreous. Collagen fibrils. Proteoglycans (mostly hyaluronic acid) of the vitreous. Plasma membrane of the muller cells & other glial cells of the retina.

51. Blood Supply Of The Retina ARTERIAL SUPPLY: Outer 4 layers upto outer nuclear layer: from choriocapillaris . 6 inner layers: from central retinal artery. Outer plexiform layer gets its blood supply from the central retinal artery & partly from the choriocapillaris by diffusion. Fovea is an avascular area & supplied by choriocapillaris . Macular region is supplied by superior & inferior temporal branch of central retinal artery. Sometimes cilioretinal artery supplies macula ( branch of ciliary system). When present, it helps to retain the central vision in the event of occlusion of the central retinal artery.

52. Blood Supply Of The Retina Retinal vessels are end arteries , usually they do not form anastomoses . So their occlusion invariably results into ischemia of the supplied part. Usually normal retinal vessels do not cross horizontal raphe . Collaterals across midline are a common finding in retinal venous occlusive diseases. Vessels course in nerve fibre layer & ganglion cell layer. Retinal arteries after 1st branch contains only endothelial cells & pericytes (1:1). No nerve fibres are seen in adventitia of the retinal vessels.

53. Blood Supply Of The Retina VENOUS DRAINAGE: Usually follows arterial supply. Retinal veins run parallel to arteries, at places they cross each other. At AV-crossings artery is present anterior to the vein & they share a common adventitial coat. And this is the reason why AV-crossing changes are seen during hypertensive retinopathy. AV-crossings are the most common sites of branch retinal vein occlusion . Central retinal vein drains into cavernous sinus so any infection of eye can travel upto brain by this route or vice versa.

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