Retina

The retina is the sensory membrane that lines the inner surface of the back of the eyeball. It's composed of several layers, including one that contains specialized cells called photoreceptors.

Photoreceptor cells take light focused by the cornea and lens and convert it into chemical and nervous signals which are transported to visual centers in the brain by way of the optic nerve.
In the visual cortex of the brain (which, ironically, is located in the back of the brain), these signals are converted into images and visual perceptions.

1. Retinal Anatomy By Getachew Kassahun Gondar, Ethioipia 1

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3. RETINA It is the light receptive inner neural coat of the eyeball consisting of outer retinal pigment epithelium (RPE) and inner sensory retina with a potential space called subretinal space between them . It lies between the choroid and the vitreous. It extends from the optic disc to the ora serrata . 3

4. The anterior pigment epithelium of the iris continues as the outer pigment epithelium of the ciliary body and later forms the retinal pigment epithelium (RPE ). • Surface area of the retina— 266 mm2 • Thickness at the ora serrata – 125 microns Thickest at the macula – 350 microns 4

5. Layers of Retina 5

6. Retina consists of 10 distinct layers. from outside inwards of the following layers: Retinal pigment epithelium (RPE ): Layer of the rods and cones(photoreceptor layer) Outer/ external limiting membrane Outer nuclear layer Outer plexiform layer Inner nuclear layer Inner plexiform layer Ganglion cell layer Nerve fiber layer Internal limiting membrane . 6

7. Retinal pigment epithelium (RPE): It is a single layer of flattened, mostly hexagonal cells which is firmly adherent to lamina vitrea of the choroid. On direct ophthalmoscopy the retina exhibits fine mottled appearance due to the following facts: The RPE cells are not equally pigmented. Pigments in each RPE cells are distributed at the periphery of the cells and the central nuclear area remains relatively pigment free. The taller and narrower pigment cells at the macula confer darker colour to this region . RPE cells transport substances to the photoreceptor cells which are needed for metabolism. 7

8. Photoreceptor cells layer Rods and cones with their nucleus and processes form the sensory receptor. arranged on the external limiting membrane like a palisade. The rods contain visual purple called rhodopsin which combines vitamin A with protein. Rhodopsin is responsible for vision in dim light ( scotopic vision) and peripheral vision. The cones are responsible for acuity of vision, vision in strong light ( photopic vision) and colour vision. 8

9. cont... No. of rods— 125 million No. of cones— 7 million Each photoreceptor, i.e. rod and cone consist of 3 parts; Outer and inner segment connected by a tubular connection called cilium in the layer of the rods and cones. Cell body and nucleus in the outer nuclear layer. Cell processes, i.e. axons that extend into the outer plexiform layer. 9

10. Outer limiting membrane: It is a fenestrated membrane formed by fibres of Müller and pierced by the processes of the rods and cones. 10

11. Outer nuclear layer: This layer contains nuclei of the rods and cones. 11

12. Outer plexiform /molecular layer: It consists of; Arborisation of the axons of the rod and cone nuclei with the dendrites of the bipolar cells Processes of the horizontal cells The fibres of Müller. 12

13. Inner nuclear layer: This layer consists of; Bipolar cells Horizontal cells Amacrine cells Nuclei of the fibres of Müller Capillaries of the central retinal artery and vein. 13

14. Bipolar cells Form the first order of neurons. Their nuclei are located in the inner nuclear layer and the dendrites arborise with the axons of the rods and cones in the outer plexiform layer. The axons of the bipolar cells synapses with the dendrites of the ganglion cells in the inner plexiform layer. The “midget” bipolar cells synapse with the individual cone feet plates while the rest of the bipolar cells synapse with both the rod spherules and cone feet. 14

15. Horizontal cells Are flat cells situated near the outer plexiform layer and send processes horizontally. 15

16. Amacrine cells Are pear shaped and send single process inwards to terminate in the inner plexiform layer. They are located near the inner plexiform layer. Both horizontal cells and amacrine cells form horizontal connections between adjacent rods and cones. 16

17. Inner plexiform /molecular layer This layer contains; Mainly arborisation of the axons of the bipolar cells with the dendrites of the ganglion cells . Processes of the amacrine cells Fibres of Müller Branches of the retinal arteries and veins 17

18. Ganglion cell layer Ganglion cells are multipolar nerve cells with clear oval nucleus and well-developed Nissl granules. Ganglion cells form the second order of neurons. Their axons pass into the nerve fibre layer and arborize on cells in the lateral geniculate body and superior colliculus (central nervous system). 18

19. Nerve fibre layer: It consists of Axons of the ganglion cells which pass through lamina cribrosa and form the optic nerve. Fibres of Müller Retinal vessels. The bundles of axons of ganglion cells run parallel to the surface of the retina and converge towards the optic disc. They are nonmyelinated . Nasal fibres reach optic disc directly. The temporal fibres pass above and below the macula to reach the optic disc. The macular fibres directly reach the temporal side of the optic disc and form the papillomacular bundle 19

20. Direction of nerve fibres reaching optic disc 20

21. Inner limiting membrane: It is a thin hyaloid membrane separating the retina from the vitreous. The feet of the fibres of Müller are attached to it. 21

22. LANDMARKS ON THE RETINA Ora serrata : It is the extreme anterior end of the retina. It is located 8 mm from the limbus and 6 mm from the equator of the eyeball. Here the retina is firmly attached to the choroid. Also the vitreous is firmly attached to ora serrata . All the retinal elements tend to cease just posterior to ora serrata . 22

23. Optic disc: It is a vertically oval structure and it is called blind spot due to it’s failure to evoke visual sensation. Only the nerve fibre layer is present here. It is pink coloured due to the presence of capillaries. There is a funnel shaped depression/ excavation at the center and slightly to the temporal side. It is called physiological cup and is lighter in colour . LANDMARKS ON THE RETINA 23

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25. Macula lutea : It is an oval area situated temporal to the optic disc. It’s centre lies 3.5 mm lateral to the optic disc . It is of the same size as the optic disc. At the macula, the ganglion cells are arranged in several layers and the outer plexiform layer is thickest . Rods progressively decrease in number towards the centre . The cones progressively increase towards the centre . It is nourished by the choriocapillaries . LANDMARKS ON THE RETINA 25

26. Anatomical land marks of the entire macular region Macula Peri fovea Para fovea Fovea(fovea centralist) Foveola Umbo 26

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28. Perifoveal and Parafoveal Areas Perifoveal area – from the outer most limit of the parafovea to the outer limit of the macula. Parafovea – the outer most limit where the ganglion cell layer, inner nuclear layer and Henle layer or outer plexiform are thickest( the retina is thickest) 28

29. cont’d The fovea has the highest concentration of cones in the retina which are estimated to be 199,000 to 300,000 per square millimeter. The number falls off rapidly as one moves away from the fovea in all directions. In this area the ratio of cones to ganglion cells approaches 1:1 which makes it a specialized area for discrimination of details and color vision. 29

30. cont’d Foveal avascular zone(FAZ) is a region within in the fovea devoid of blood vessels. The geometric center of FAZ is often taken to be the center of macula and thus the point of fixation. And its an important land mark for flouresceinangiography. 30

31. Foveola Central floor of fovea Diameter: approximately 0.35mm A region where the cones are slender, elongated and densely packed. Umbo is a tiny depression in the center of foveola. 31

32. Fovea centralis : It is the central depression of the macula lutea . Only cones are present in the fovea. It is the thinnest area of the retina and characterised by: Absence of rods, fibres of Müller, ganglion cells and nerve fibre layer. Extreme thinness of inner plexiform layer, outer plexiform layer and inner nuclear layer. Tall and slender cones. 32

33. Anatomical land marks of the macula 33

34. Most central region of the macula 34

35. Neuroglial cells of the retina Neuroglial cells are not actively involved in the transfer of neural signals. Provide structure and support and play a role in the neural tissue reaction to injury or infection. Types of neuroglial cells in the retina are; A. Muller cells B. Microglial cells and C. Astrocytes 35

36. A. Muller cells Large neuroglial cells extending throughout the retina. Their apex is in the PRC layer and the basal aspect in the inner retinal surface. They act as a buffer in regulating concentration of potassium ions ( k + ), GABA, and glutamate in the extracellular space. They are also involved in the glucose metabolism by synthesizing and storing glucose. 36

37. Muller cells 37

38. B. Microglial cells Are highly specialized, mononuclear phagocytic cells in the retina. Found anywhere in the retina. Their cell bodies are located in NFL-GCL, INL, and OPL. They increase in response to injury and inflammation. 38

39. Cont’d The function of microglial cells; 1) An immune effectors through release of chemokines and cytokines. 2) As a potential antigen presenting cells in the retina. 3) As immune modulator limiting leukocyte infiltration of the retina. 39

40. C . Astrocytes . Are star-shaped fibrous cells. Found in the NFL and GCL. They may contribute to ILM and perform the same functions as Muller cells. 40

41. Neurotransmitters Are either inhibitory or excitatory chemicals released by the neural cells in the retina. Glutamate Gamma amino butyric acid( GABA) Glycine Acetyl cholinE Dopamine 41

42. Blood supply of the retina The retina is highly metabolically active sheet of the neural tissue with the highest oxygen consumption of any human tissue. Like brain the retina has a highly selective blood tissue barrier, called Blood Retinal Barrier(BRB). BRB is formed by the junction of capillary endothelium with each other and tight junction of RPE. 42

43. Cont’d BRB – has the following importance ; Primarily it regulate the optimal extracellular environment to facilitate neural transmission. It also control the passage of pathogens and intravascular leucocytes. Partly protect the neural environment from surveillance by immune cell. 43

44. Cont’d The retina gets its blood supply from the following source:- 1 . The central retinal vessels (Artery and vein) 2 . The choroid capillaries . 44

45. 1. The central retinal artery It’s a direct branch of the ophthalmic artery. The artery enters the retina through the optic disc usually slightly nasal of center and branches into superior and inferior retinal artery each of which divides further into nasal and temporal branches and these vessels continue to bifurcate. The nasal branches run a relatively straight course towards the ora serrata. 45

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47. Cont’d But the temporal vessels arch around the macular area in route to the periphery. The vessels are located in the NFL just below the transparent internal limiting membrane. The central retinal artery supply 2/3 of the inner retinal layers such as INL, IPL , GCL , NFL and ILM. Has a flow rate of 25mm/s with high oxygen exchange. 47

48. Cont’d Retinal vessels are said to be end vessels. The retinal vessels terminate in delicate capillary arcades approximately 1mm from the orr-serrata. The central retinal artery is 1/3 to 1/4 of the central retinal vein. The central retinal artery lies superficial to the vein. The retinal blood vessels are readily visible with ophthalmoscope. 48

49. 2. The choroidal capillaries A capillary system of choroidal arteries supplied by the short posterior ciliary artery. Supply the outer 1/3 of the retina such as RPE , PRC , ELM , ONL and OPL. Macula lutea and fovea are exclusively supplied by choriocapillaries. Supply majority of oxygen for the retina. 49

50. Cont’d A Cilioretinal artery- is derived from the choroidal circulation which emerges from the temporal margin of optic disc and supplies a portion of the inner retina between the optic disc and the center of macula. 50

51. Anatomy of cilioretinal artery 51

52. Thank you 52