Anatomy of the Retina (ophthalmology).pptx

Retina on which image is formed in the human eye
One should no the anatomy of retina

1. RETINA PRESENTED BY DR OMDEEP GUPTA (JUNIOR RESIDENT) M.L.B MEDICAL COLLEGE JHANSI

2. INTRODUCTION IT IS THE INNERMOST TUNIC OF THE EYEBALL IT IS THIN, DELICATE AND TRANSPARENT MEMBRANE HIGHLY DEVELOPED TISSUE OF THE EYE

3. GROSS ANATOMY Extends from Optic disc to the ora serrata Surface area : 266 mm2 Thickness : at peripapillary region : 0.56 mm at the equator : 0.2 mm at the ora serrata : 0.1mm Colour : Purplish-red due to the visual purple of Rods

4. Fundus can be divided into three regions 1. Optic disc 2. Macula lutea 3. Peripheral retina

5. OPTIC DISC Optic disc is pale pink well defined circular area of 1.5 mm in diameter In comparison to the rest of retina, the optic disc appears pale pink due to lamina cribrosa , medullated nerve fibres behind it and absence of vascular choroid All retinal layers terminate at optic disc except the nerve fibres , which passes through the lamina cribrosa to run into optic nerve Depression in disc is a physiological cup, retinal vessels emerges through it

6. Cup varies in size, shape, position and depth in different eyes Normal C/D ratio 0.3 to 0.4.

7. MACULA LUTEA Dark area at posterior pole Situated temporal to disc It is 5.5 mm or 3.5DD in diameter, FOVEA, central part of macula, 1.5mm in diameter, entirely of cones FOVEOLA, central part of fovea, 0.35 mm in diameter UMBO, small central concavity of the floor of the foveola,0.15 mm in diameter

8. Peripheral Retina It can be divided in 4 regions: Near periphery : 1.5mm width around macula Mid periphery : 3 mm width around near periphery, its outer limit corresponds to equator Far periphery : extends from equator to orra serrata , average width is about 6 mm 1 clock hour corresponds to 5 to 6 mm of far peripheral circumference so, peripheral retina can be divided into 12 squares of 6*6 mm in size 4. Extreme periphery : refers to the area of ora serrata and pars plana

9. Ora serrata is the serrated peripheral margin where the retina ends and ciliary body starts it is 2.1mm wide temporally and 0.7mm wide nasally from limbus it lies 6mm nasally and 7mm temporally Ora serrata is the water shed zone between anterior and posterior vascular system. So, peripheral retinal degenerations are more common

10. Microscopically Retina consists of 10 layer: Retinal pigment epithelium (outermost layer) Layer of rods and cones ( neuro epithelium) External limiting membrane Outer nuclear layer Outer plexiform layer Inner nuclear layer Inner plexiform layer Ganglion cell layer Nerve fibre layer Internal limiting membrane (inner most layer)

11. RETINAL PIGMENTED EPITHELIUM Outermost layer of retina It consists of a single layer of Hexagonal shaped cells containing pigment Firmly adhere to BRUCH’S membrane (basal lamina of choroid) Loosely attached to the layer of rods and cones creating potential space between RPE and sensory retina called Sub retinal space. A separation of the RPE from the sensory retina is called retinal detachment and the fluid between the two layers is called subretinal fluid

12. It forms outer blood retinal barrier , as adjacent cells are adhere to each other by tight junctions RPE cells in cross section can be differentiated into apical and basal configuration Melanin granules are concentrated in the apical end, this pigment serves to absorb stray light and minimize scatter within the eye Other major pigment is lipofuscin , which accumulates gradually with age Basal membrane lies in contact with Bruch’s membrane and has convulated infolds to increase surface area for the absorption and secretion of material

13. FUNCTIONS OF RPE PHOTORECEPTORS RENEWAL AND RECYCLING OF VITAMIN A MAINTAINS INTEGRITY OF SUB RETINAL SPACE BY ACTIVELY PUMPING OUT WATER AND IONS FROM SPACE TRANSPORT OF NUTRIENTS AND METABOLITES THROUGH BLOOD RETINAL BARRIER PHAGOCYTIC ACTION MECHANICAL SUPPORT TO PHOTORECEPTORS REGENERATIVE AND REPAIRING FUNCTION AFTER INJURY OR SURGERY ELECTRICAL HOMEOSTASIS

14. NEUROEPITHELIUM(RODS AND CONES) There are about 120 million rods and 6.5 million cones They transform light energy to visual impulse Rods contain a photosensitive substance visual purple( rhodopsin ) and subserve the peripheral vision and low illumination vision ( scotopic vision), are absent at fovea Cones are responsible for central vision ( photopic vision) and colour vision, highest density is at fovea

15. Structure of photoreceptor Cell body and nucleus lies in the outer nuclear layer, cell process extends into outer plexiform layer. RODS 40 to 60 um long 2 segments, outer and inner segment Outer segment : cylindrical, highly refractile , transversely striated and contains visual purple , attached to inner segment by a cilium with a characteristic of 9+0 configuration.

16. Inner segment : thicker than the outer segment, consist of two regions ELLIPSOID and MYOID Ellipsoid is adjacent to the outer segment and contains abundant mitochondria Myoid contains glycogen and organelles

17. CONES : 40 to 80 um long (largest at fovea and shortest at periphery) 2 segments Outer segment: conical in shape, shorter than rod, contains the iodopsin Inner segment : similar to rod structure , however cone ellipsoid is very plump and contains a large number of mitochondria the inner segment of the cone becomes directly continous with its nucleus and lies in outer nuclear layer, inner cone fiber runs from the nucleus which at the end forms cone pedicle/foot

18. External limiting membrane it appears as a fenestrated membrane extending from the ora serrata to the edge of the optic disc , through which pass processes of rods and cones Electron microscopy shows that external limiting membrane is formed by the zonulae adherentes between the cell membrane of photo receptors and muller cells

19. Outer nuclear layer Primarily formed by nucleus of rods and cones Cone nuclei (6-7 um)are larger than the rod nuclei (5.5um)and arranged in a single layer While rod nuclei form the bulk of the multilayered outer nuclear layer except in foveal region

20. Outer plexiform layer This layer contains the synapses of rod spherules and cone pedicle with the dendrites of the bipolar cells and processes of the horizontal cells It is thickest at the macula (51 um) and consists predominantely of oblique fibres that have deviated from the fovea and is also known as HENLE’S layer

21. Inner nuclear layer This layer resembles the outer nuclear layer except that it is very thin This layer disappears at fovea and in rest of the retina consist of the following : Bipolar cells Horizontal cells Amacrine cells Soma of muller’s cells Capillaries of central retinal vessels

22. Bipolar cells These are neurons of first order of vision Body of bipolar cells consist entirely of the nucleus which lies in the inner nuclear layer There dendrites arborize with the rod spherules and cone pedicles in the outer plexiform layer and there axon arborize with dendrites of ganglion cells in the inner plexiform layer

23. On the basis of morphology and synaptic relationships, 9 types of bipolar cells are seen under light microcopy: Rod bipolar cells 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 invaginating bipolar cells

24. Amacrine cells These cells are situated with in the innermost part of the inner nuclear layer These have a piriform body and a single process which passes inwards in the inner plexiform layer and forms connections with the axons of the bipolar cells and the dendrites and soma of the ganglion cells Thus they perform an integrative function similar to that of the horizontal cells

25. Muller’s cells Nucleus and cell bodies of the muller’s cells are located within the inner nuclear layer Fibres from their outer ends extend upto the external limiting membrane and those from there inner ends reach upto the internal limiting membrane Muller’s cells provide structural support and contribute to the metabolism of the sensory retina

26. Inner plexiform layer This layer essentially consist of synapses between the axons of bipolar cells, dendrites of ganglion cells and the processes of integrative amacrine cells Fibres of the Muller’s cells courses vertically through this layer and their side branches form the the horizontal extending reticulum This layer is absent at the foveola

27. Ganglion cell layer Second order neuron of visual pathway The cell bodies and the nuclei of the ganglion cells lie in this layer Made up of single layer of cells, except in the macular region where it becomes multilayered (6-8 layers of the cells) and on temporal side of the disc where it has 2 layers. Ganglion cell layer is absent in the region of foveola

28. Classification of ganglion cells Monosynaptic ganglion cells : these cells predominate in the central retina, dendrite of each cell synapses with the axon of the single midget bipolar cell Polysynaptic ganglion cells : lies predominantly in peripheral retina , synapses with multiple bipolar cells The axons of the ganglion cells form the nerve fibre layer and then after passing through optic nerve, chiasma and optic tracts, ultimately synapse with cells in the lateral geniculate body (third order neurons)

29. Nerve fibre layer Aka Stratum opticum Consists of the unmyelinated axons of the ganglion cells which converge at the optic nerve head , pass through lamina cribrosa and become ensheathed by myelin posterior to lamina This layer contains axons of the ganglion cells, centrifugal nerve fibres , processes of muller’s cells , neuroglial cells , retinal vessels

30. Thickness of nerve fibres varies from 0.5 um to 2 um and are non- myelinated Arrangement of nerve fibre in the retina : Fibres from the nasal half of the retina come directly to the optic disc as superior and inferior radiating fibres Fibres from the macular region pass straight in the temporal part of the disc as papillomacular bundle Fibres from temporal retina arch above and below the macular and papillomacular bundle as superior and inferior arcuate fibres with a horizontal raphe in between

31. Arrangement of nerve fibres of optic nerve head Optic nerve comprises about 1.2 million nerve fibres Fibres from the peripheral part of the retina lie deep in the retina but occupy the superficial part of optic disc Fibres originating closer to the optic nerve head lies superficialy in the retina and occupy a more central (deep) portion of disc

32. Thickness of nerve fibre layer around optic disc margin progressively increases in following order Most lateral quadrant (thinnest) Upper temporal and lower temporal quadrant Most medial quadrant Upper nasal and lower nasal quadrant (thickest) CLINICAL SIGNIFICANCE: Papilloedema appears first of all in the thickest quadrant and last of all in thinnest quadrant Arcuate nerve fibres are most sensitive to the glaucomatous damage Macular fibres are most resistant to glaucomatous damage and explain the retention of central vision till end

33. Internal limiting membrane It mainly consists of a PAS positive true basement membrane that forms the interface between retina and vitreous ILM consists of four elements Collagen fibrils Proteoglycans Basement membrane Plasma membrane of muller cells and other glial cells of retina

34. Blood supply of the Retina Outer four layers get their supply from the choriocapillaries Inner six layers get their supply from central retinal artery Outer plexiform layer gets its blood supply partly from central retinal artery and partly from choriocapillaries by diffusion Fovea is an avascular area mainly supplied by choriocapillaries Arterial blood supply

35. Macular region gets its blood supply by small twigs from superior and inferior temporal branches of central retinal artery Sometimes cilioretinal artery; seen in 20% population (branch of ciliary artery) is seen in a hook shaped manner with in the temporal margin of the disc , it runs toward the macula and supplies it, thus when present it help to retain the central vision in the event of occlusion of central retinal artery. Retinal vessels are end arteries ,they donot anastomose with each other.

36. Bibliography A.K. Khurana , Indu Khurana , Anatomy and physiology of eye, 3 rd edition, CBS Publishers and distributers, Page no. 166 to 179.

37. Thank you