ANATOMY OF RETINA

EMBRYOLOGY

embryology

The retina is derived from the two layers of optic vesicle.

RPE-develops from the outer layer &sensory retina from the inner layer.

The potential space of the optic vesicle b/w the 2 layers is bridged by the processes of pigment cells.

anatomy

Rete in Latin means Netthe retina proper is a thin, delicate layer of nervous tissue

that has a surface area of about 266mm2.

The major landmark of the retina are: The optic disc The retinal blood vessels The area centralis with the fovea and foveola The peripheral retina (which includes equator) The ora serrata

the retina is thickest near the optic disc,measuring 0.56mm,it becomes thinner towards the periphery,measuring 0.18mm at the equator and to 0.1mm at the ora serrata

THE OPTIC DISC

The optic disc is a circular to slightly oval ,situated 1mm above and 3mm nasal to the posterior pole

measures approx 1.5mm horizontally and 1.75mm vertically.

It corresponds to the scleral opening through which the axons of the ganglion cells leave to form the optic nerve.

THE AREA CENTRALIS(MACULA LUTEA)

The area centralis or central retina is divisible into the fovea and foveola,with a parafoveal and a perifoveal ring around the fovea.

Avg diameter of about 5.5mm This region in demarcated

approx by the upper and lower arcuate temporal retinal vessels and has an ellipitical shape horizontally.

It corresponds to approx 15 degress of the visual field.

Yellow colouration is derived from the presence of the carotenoid pigment,xanthophyll in the ganglion and bipolar cells.

Contd…area centralis

The macula lutea can be best visualized in red free light and in darkly pigmented individuals

Parafovea-this is 0.5mm wide zone around the fovea.it has the maximum no. of nerve cells especially the ganglion cells &cells of the inner nuclear layer.the cone:rod ratio is 1:1

Perifovea-this is the outermost ring of area centralis.1.5mm wide,the cell density is less in this area. The cone:rod ratio is 1:2

foveal avascular zone or capillary free zone-is approx 500microm in diameter & is often taken to be the centre of the macula and thus the point of fixation.

- is an important clinical landmark in the treatment of subretinal neovascular membrane by laser photocoagulation.

The fovea It marks the approximate

centre of the area centralis,is located at the posterior pole of the globe,4mm temporal to the centre of the optic disc and about 0.8mm below the horizontal meridian.

1.85mm in diameter and 0.25mm in thickness.

It corresponds to the central 5 degrees of the visual field.

At the fovea, the only layers that are present are RPE,photoreceptors( only cones), the ext limiting membrane, the outer nuclear layer, the inner fibers of the photoreceptors(Henle’s fibre layer) and the internal limiting membrane.

The foveolaRepresents the area of the

highest visual acuity in the retina because of the sole presence of cones and its avascular nature.

Correspond to the central 1 degree of the visual field.

Measures 0.35mm in diameter and 0.1mm in thickness,thinnest part of the retina,is devoid of ganglion cells. Appears deeper red than does the adjacent retina because of the rich choroidal circulation of the choriocapillaries which shines through it.

The colour of the fovea persists and is called ‘cherry-red spot’ when the surrounding retina becomes cloudy which occurs after obstruction in the central retinal vasculature and in certain metabolic storage diseases.

The peripheral retinaThe peripheral retina increases the field of vision

and is divided into four regions: The near periphery The mid-periphery The far periphery and The ora serrata the near periphery is a 1.5mm around the

area centralis and the mid-periphery is a 3mm wide zone around the near periphery.

the far periphery is a region that extends from the optic dics,9-10mm on the temporal side and 16mm on the nasal side in the horizontal meridian.

Ora serrataIt is the most anterior region of the

retina, which consists of a dentate fringe and which denoted the termination of the retina. it terminates by becoming continouous with the non-pigmented epithelium of pars plana.

Here the nervous tissues of the retina end.

2.1mm wide temporally ,0.7-0.8mm wide nasally

Located 6mm nasally and 7mm temporally from the limbus

6-8mm from the equator and 25mm from the optic nerve on the nasal side.

the external landmark of the ora serrata is the location of the insertions of the recti muscles(except for the superior rectus which inserts post to the ora)

The vitreous base is attached to the retina at the ora.

Varous anolamies at the ora which can predispose to retinal tears and vitreoretinal detachments include;1.retinal tufts

2.meridional folds 3.meridional complexes. beginning at the young age,cystoid

degeneration occurs,usually in the outer plexiform layer at the ora serrata.the cystic spaces become pronounced in the elderly and are more marked on the nasal than on the temporal aspect.later they extend betn the inner and outer limiting membranes and may communicate with the vitreous and give rise to retinal detachment.

It is in a watershed zone betn the anterior and posterior vascular system,which may in part explain why peripheral retinal degeneration is relatively common.

Contd…

The ora has distinct anatomical properties because of-

1.It is the point of peripheral retina termination.

2.It is thin.3.It is avascular.4.Its intimate relationship to the

vitreous.5.External landmark of the insertion

of the recti muscles.

Factors keeping the retina attached

1.MECHANICAL FORCES OUTSIDE THE SUB RETINAL SPACE:

A.FLUID PRESSURE:hydrostatic &osmotic fluid is driven from the vitreous towards the

choroid.but the posterior route is limited bcos the retina & RPE provide substantial resistance to water movement. hence as an effect of this outward push of fluid the retina remains on the wall of the eye.

B.PRESSURE DIFF ACROSS THE RETINA: C.VITREOUS ADHESIONS: vitreous has a physical structure of a gel that may

help to keep the retina in place. even in syneresis a thin cortical layer remains aiding fluid pressure keeping the retina in place.

2.FORCES IN THE SUB RETINAL SPACE:1.RPE PUMP: the RPE can pump fluid out of the subretinal space to

the choroid at the rate of 0.3microL\hr\sqmm.this is an active energy dependent process and keeps the subretinal space dry.

2.MECHANICAL INTERDIGITATION:RPE microvilli wrap closely around the tips of the outer

segments of the photoreceptors.C.INTERPHOTORECEPTOR MATRIX: 1.Cell-cell adhesion mediated by CELL ADHESION

MOLECULES, which are intrinsic membrane glycoproteins.

2.Cell-matrix adhesion mediated by matrix molecules like fbronectin,laminin,collagen and proteoglycans.

On cross section,from outer to inner retina,retinal layers are

RPE and its basal lamina Rod and cone inner and outer

segments External limiting membrane Outer nuclear layer(nuclei of

photoreceptors) Outer plexiform layer Inner nuclear layer Inner plexiform layer Ganglion cell layer Nerve fibre layer(axons of the

ganglion cells) Internal limiting membrane

Retinal pigment epithelium The RPE consists of a monolayer of

hexagonal cells that extends anteriorly from the optic disc to the ora serrata, where it merges with the pigmented epithelium of the ciliary body.

4-6 million cells\eye It is 10-14 microns thick at the macula

and becomes thinner towards the ora. 3 parts- a.base(plasma membrane,mitochondria) b.body-(nucleas,ER,lipofuscin) c.apex-(pigment,ingested outer

segment{phagosomes}) -microvilli -terminal bars. The apical portion of each RPE has a

villous process that envelops the outer segment of the photoreceptor cells whereas the basal surface shows a rich infolding of the plasma membrane.the basal portion is rich in mitochondria and annulate lamellae which are active in protein synthesis.

The brown colour of the pigment is due to melanin granules which are present towards the apex.

RPE contd…

The cells joined at their lateral apical margins by terminal bars which constitute gap junctions,ZONULA OCCLUDENS &ZONULA ADHERENCE,which maintain the blood-retinal barrier.the space not occupied by the terminal bars is filled with extracellular matrix & forms VERHOEFF”S MEMBRANE.

The lipofuscin granules are present in the central and basal parts ,probably arise from the discs of photoreceptor outer segments.

Throughout the life,incompletely digested residual bodies,lipofuscin pigment,phagosomes and other materials are excreted beneath the basal lamina and form drusen

Functions of RPE

Vitamin A metabolism Maintenance of the outer blood-retinal

barrier Phagocytosis of the photoreceptor outer

segments Absorption of light (reduction of scatter) Heat exchange Formation of the basal lamina Production of the mucopolysaccharide

matrix surrounding the outer segments Active transport of materials in and out of

the RPE

NEUROSENSORY RETINA

1.PHOTORECEPTOR CELLSa.outer segment (rods and

cones)b.Ciliumc.Inner segments Ellipsoid Myloid Outer fiber Cell body(NUCLEUS) Inner fiber Synaptic vesicle2.MODULATOR CELLS (NUCLEI

FROM INNER NUCLEAR LAYER)

a.bipolar b.horizontal c.amacrine3. TRANSMITTER CELLSganglion 4.SKELETAL SUPPORT a.muller calls(nuclei form inner

nuclear layer) b.astroglia nerve fiber layer

The layer of rods and cones

Number of rods 110-125 million Number of cones 6.3- 6.8 the density of rods and cones varies in diff parts of

retina. The density of cones is maximal at the fovea ,with an

avg of 199000\sq mm the numbers decreases at the periphery.

The rods are absent at the fovea, rising rapidly towards the periphery and then slowly diminishing at the extreme periphery.

Rods & cones have diff visual pigments,the rods have rhodopsin which is sensitive to light having a wavelenth of 493mm,cones have 3 diff types of iodopsins-

Blue cones-444mm Green cones-540mm Orange\red cones-577mm

OUTER SEGMENTS: RODS: cylindrical in shape and have

stacks of flat,double lamellae in the form of discs,which have no atttachments to each other or to the ensheating plasma membrane.visual pigments are arranged on the surface of these lamellae.

CONES:they are short and cone like at the periphery .there are more discs and they are attached to the plasma membrane.

the outer and inner segments are joined to each other by a narrow cilium .at the inner end the cilium has 9 doublets of microtubules and they become 9 singlets at the outer segments.

INNER SEGMENTS: THEY HAVE AN OUTER ELLIPSOID

PART AND INNER MYLOID part. ELLIPSOID PART-is eosinophili,rich in

mitochondria and smooth endoplasmic reticulum.

MYLOITD PART- basophilic,has ribosomes and rough ER.It carries out protein synthesis.

The external limiting membrane

It is not a true membrane as small molecules pass freely through the junctional complexes.

Main function is to provide a selective barrier for nutrients that pass betn the adjacent muller cells as well as stabilization of the position of the transducing portion of the photoreceptors.

The outer nuclear layer

It lies internal to the ext limiting membrane and contains the soma and nuclei of the photoreceptor cells.

Nasal to the disc the outer nuclei layer is 45microm thick and has 8-9rows of nuclei,at the temporal disc only 4 rows of nuclei are present and the thickness is reduced to 22micrm and in the fovea 10 rows of cone nuclei increases the width of the nuclear layer by 50microm.

The diff nuclei of the two types of photoreceptors may be distinguished because the nuclei of rods stain orange and those of the cones red with Malory’s stain.

The outer plexiform layer

This layer marks the junction of the first and second order neurons in the retina.

The outer 2\3rd of this layer is composed of the inner fibres of the photoreceptors surrounded by the processes of the Muller cells and the remaining 1\3rd consists of the dendrites of the bipolar and horizontal cells as well as Muller cell processes.

The OPL in macular region is known as fibre layer of Henle,accumulation of lipid and other blood products within the layer of Henle accounts for the star pattern observed ophthalmoscopicaly at the macula in some cases of sys hypertension

The inner nuclear layer This layer consists of 8-12 rows of clensely packed nuclei of the bipolar cells,horizontal

cells,amacrine cells,interplexiform cells and supportive Muller cells. HORIZONTAL CELLS the flat horizontal cells serve to modulate and transform visual information received

from the photoreceptors. Their concentration is highest at the fovea and their number decreases towards the

periphery but their processes branch extensively as from the the central retina towards the ora serrata.

A characteristic feature of these cells is kolmer crstalloid-stacks of parallel dense tubules with ribosome like particles.

BIPOLAR CELLS They are the second order neurons oriented radially in the retina,the perikarya of these cells are located in the inner

nuclear and their processes extend to the outer and inner plexiform layers. On the basis of morphology and synaptic relationships,there are 9 main types of bipolar

cells; rod or mop-connect several rod cells to one to four ganglion cells. Invaginating midget Flat midget-connect a single cone cell with a single midget ganglion cell. Flat diffuse or brush –connect many cone cells with many ganglion cells. Invaginating diffuse ON-centre blue cone OFF-centre blue cone Giant bistratified Giant diffuse invaginating

MULLER CELLS

Most of the inner intermediate layer of the inner nuclear layer is occupied by the cell bodies of Muller cells, although their perikarya can be present at any sublayers.

Embryonically,they are derived from the inner layer of the optic vesicle.

As the principal glial cells of the retina, they conserve the structural alignment of its neuronal elements.

They are the largest of all the cells in the retina and extend from the ext to int limiting membrane.

Muller cells are characterized by their cytoplasmic expansions which fill all intercellular spaces and envelop the cell bodies of the neurons.

They form 4 types of processes- radial in the inner plexiform layer Fine horizontal processes in the nerve fibre & both the

plexiform layers. Thin villous processes around the inner segments. Honey comb mesh work enveloping ganglion cell bodies &

cells of inner nuclear layer.

AMACRINE CELLS

They were so named because it was believed that they had no axons.

They can be recognized by their large cell bodies with abundant cytoplasm and lobulated indented nuclei.

They are situated close to the ganglion cells and their long processes radiate widely and synapse with one another and with the dendrites of the ganglion cells and axonal endings of the bipolar cells.

The neurotransmitter associated with amacrine cell function include ach,GABA,glycine,dopamine,serotonin,cholecystokinin,glucagon, etc..

Most amacrine cells contain GABA and glycine which have inhibitory action on ganglion cells.

THE INNER PLEXIFORM LAYER

This layer marks the junction of the second order neuron ,the bipolar cells with the third order neurons, the ganglion cells.

THE GANGLION CELL LAYER

This layer is composed mainly of the cell bodies of the third order ganglion cells,although processes of Muller cells,other neuroglia and branches of retinal vessels are also present.

These cells form a single layer in the peripheral retina but 2 layers are formed at the temporal side of optic disc and 6-8 layers at the edge of the foveola.

At the foveola and optic nerve head this layer is absent.

two major types of ganglion cells are; M ganglion cells P ganglion cells-p1 p2

THE NERVE FIBRE LAYER

This layer is formed by axons of the ganglion cells.Normally,they do not become myelinated until they pass thorugh the lamina cribrosa ,the myelin sheath being formed by the oligodendrocytes.

THE INTERNAL LIMITNG MEMBRANE

It forms the innermost layer of the retina and the outer boundary of the vitreous.this is also not a true membrane

Both the retina and vitreous contribute to the formn of this membrane,which consists of following elements;

Collagen fibril and Proteoglycans (mostly hyaluronic acid)of

the vitreous The basement membrane The plasma membrane of the Muller cells.

Overall,cells & their processes in the retina are oriented perpendicular to the plane of the RPE in the middle & outer layers but parallel to the retinal surface in the inner layers.

For this reason,deposits of blood or exudates tend to form round blots in the outer layers (whr small capillaries are found) and linear or flame shaped patterns in the nerve fibre layer.

At the fovea,the outer layers tend to be parallel to the surface(Henle’s layer).as a result,radial or star-shaped patterns may arise when these extracellular spaces are filled with serum and exudate.

Retinal blood supply

Arterial supply of retina is from 2 systems:1.The choroidal circulation supplies the

outer1\3rd of the retina.2.The retinal circulation supplies the inner

2\3rd of the retina. the retina is most exclusively supplied by

the central retinal artery, a branch of the ophthalmic artery.

The retinal arteries are anatomic end arteries and there are no arteriovenous anastomoses.

Contd…blood supplyCAPILLARY NETWORK-2 capillaries plexus are given off which in

turn anastomose with one another. 1.superficial or inner capillary network- this is present in

the superficial part of the nerve ,are arranged in 2-dimensional pattern.

2.deep network- this is more complex and lies betn the inner nuclear &outer plexiform layer.

the capillary network is more dense around the veins. The 2 layered pattern is marked at the equator & is modified

near the disc, periphery &around macula.Peripapillary the area. around the macula, the superficial radial

network becomes thicker & 3 dimensional, it forms 3 separate layers around the disc excluding the macula.

1.superficial radial network 2.superficial plexus proper. 3.reduplication of the above layer at inner nuclear

layer.

Contd………

PERIPHERAL RETINA-here the deep network becomes intermittent being represented only around the veins. at the periphery it disappears totally. the capillaries of superficial network also loop around in arches to become continuous with veins 1 mm behind the ora,thus making the extreme end of retina avascular.

the terminal venules bend around to form an incomplete ring near the ora.they run independently for some distance and then in close connection with the arteries, the retina is eventually drained by the central retinal vein which joins the superior ophthalmic vein.