Ординатура / Офтальмология / Английские материалы / Basic Sciences in Ophthalmology_Velayutham_2009

the two segments is marked by a constriction, which lies exactly between the external limiting membrane and RPE. The plasma membrane of inner and outer segments is continuous. Extensions of Muller’s cells called the fiber baskets of Schultz separate adjacent inner segments.

The rods are 40-60 microns in length. The outer segment is cylindrical and thin. It is refractile and shows striations on the surface, which run both transversely and longitudinally. The rhodopsin in rods is called the visual purple. The inner segment is slightly thicker and has transparent protoplasm. From the inner end of this segment a thin sinuous fiber exits and crosses the external limiting membrane. The nucleus of the cell which, is situated in the outer fiber is located in the outer nuclear layer. This nucleus is called the rod granule. The inner fiber extends further and ends in the outer molecular layer. The terminal end is bulbous and is called rod spherule. It transmits impulses to the dendrites of the bipolar cells.

The cones are larger in the foveal region, i.e. about 85 microns. In the periphery they are about 40 microns in size. The outer segments of the cones are conical and shorter than that of rods. The inner portion of the cones is in close contact with the nucleus, which lies in the outer nuclear layer. The cone fibers which are thicker than the rod fibers end in a pedicle. The lateral processes of the cone pedicle meet the dendrites of the bipolar cells in the outer molecular layer.

The rod and cone outer segments contain a series of lamellae, which appear discoid. These discs have larger diameter close to the inner segment. The inner segment has a segment called ellipsoid and a basal myoid.

The cell bodies of the cones are arranged in a single layer where, as that of the rods are in multiple rows (Fig. 7.3).

Fig. 7.3: Arrangement of nerve fibers

The outer segments of rods and cones are shed everyday. Rods shed the discs one-hour after exposure to light in the morning. The outer segments of the rods are entirely replaced in 10 days.

At the fovea only cones are present. 50% of the cones are present in the macula. Highest number of rods is present in the elliptical ring that passes through the optic nerve. Rods degenerate as age advances.

EXTERNAL LIMITING MEMBRANE

It extends from the ora serrata to the optic disc. It is pierced by the processes of the rods and cones and hence it is fenestrated. It is actually formed by the terminal expansion of the fibers of the Muller’s cells and junction between the rod and cone processes. It separates the photoreceptor layer from the outer nuclear layer.

OUTER NUCLEAR LAYER

This lies internal to external limiting membrane and it contains the nuclei of the rods and cones. The nuclei of the cones are close to the external limiting membrane. The axons of these cells synapse with horizontal and bipolar cells in outer plexiform layer. There are 4 rows of nuclei on the temporal side of the disc, 8 or 9 on the nasal side and 10 rows of cone nuclei in the foveal area. The rest of the retina has one row of cone and four rows of rod nuclei.

OUTER PLEXIFORM LAYER

This is the junctional zone between photoreceptors and first order neuron, i.e. the bipolar cells. It is thickest around the fovea. A middle limiting membrane exists in this layer and it helps in demarcating the outermost extent of normal retinal vasculature.

The following synaptic units are present in this layer:

a.Presynaptic elements – spherules and pedicles,

b.Synaptic elements – bipolar and horizontal cells are in contact with the spherules and pedicles. Except for the fovea in other places many

photoreceptors connect with one bipolar cell.

c. Postsynaptic elements – bipolar dendrites and horizontal cell processes.

INNER NUCLEAR LAYER

This layer has four layers of cell bodies from within outward. These are:

a.The nuclei of amacrine cells,(so called as they lack axons)

b.Muller’s cells,

c. Bipolar cells and

d.Horizontal cells

Bipolar cells represent the 1st order neuron and transmit nerve impulse

from photoreceptors to ganglion cells. Horizontal and amacrine cells have long branches, which extend horizontally and act as integrating circuits. The Muller’s cells are supportive and also supplies nutrition. They also help in transmission and modification of nerve impulses. The capillaries of central retinal vessels are present in this layer.

poles. Fibers from the macula and fovea go directly forward to the temporal side of the disc forming the papillomacular bundle. Besides the nerve fibers there are:

a.centrifugal fibers

b.fibers of Muller cells

c. neuroglial cells and

d.retinal vessels in this layer.

The nerve fiber is thinnest on the temporal side of the disc. The upper lateral and lower lateral come next followed by the medial part. The thickest parts are the upper medial and lower medial quadrant. When papilloedema occurs the thickest parts are affected first, so the upper medial and lower medial quadrants develop edema first. In the foveal region just like other layers it is almost absent.

Normally fibers in the nerve fiber layer are nonmyelinated because during development, myelinization of optic nerve stops at lamina cribrosa. Sometimes myelinated fibers are seen clinically in otherwise normal fundi as patch of white fibers with a feathery edge usually radiating out of the disc or rarely as isolated patch anywhere in the retina.

This layer contains some centrifugal fibers, fibers of Muller cells, neuroglial cells and retinal vessels.

INTERNAL LIMITING MEMBRANE

In between the retina and vitreous there is a membrane called the internal limiting membrane of the retina or the hyaloid membrane of the vitreous.

This is a PAS positive filamentous basement membrane of the Muller’s cells and stains with Mallory’s triple stain. The inner surface of the membrane is even but the retinal side is irregular conforming to the irregularity of basal plasma membranes (foot-plates) of Muller’s cells. But over a vessel it is smooth on both sides.

GLIAL SYSTEM

Muller’s Cells

These are the largest retinal cells. They span the entire retina inner limiting membrane to even beyond external limiting membrane. They synthesize and store glycogen and provide glucose to the surrounding neurons. Its cell body is in the inner portion of the inner nuclear layer.

Accessory Glia

Cells like astrocytes, oligodendrocytes and microglia are also present in the retina but only in the nerve fiber layer, ganglion cell layer and inner plexiform layer. Microglia are phagocytic, while the star shaped astrocytes send processes

around the nerve cells and blood vessels giving support to them. The oligodendrocytes in the retina do not produce myelin.

BLOOD SUPPLY

The retina internal to middle limiting membrane receives its blood supply from branches of the central retinal artery. The central retinal artery and vein divide into 4 main branches, the superior and inferior nasal and temporal vessels. Subsequent branching tends to be at right angles in the posterior part of the eye and dichotomous or γ shaped in the periphery. In 20% of normal eyes a cilio retinal vessel arises from the choroidal circulation and supplies the foveal region. The retina external to the middle limiting membrane is nourished by choriocapillaries.

Beyond the first bifurcation, vessels become arterioles, i.e. they loose their continuous muscular coat and internal elastic lamina. The venules, which course in the nerve fiber layer ultimately break up into an extensive capillary network and lie at all levels of retina internal to the middle limiting membrane present in the outer plexiform layer.

Arterioles differ from venules. They are more circular in cross-section and are thick walled. Arterioles lie internal to venules where they cross and have a broader surrounding avascular zone. Capillaries have a single layer of continuous endothelial cell lining surrounded by an interrupted layer of pericytes. A basement membrane derived from the endothelial cells and pericytes completely surrounds the outer surface of endothelial cells and cell surface of pericytes.

Around the disc is a distinct capillary network called the radial peripapillary network. This layer of capillaries follows the nerve fiber layer and supplies the axons arising temporal to the nerve head. The pattern of distribution of these capillaries is responsible for the arcuate scotoma seen in glaucoma (Fig. 7.4).

Fig. 7.4: Arrangement of posterior ciliary vessels and nerves

REGIONAL ANATOMIC VARIATIONS

Central Retina

Contains the macula and the optic disc.

‘The optic disc is a circular to oval structure 1.5-mm in diameter.

The macula is clinically seen as an ill-defined yellow zone. This is free of capillaries and located temporal and slightly below the center of the disc. In the middle of this yellow region is a depression called the fovea. The sidewall of this depression is called the clivus. The edge of this slope shines bright when light is shone on the retina with an ophthalmoscope. A tiny bright reflex is seen in the center of this annular reflex. That is the region of the foveola. The macula around the fovea is divided into the inner parafoveal and outer perifoveal area.

The macula contains 2 or more layers of ganglion cells with an obliquely oriented outer plexiform layer. There is a high concentration of cones and the internal limiting membrane here is very thick.

The fovea is depression in the center of the macula. Its size is equal to the optic disc. Along the slope of the fovea called the clivus there is a transition from the thick internal limiting membrane to a thinner membrane. The retina is also avascular here. The ganglion cells are also smaller compared to other areas. The ganglion cells are 5 to 7 layers in thickness at the periphery and disappear gradually as the foveola is approached. The outer nuclear layer is also 8 layers thick in the periphery and only 2 or 3 cell layers in the center and are displaced towards the inner limiting membrane. The outer plexiform layer consists exclusively of cones and a few internally displaced photoreceptor nuclei. There is no nerve fiber layer, ganglion cell layer, inner plexiform layer or inner nuclear layer. As the inner photoreceptor fibers travel horizontally away from the center to reach the displaced bipolar neurons they form a plexiform lamina known as the Henle’s layer. Because of this any edema or deposition of exudates take a radial pattern here.

The masking of choroidal fluorescence during FFA is due to the presence of xanthophyll pigments as well as the increased pigmentation of the retinal pigment epithelium at the fovea.

Rods are absent in the fovea. This is the reason for physiological central scotoma in dim light.

Peripheral Retina

Neural retina ends anteriorly at the ora serrata. It is a serrated zone approximately 2 mm wide temporally and 1mm wide nasally. The dentate processes, which produce serrations, are prominent nasally. They extend into the pars plana and point towards the valleys between ciliary processes. It is 8.5 mm from the limbus, 6 mm from the equator and 25 mm from the optic nerve.The peripheral

the future macula the nerve fibers grow out of them and reach the optic stalk. By the fifth week, two layers of neuroblastic cells the inner and outer are seen with the cell processes in between. The dendritic processes from the ganglion cells create a non-nuclear zone, which forms the future inner plexiform layer. The ganglion cells are the first cells of retina to become clearly differentiated. Initially there are a large number of ganglion cells. These later undergo apoptosis and reduce in number but increase in size.

The outermost cells of the nuclear zone give rise to the rods and cones. High mitotic activity is seen between the 4th and 12th week. The outer plexiform layer develops by the 5th month. Differentiation of cones begins at 5th month. Rods develop later around the 7th month. No new retinal cells are formed after the 6th month. The development of the fovea starts at the end of 6th month by thinning of ganglion cells. This thinning causes a depression in the center. Later the inner nuclear layer also becomes thinner. The cones appear later in the macular region and they are longer and thinner than the cones in other areas. Only when the child is several months old the cones become mature and they continue to differentiate until the fourth year of life. The location of the fovea is same at birth as in an adult.

Amacrine cells develop by 14th week and bipolar cells by 23rd week. The pigment epithelium develops from the outer wall of the optic cup.

These cells which are at first cylindrical gets pigmented at about three weeks. These are the earliest cells to become pigmented in the body. The anterior portion is pigmented first. The cylindrical cells then become cubical and are arranged in a single row. The pigment epithelial cells induce growth of choroid, sclera and neurosensory retina. So if growth of this layer is affected other layers also will be affected.

•Medially the frontal bone, maxillary bone

•Inferiorly maxillary and zygomatic bones and

•laterally by the zygoma and frontal bones.

The margin is like a spiral, as the superior margin is continuous with the posterior lacrimal crest while the inferior is continuous with the anterior lacrimal crest. Each side is approximately 40 mm in size, but the width is slightly more than the height. The ratio between the two varies in different races and is called the orbital index.

The supraorbital notch or foramen transmits the supraorbital nerve and vessels. This can be felt through the skin. Local anesthetic agents for ptosis surgeries or suturing cuts near the brow can be injected here. Medial to this groove is another one for the supratrochlear nerve and vessels.

The inferior margin is above the floor of the orbit. The suture between the zygoma and maxilla forming this margin is marked by a tubercle. This is situated about the middle of the lower margin. The infraorbital foramen transmitting the infraorbital nerves is situated just below the tubercle. Hence this serves as a landmark when the infraorbital nerve has to be anesthetized.

The four walls of the orbit (Fig. 8.1B).

Fig. 8.1B: The four walls of orbit