Normal




Retinal Histology


The sensory retina extends to the ora serrata, where it is continuous with the nonpigmented ciliary epithelium of the pars plana. The ora serrata is 2.1 mm wide temporally and 0.7–0.8 mm wide nasally and is located more anteriorly on the nasal versus the temporal side. The nasal ora has a irregular, or serrated border and is about 6 mm posterior to the limbus while the temporal ora has an regular or smooth border and is about 6.5 mm posterior to the limbus. The average distance from the ora serrata to the optic nerve is 32.5 mm temporally and 27 mm nasally, and 31 mm superiorly and inferiorly. The retina itself is a thin transparent tissue that is thickest near the optic nerve where it measures 0.56 mm and becomes progressively thinner at the equator (0.18 mm) and the ora serrata (0.1 mm). At the fovea, the retina is thin and measures 0.2 mm in thickness. The nerve fiber layer increases toward the edge of the disc and is the only retinal structure that continues into the disc to become the optic nerve. The sensory retina is composed of nine contiguous microscopic layers, linked by synaptic connections between axons and dendrites in the inner and outer plexiform layers. The neuronal cells are supported by Müller cells and the astrocytes from the inner portion of the retina. The retinal pigment epithelial layer is a monocellular tissue of irregular density. It has a cuboidal and hexagonal shape with villous processes that envelop the photoreceptor outer segments. It also contains melanin granules and is taller, more densely pigmented, and columnar in shape in the central macula.


Bruch membrane refers to a sheet-like condensation of the innermost portion of the choroidal stroma that consists of two layers of collagen on either side of a central layer of elastic tissue. The basement membrane of the retinal pigment epithelium (RPE) and the choriocapillaris endothelium are the boundaries of Bruch membrane, although this interpretation is controversial. Some consider Bruch membrane as a part of the choroidal stroma. The choroidal circulation is supplied by the short ciliary or choroidal arteries that are concentrated in the macula and peripapillary region. A dense anastomotic network of vessels comprises the choriocapillaris bordered by the outer part of Bruch membrane. In the macula, the choriocapillaris is composed of a lobular pattern of highly concentrated and interconnecting capillary segments supplied by a central arteriole and drained by circumferential venules.




Left: fundus photograph matched with a horizontal section of the macula, delineating the (a) foveola, (b) fovea, (c) parafovea, and (d) perifovea. Right: schematic diagram showing the relative dimensions of the fovea, foveola, macula (area centralis), and peripheral fundus. ILM, internal limiting membrane; ELM, external limiting membrane.





The histology of the macula is represented in this image and is defined by the multilayered ganglion cell layer. The retina is bordered anteriorly by the internal limiting membrane (ILM) comprised of Müller cell footplates. Note the nerve fiber layer (NFL) and its ganglion cells (GC), the inner plexiform layer (IPL), the inner nuclear layer (INL), middle limiting membrane (MLM), the outer plexiform layer (OPL), the outer nuclear layer (ONL), the external limiting membrane (ELM), the inner segments of the photoreceptors (IS), the outer segments of the photoreceptors (OS), and the retinal pigment epithelium (RPE). The ELM is comprised of Müller cell attachments to the inner segments.





These are histological specimens of the optic nerve (top), the foveal area including the sclera (middle), and the peripheral retina (bottom) defined by the single layer of ganglion cells.




The Fundus





This is a color montage of a blond fundus. The choroidal circulation is visible through a mildly pigmented retinal pigment epithelium. Note the 4 vortex veins (arrows) in the outer choroidal circulation, which accommodate the very high flow supplied posteriorly by 10–20 short posterior ciliary branches of the ophthalmic artery. Nasal and temporal long posterior ciliary arteries supply the anterior choroid and uvea.




Vitreous


The vitreous body extends from the posterior lens to the surface of the retina. It is slightly less than 3.9 mL in volume, comprising approximately 2/3 to 3/4 of the adult globe. It is spherical posteriorly and saucer-shaped anteriorly due to a depression caused by the convexity of the posterior lens surface. The vitreous cortex is made of 3 visible components: (1) collagen-like fibers, (2) cells, and (3) mucopolysaccharides and other proteins. The vitreous cortex is covered by the hyaloid membrane, a thin enveloping structure. In the posterior pole, there is a precortical vitreous pocket, referred to as the premacular bursa, which may extend to the retinal vascular arcades.




Note the posterior precortical vitreous pocket (PPVP) that forms the premacular bursa and that is located immediately anterior to the posterior fundus surrounded by the temporal vascular arcades (arrows). The posterior wall of the PPVP is composed of a thin layer of vitreous cortex. The rest of its border is contoured by formed vitreous. Occasionally, the PPVP expands to become confluent with adjacent lacunae in the vitreous. This structure is inconsistently detectable clinically when there is posterior vitreous detachment. Otherwise it is consistently present in normal eyes.

Courtesy of Dr. Lennart Berglin, Dr. Louise Bergman, and Dr. Henry F. Edelhauser




Retina


The retina lines the inner surface of the eye with neuronal connections extending to the optic nerve and the central nervous system. It is a layered diaphanous structure comprised of neurons and interconnected synapses with principal light-sensitive cells at its outer aspect in the photoreceptor layer, which contains rods and cones. There are approximately 6 million cones, most densely packed within the fovea, and 125 million rods positioned predominantly in the eccentric macula and peripheral retina.




This image illustrates the distribution of the retinal vessels throughout the retina. The retinal venules are darker and more dilated (in a 3:2 ratio) than the brighter arterioles. Note the 4 major vascular arcades, 2 temporal and 2 nasal.




Macula


The macula refers to an area that includes the parafoveolar area (about 2.85 mm in diameter), but some equate the macula to the foveal area (about 1.8 mm in diameter). The fovea is a 1.5 mm depression in the center of the macula. It is located about 4 mm temporal and 0.8 mm inferior to the horizontal raphe. The average thickness of the fovea is about 0.25 mm, roughly half that of the adjacent parafoveal area. The central 0.35 mm of the fovea is the foveola, which is located in a capillary-free zone (i.e. the foveal avascular zone) that measures about 0.5 mm in diameter. A small protuberance in the center of the foveola is called the umbo, where there is a dense concentration of cell bodies of elongated cones referred to as the cone bouquet of Rochon–Duvigneaud. A 0.5 mm wide annular zone surrounding the fovea is the area where the ganglion cell layer, nuclear layers, and outer plexiform layer of Henle are the thickest. This is referred to as the parafoveal area. This area is surrounded by a 1.5 mm ring zone called the perifoveal area where the ganglion cell layer is reduced from 5–7 layers to a single layer of nuclei, as noted elsewhere in the peripheral retina.


There are several modifications in the retinal architecture in the macular area, beginning with the absence of retinal vessels in the central foveal region (i.e. foveal avascular zone). There are no rods in the foveola, and the cones are so modified that they resemble rods in form. The outer segments of the cones are long and approach the apical side of the RPE cells. At the edge of the fovea, the ganglion cell layer and the inner nuclear layer thicken, but both layers disappear within the fovea. In the foveolar area, only photoreceptor cells and Müller cell processes are present. Each cell is united with a single bipolar cell and possibly a single ganglion cell, yielding maximal transmission of the visual stimulus.




The morphological landmarks of the macula are not very distinct clinically. However, a dark zone surrounding the fovea is clearly evident due to the intrinsic pigmentation of the retina (xanthophyll) and, above all, the retinal pigment epithelium (melanin). The foveal pit and the umbo are noted by the characteristic central foveolar light reflex.

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Jul 30, 2019 | Posted by in OPHTHALMOLOGY | Comments Off on Normal

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