Verkkokalvon rakenne

Lisätietoja The central part of the retina (macula) corresponds to the "posterior pole" of the eye. It borders to the optic disk and the major temporal vascular arcades of the retina. It is 5-6 mm in diameter. In the macula, the ganglion cell layer is at least two cells thick and it is functionally the area of sharp (reading) vision of the retina.

The fovea and, especially, the foveola at the center of the macula are responsible for the sharpest vision. The yellow spot (macula lutea) is also located in the macula. Its colour is caused by a xanthophyll pigment. The diameter of the fovea is 1.5 mm and when the fundus is examined ophthalmoscopically the fovea is seen as an area that is darker red in colour than the orange fundus. In addition to the xanthophylls pigment this is caused both by the thinness of the retina in the foveal area and by the thickness of the pigment epithelium in this area; the redness of the choroid and the thick pigment epithelium are seen more clearly through a thin retina. The diameter of the foveola is 0.35 mm and it is visible especially in young people as a tiny light reflection in the middle of the fovea. The thicker rims of the fovea can induce a circular light reflection. Sometimes the clinicians call the anatomical fovea the macula and the anatomical foveola the fovea.

Two objects are resolved from one another if they excite individual photoreceptors that are separated a third one. In the macular area photoreceptor cells have been packed as close to each other as possible to increase the visual acuity. All the receptor cells in the fovea are cones, which are, exceptionally, rod-shaped. Only a couple of thousand of them will fit into the foveola. Other layers of the retina have been displaced from the fovea in order that the rays of light would have an unrestrained access to these cells. The foveola is thin because the central part of the retina contains only photoreceptor cells. The thickest part of the retina is the rim of the fovea, where other layers have been displaced from its central part. To improve discrimination, the central parts of the retina around the foveola lacks blood vessels.

Verkkokalvon rakenne - sahalaita The periphery of the retina (periferia) is thinner, the closer one comes to the ciliary body.choroid and the ciliary body the retina ends and becomes an unpigmented epithelium in the inner part of the ciliary body. This boundary is even on the temple side but sawed on the nose side of the eye and it is called the serrated ora of the retina (ora serrata). In the serrated ora the retina fastens on the fundus of the eye as tightly as on the optic papilla. The other parts of the retina have only pressed against the pigment epithelium. The pigment cells pump continuously fluid away from beneath the retina, and together with the viscous intercellular material between the photoreceptor cells and the pigment epithelium, this suction keeps the retina in place. If fluid is secreted under the retina or a puncture occurs in it where fluid gets through the opening under the retina, it will detach.

The retina is divided into ten layers which consist of its cells and their processes:

  1. The inner limit membrane (membrana limitans interna) is restricted to the hyaloid membrane. It consists of the terminal disks of the radial gliacytes in other words Müller’s cells .

  2. The nerve fibre layer consists of the axons of the ganglion cells of the retina. It is thickest near the optic papilla. The nerve fibres which arrive from the area of the exact vision (macula) and the retina on the nose side are nearly vertically directed and the nerve fibres arriving from elsewhere are curvedly directed towards the optic papilla. The bleedings of the nerve fibre layer settle between the radial nerve fibres and are therefore also radial, flame-shaped. The bleeds are spot-shaped in other layers of the retina.

    Verkkokalvon histologia

  3. In the ganglion cell layer there is the somas of the large ganglion cells of the retina. In the periphery of the retina it is one cell layer strong, there are about ten layers of ganglion cells in the area of the exact vision. The astrocytes, which toughen the retina and participate in the metabolism, are also located in the ganglion cell layer.P>
  4. The inner plexiform layer consists of the dendrites of the ganglion cells, the axons of the bipolar cells and short fibrous amacrine cells and synapses between them.

  5. The inner nucleus layer (the inner granular layer) contains the somas of the bipolar cells, the amacrine cells and the horizontal cells. The bipolar cells transmits the nerve impulse from the sensor cells to the ganglion cells. They infuse most of the inner nucleus layer. The somas of the amacrine cells are located on the same side as the rim of the inner plexiform layer, the somas of the horizontal cells on the same side as the rim of the outer plexiform layer. Both of the cells are interneurons which edit nerve impulses. The somas of the radial gliacells of the retina, in other words Müller’s cell’s are also located in this layer. Their processes reach through the inner boundary membrane of the retina to the outer one. Müller’s cells toughen the retina and support the metabolism of its nerve cells. Their cytoplasm surrounds all the nerve cells of the retina.

  6. The outer plexiform layer consists of the axons of the photoreceptor cells as well as the dendrites of the bipolar cells and the horizontal cells and from the synapses between them.

  7. The somas of the photoreceptor cells -cones and rods- are in the outer nucleus layer (the outer granular layer). The somas of the cones are located immediately inside the outer limit membrane in one cell layer, the somas of the rods form the rest of the outer nucleus layer.

  8. The receptor parts of the cones and rods are in the photoreceptor layer. They consist of an inner segment and an outer segment which are connected through cilia. The inner segments of the cones are thick and the inner segments of the rods are thin. There are many mitochondria, ribosomes and other common organelles in them. There are membrane sack piles in the outer segments where the photosensitive visual pigments are located. The pigments of cones, opsins, are sensitive to either blue, green or red light depending on the cone. The pigment of rods rhodopsin, is extremely sensitive to any coloured light and is also able to operate in crepuscular light. Only the rods operate between the dark and the twilight, and the colours cannot be seen then. There are only cones in the central part of the exact vision in macula, in the periphery of the retina the rods dominate.

  9. The surface of the outer limit membrane (membrana limitans externa) looks like membrane in the light microscope and forms out of the tight intermediate junctions between the inner segments of the photoreceptor cells and Müller’s cells . In reality it is not a membrane.

  10. Verkkokalvon rakenne - pigmenttiepiteeli The pigment epithelium is a one layered neuroepithelium which is restricted to Bruch’s membrane. It consists of regularly hexagonal cells in which there are many melanin granules. There are a lot of microvilli in the cell membrane on the tip side of the photoreceptor cells which thrust between the segments of the photoreceptor cells. There is a plenty of intercellular material between the pigment epithelium and the photoreceptor cells which their metabolites exchange through. The task of the pigment epithelium is to phagocytize constantly regenerating tips of the outer segments of the photoreceptor cells and the membrane sacks. The pigment epithelial cells do not regenerate. The pigment epithelium receives the visual pigments with the membrane sacks and returns them to the photoreceptor cells.

The metabolism of the retina, especially of the photoreceptor cells is among the most active in the body. The retina gets nourishment and oxygen in two ways. Two thirds (from the nerve fibre layer into the inner nucleus layer) of the retina is nourished by the branches of the central retinal artery (arteria centralis retinae). This terminal artery starts from the ophthalmic artery (arteria ophthalmica), which is in turn the branch of the inner carotid artery. An outermost third of the retina (from the outer plexiform layer to the photoreceptor layer) receives its nourishment with diffusion through the pigment epithelium and the Bruch’s membrane from the capillary network of the choroid (choriocapillaris), which originates from the ciliary arteries (arteriae ciliares posteriores breves et longae, arteria ciliares anteriores).

The general deviation is a so-called retinal ciliary artery, which originates from the ciliary artery, but goes through the optic papilla to the retina instead of the choroid towards the macula. Its circulation continues even if the central retinal artery is clogged.

The capillaries of the retina are close unlike the capillaries of the uvea (an iris, a ciliary body and a choroid). The capillaries of the retina form an inner blood-retina barrier comparable to the blood-brain barrier. Similar outer blood-retina barrier consists of the tight junctions between the epithelial pigment cells of the retina.

The arteries and veins of the retina form four large arcades, each of which vascularisates one quadrant of the retina. The large blood vessels branch very individually and the photo of the fundus of the eye is as reliable in the identification of the individual as the fingerprints. The walls of the blood vessels of the retina are as transparent as the retina itself. When the fundus of the eye is examined, only the blood columns in the blood vessels are observed. The blood column is thinner and bright red in the arteries, thicker and dark red in the veins. There is a combined tunica externa (adventitia) in sections where the artery and the vein cross. The artery goes usually on top of the vein and if the artery hardens, it flattens and obstructs the vein easily. This exposes the crossings of the blood vessels to the phlebothromboses.

PähkinänkuoressaIn a nutshell

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© 1999-2003 University of Helsinki, Clinical Institute, Department of Ophthalmology

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