FOVEOLA
The bouquet of central cones is surrounded by the fovea bottom, or foveola ,which measures 350 um in diameter and 150 um in thickness. This avascular area consists of densely packed cones that are elongated and connected by the external limiting membrance. as a result of the elongation of the outer segments,the external limiting membrance is bowed vitreally, a phenomenon that has been fovea externa.
Both umbo and foveola represent the most visible part of the outer retina ; however , to the level of the external limiting membrance , all cones and their axons are enveloped by the processes of Muller cells, which form the vitreal inner layer and elaborate and support the internal limiting membrance. Thus, the apex-to apex arrangement of the optic cup is maintained by the processes of mullerian glia that face the apices of the pigment epithelial cells in the foveola. The high metabolic demands of the central cones are met by direct contact with the pigment epithelium , as well as through the processes of glia whose nuclei lie peripheral in the inner nuclear layer and closer to the perifoveal vascular arcades.
FOVEA
The fovea consists of the thin bottom a 22 declivity ( the clivus ) and a thick margin . The bottom , or foveola , was described earlier. The declivity of 22 denotes the lateral displacement of the second and third neurons in the inner nuclear layer, which includes most of the nuclei of its mullerian glia . The avascular foveola is surrounded by the vascular arcades, a circular system of capillaries. These vessels are located at the level of the internal nuclear layer and leave an avascular zone of 250- 600 um between them. The declivity also is associated with an increase in basement membrance thickness, which reaches a maximum at the foveal margin. Internal limiting membrance thickness and strength of of vitreal attachment are inversely proportional : that is , adhesions are strongest in the foveola.
PARAFOVEA
The parafovea is a belt that measures 0.5 mm in width and surrounds the foveal margin.
At the distance from the centre, the retina features a regular architecture of layers, which includes 4-6 layers of ganglion cells and 7-11 layers of bipolar cells.
PERIFOVEA
The perifovea surrounds the parafovea as a belt that measures 1.5 mm wide. The region is characterized by several layers of ganglion cells and six layers of bipolar cells .
MACULAR OR CENTRAL AREA
The umbo, foveola , fovea parafovea , and perifovea together constitute the macula , or central area . The central area can be differentiated from the extra-areal periphery by the ganglion cell layer . In the macula , the ganglion cell layer is several cells thick: however, in the extra- areal periphery, it is only one cell thick . The macular border coincides with the course of the major temporal arcades and has an approximate diameter of 5.5 mm . As measured along the outside of the sclera, the centre of the macula lies 4.5 mm temporal to the centre of the optic nerve and 3.0 mm temporal to the temporal margin of the optic nerve. The surface landmark of the macula is in close proximity to the insertion of the inferior oblique muscle , being 1 to 2 mm posterior and 1 mm above the nasal limit of insertion of the muscle into sclera.
The bouquet of central cones is surrounded by the fovea bottom, or foveola ,which measures 350 um in diameter and 150 um in thickness. This avascular area consists of densely packed cones that are elongated and connected by the external limiting membrance. as a result of the elongation of the outer segments,the external limiting membrance is bowed vitreally, a phenomenon that has been fovea externa.
Both umbo and foveola represent the most visible part of the outer retina ; however , to the level of the external limiting membrance , all cones and their axons are enveloped by the processes of Muller cells, which form the vitreal inner layer and elaborate and support the internal limiting membrance. Thus, the apex-to apex arrangement of the optic cup is maintained by the processes of mullerian glia that face the apices of the pigment epithelial cells in the foveola. The high metabolic demands of the central cones are met by direct contact with the pigment epithelium , as well as through the processes of glia whose nuclei lie peripheral in the inner nuclear layer and closer to the perifoveal vascular arcades.
FOVEA
The fovea consists of the thin bottom a 22 declivity ( the clivus ) and a thick margin . The bottom , or foveola , was described earlier. The declivity of 22 denotes the lateral displacement of the second and third neurons in the inner nuclear layer, which includes most of the nuclei of its mullerian glia . The avascular foveola is surrounded by the vascular arcades, a circular system of capillaries. These vessels are located at the level of the internal nuclear layer and leave an avascular zone of 250- 600 um between them. The declivity also is associated with an increase in basement membrance thickness, which reaches a maximum at the foveal margin. Internal limiting membrance thickness and strength of of vitreal attachment are inversely proportional : that is , adhesions are strongest in the foveola.
PARAFOVEA
The parafovea is a belt that measures 0.5 mm in width and surrounds the foveal margin.
At the distance from the centre, the retina features a regular architecture of layers, which includes 4-6 layers of ganglion cells and 7-11 layers of bipolar cells.
PERIFOVEA
The perifovea surrounds the parafovea as a belt that measures 1.5 mm wide. The region is characterized by several layers of ganglion cells and six layers of bipolar cells .
MACULAR OR CENTRAL AREA
The umbo, foveola , fovea parafovea , and perifovea together constitute the macula , or central area . The central area can be differentiated from the extra-areal periphery by the ganglion cell layer . In the macula , the ganglion cell layer is several cells thick: however, in the extra- areal periphery, it is only one cell thick . The macular border coincides with the course of the major temporal arcades and has an approximate diameter of 5.5 mm . As measured along the outside of the sclera, the centre of the macula lies 4.5 mm temporal to the centre of the optic nerve and 3.0 mm temporal to the temporal margin of the optic nerve. The surface landmark of the macula is in close proximity to the insertion of the inferior oblique muscle , being 1 to 2 mm posterior and 1 mm above the nasal limit of insertion of the muscle into sclera.
No comments:
Post a Comment