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The Horizontal Raphe of the Human Retina and Its Watershed Zones

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Review

The Horizontal Raphe of the Human Retina and Its Watershed Zones

Christian Albrecht May et al. Vision (Basel).

Abstract

The horizontal raphe (HR) as a demarcation line dividing the retina and choroid into separate vascular hemispheres is well established, but its development has never been discussed in the context of new findings of the last decades. Although factors for axon guidance are established (e.g., slit-robo pathway, ephrin-protein-receptor pathway) they do not explain HR formation. Early morphological organization, too, fails to establish a HR. The development of the HR is most likely induced by the long posterior ciliary arteries which form a horizontal line prior to retinal organization. The maintenance might then be supported by several biochemical factors. The circulation separate superior and inferior vascular hemispheres communicates across the HR only through their anastomosing capillary beds resulting in watershed zones on either side of the HR. Visual field changes along the HR could clearly be demonstrated in vascular occlusive diseases affecting the optic nerve head, the retina or the choroid. The watershed zone of the HR is ideally protective for central visual acuity in vascular occlusive diseases but can lead to distinct pathological features.

Keywords: anatomy; choroid; development; human; retina; vasculature.

Conflict of interest statement

The author declares no conflict of interest. The sponsors had no role in the design, execution, interpretation, or writing of the study.

Figures

Figure 1
Figure 1
A series of fundus photographs and Humphrey visual fields (24–2) demonstrating the progress of an arteritic ischemic optic neuropathy in the left eye of a 91-year-old female. Note the big red spot next to the optic nerve (A), which became smaller after one week (B) and disappeared after one month (C). The initial scotoma of the inferior temporal quadrant (D) increased continuously after one year (E) and three years (F).
Figure 2
Figure 2
A series of fundus photographs and corresponding Humphrey visual fields (10–2) demonstrating the development of a cilio-retinal artery occlusion in the left eye of an 81-year-old female. The initial scotoma (A,D) in the inferior visual field improved somewhat after three months (B,E) but remained constant after four years (C,F).
Figure 3
Figure 3
Retinal angiogram image (Figure 3) as a courtesy of Dr. Jeremy Chess, New York. Note the occluded retinal artery branch (black vessel in the inferior temporal quadrant). The functioning retinal capillary anastomoses fill the precapillary venules of the ischemic side (1 and 2 as examples). The capillary ring around the avascular foveal region appears complete.
Figure 4
Figure 4
Fundus photographs of the right eye of a female with high myopia at age 45 (A) and age 57 (B). Note the increasing peripapillary defect suspecting a choroidal retinal artery occlusion. The Humphrey visual field (24–2) measurement at age 57 (C) shows a superior hemiretinal visual field defect (dark grey areas).
Figure 5
Figure 5
Fundus photographs of a 92-year-old male left eye at the onset of vein occlusion (A) and 10 months later (B), revealing the development of hemorrhage accumulations somewhat recessing the fovea centralis. (C) A photo collage at the second time point shows the symptoms in both the temporal and nasal quadrants of the inferior half.
Figure 6
Figure 6
Schematic summary of the structures influencing the development of the horizontal raphe. 1 = scleral condensation, 2 = long posterior ciliary artery appearance, 3 = short posterior ciliary artery, 4 = immature choriocapillaris, 5 = Müller cells, 6 = retinal ganglion cells, 7 = retinal nerve fiber layer, 8 = immature deep retinal capillary bed, 9 = immature superficial retinal capillary bed, 10 = inner limiting membrane, 11 = chorio-retinal interface.

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