Adaptation of the central retina for high acuity vision: Cones, the fovea and the avascular zone

Abstract

Presence of a fovea centralis is directly linked to molecular specification of an avascular area in central retina, before the fovea (or `pit') begins to form. Modeling suggests that mechanical forces, generated within the eye, initiate formation of a pit within the avascular area, and its later remodeling in the postnatal period. Within the avascular area the retina is dominated by `midget' circuitry, in which signals are transferred from a single cone to a single bipolar cell, then a single ganglion cell. Thus in inner, central retina there are relatively few lateral connections between neurons. This renders the region adaptable to tangential forces, that translocate of ganglion cells laterally / centrifugally, to form the fovea. Optical coherence tomography enables live imaging of the retina, and shows that there is greater variation in the morphology of foveae in humans than previously thought. This variation is associated with differences in size of the avascular area and appears to be genetically based, but can be modified by environmental factors, including prematurity. Even when the fovea is absent (foveal hypoplasia), cones in central retina adopt an elongated and narrow morphology, enabling them to pack more densely to increase the sampling rate, and to act as more effective waveguides. Given these findings, what then is the adaptive advantage of a fovea? We suggest that the advantages of having a pit in central retina are relatively few, and minor, but together work to enhance acuity.

The optic nerve and peripapillary vasculature in albinism

“…We also found significantly more patients with albinism with situs inversus and a nasally directed artery compared to controls, but any vascular changes related to vision loss are more likely to occur in the foveal area which is known to have an abnormal foveal avascular zone in albinism [19]. The amount of decussation was not studied in this report and may also contribute to vision loss.…”

“…Optical coherence tomography has further defined the arrest in foveal development [13][14][15][16]. However, it has recently been questioned whether foveal dysmorphology can completely account for the reduced visual acuity found in individuals with albinism [17][18][19]. The presence of granular melanin pigment in the retinal pigment epithelium has been associated with better vision in albinism, so presence or absence of macular pigment could also play a role [20][21][22].…”

Is Angiostatin Involved in Physiological Foveal Avascularity?

“…The fovea is devoid of retinal vessels, and is known as the foveal avascular zone (FAZ). 1 According to previous investigations, vascular endothelial growth factor (VEGF) is the key factor that induces and maintains the ingrowth of retinal vessels during development. [2][3][4][5] Retinal ganglion cells (RGCs) express VEGF strongly during retinal development and inhibit the migration of astrocytes and endothelial cells into the macula.…”

“…6 Furthermore, it has been proposed that angiostatic factors are expressed in the macula during development, regulating vascular growth through repellent mechanisms. 7 Although the nature of the inhibitory factor expression involved in the avascularity of the fovea throughout life has yet to be clarified, 8 expression of several angiostatic proteins including ephrins A1 and A4 and their receptor EphA6, brain natriuretic peptide (BNP), and pigment epithelium-derived factor (PEDF) 1,[7][8][9][10] was associated with avascularity. In particular, the critical balance between the angiogenic VEGF and the angiostatic PEDF appears to be essential for preserving 11 and stabilizing 5,12,13 the avascular environment of the outer retina.…”

Gap Junction Coupled Cells, Barriers and Systemic Inflammation

“…Retinal astrocytes are found around blood vessels and the distribution depends on the density of nerve fibers. They are absent in the fovea and the avascular zone to maintain high acuity vision [31]. A main difference between astrocytes and Müller cells is that only the astrocytes are formed with gap junctions, where the astrocytes are coupled in syncytium and have a Ca 2+ signaling system.…”