Retinal Macrostructure

Embryology

• Day 25: optic vesicle formed by lateral out-pouching of the diencephalon
• Proximal constricted part: optic stalk
• Dilated distal part: optic cup
• Week 4-5: optic cup and stalk form choroidal fissure

• Week 5: edges of the choroidal fissure close: cup first, then stalk, from forebrain to cup direction. 

• Thickened inner portion of the optic vesicle is known as the retinal disk: future neural retina

• Outer nuclear zone: mitosis is maximal here, completed by week 15
• Inner marginal zone: once mitosis completes differentiation occurs here

• Inner and outer neuroblastic layers are formed from the above by week 7, separated by the transient layer of Chievitz

• Inner neuroblastic layer forms ganglion cells, Muller cells and amacrine cells
• Outer neuroblastic layer forms bipolar cells, horizontal cells, rod and cone nuclei
• Photoreceptors derived from cilia in the outer nuclear/neuroblastic layer
• Lamination completes by 4.5 months and ora serrata formed by 6 months.

• Week 3: thinner outer portion of the vesicle contains pseudostratified columnar cells with pigment granules: future RPE

• The cilia disappear as melanogenesis begins

• These two layers are continuous at the optic cup margin (future iris and CB)

• The opposing surfaces of these layers is initially ciliated: the outer portion (RPE) cilia degenerate but the inner portion cilia contribute to photoreceptor formation (as above)

• 4-5 months: maculogenesis: localised increase in ganglion cell density at the posterior pole

• 8-9 layers of ganglion cell nuclei at month 6
• Peripheral displacement of ganglion cells due to axonal elongation at month 7
• Foveal cones increase in length and decrease in width (and increase density)

• At birth: single layer of ganglion cells remain as internal nuclear layer over macular cones

• At 4 months of life: these ganglion cells are displaced to leave central cones ‘uncovered’

Retinal perfusion

• Retinal vessels supply (high oxygen exchange, low flow: 25mm/s):
• Nerve fibre layer
• Ganglion cell layer
• Inner plexiform layer
• Inner third of the inner nuclear layer
• Choroid vasculature supplies (low oxygen exchange, high flow: 150mm/s):
• Outer two thirds of the inner nuclear layer 
• Outer plexiform layer
• Outer nuclear layer
• Photoreceptors
• RPE

• Retinal capillaries are surrounded by
• Astrocyte foot processes
• Contractile pericytes
• Thick basal lamina

• Retinal blood flow in the central retinal artery is autoregulated (same for iris and ciliary body perfusion) so does not vary with changing perfusion pressures (ie. is constant between ocular perfusion pressures of 45-145mmHg)

• Responsive to hyperoxia (vasoconstriction) and hypercapnia (vasodilation)
• Responsive to eicosanoids and nitric oxide