Summary

The fundus is the inside, back surface of the eye. It is made up of the retina, macula, optic disc, fovea, and blood vessels. In this article we will break down the anatomy of the fundus, in addition to the vitreous body and optic nerve.

Retinal topography

The retina represents the inner layer of the three coats of the eye, and it’s primary function is to convert light into neural impulses which are transmitted to the brain.

We can divide the retina into two main parts: the inner neurosensory retina (which develops from the inner layer of the neuro-ectoderm), and the retinal pigment epithelium, or RPE, (which develops from the outer layer of the neuro-ectoderm). The RPE is poorly adherent to the neurosensory retina resulting in a potential space between them: the subretinal space. The retina has it’s anterior attachment to the ora serrata, and attaches to the optic nerve head posteriorly.

<aside> 💡 The retina is very strongly adherent to the ora serrata, which is why retinal detachment seldom extends anterior to this point.

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At the external surface of the retina is the Bruch’s membrane, the innermost layer of the choroid. At the internal aspect, we have the vitreous.

The human retina measures 32 mm from ora to ora, with a surface area of 1,100 mm2, and an average thickness of 200 um, which increases around the optic nerve and macula, and decreases around the ora serrata and fovea. It can be divided topographically into the following areas:

• Area centralis: a zone measuring around 5.5 mm, between the superior and inferior temporal arteries. Also known as the ‘posterior pole’. It contains the following structures:
  • Macula lutea: a 5 mm diameter area, which lies 3mm lateral to the optic disc, corresponding to 15 degrees of the visual fields. It’s main function is colour vision.
  • Fovea: a 1.5 mm diameter area which marks the centre of the macula lutea, located 4 mm temporal to the optic disc and 0.8mm below the horizontal meridian, corresponding to 5 degrees of the visual field.
  • Foveola: a 0.35 mm diameter depression at the centre of the fovea where the retina thins, representing 1 degree of the visual field.
  • Optic disc: lies 3mm medial to the center of the macula (the fovea). The retinal layers here are disrupted by the ganglion cell axons piercing the sclera to enter the optic nerve, therefore this is known as the blind spot.
• Equator: this is an imaginary circular line drawn through the ampullae of the vortex veins, dilations of the vortex veins which the choroidal veins drain into, situated around 15 mm from the limbus.
• Peripheral retina: this is the area outside of the area centralis, making up 95% of the retina. The peripheral retina is the thinnest (110-140 micrometres).

Topography of the retina. Image source.

<aside> 💡 Pro tip: the macula lutea is yellow as a result of yellow xanthophyll carotenoid pigments (zeaxanthin and lutein) in the ganglion and bipolar cells.

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Layers of the retina

Diagram showing the layers of the retina. Image source.

The retina is made up of 10 layers:

1. Retinal pigment epithelium
2. Photoreceptor layer
3. External limiting membrane