Corpus: Retina

The retina is a layer of tissue about 200 µm thick that lines the inside of the eye like wallpaper. It is the sensory area of the eye and is used to perceive light stimuli.

The outermost layer of the retina is directly adjacent to the Bruch's membrane, a membrane-like thickening of the choroid, from which the retina can be easily detached. The innermost layer borders on the vitreous body (corpus vitreum). Morphologically and functionally, the retina can be subdivided according to several aspects:

2.1. ...according to light sensitivity

• visual retina: The "seeing" part of the retina. It lines the back of the eye (fundus oculi) and lies against the choroid from the inside.
• nonvisual retina: The "blind" part of the retina. It has no photoreceptors and can in turn be divided into a ciliary part on the dorsal side of the ciliary body and an iridial part on the back of the iris.

The transition between the blind and the seeing part of the retina takes place in the anterior part of the eyeball and is recognisable by its jagged course. The circular line inside the eye is also known as the ora serrata.

2.2. ...by morphology

The visual retina consists of various cell types, including photoreceptors, bipolar cells, horizontal cells, ganglion cells, Müller cells and amacrine cells. Together, they form a complex three-dimensional cell network that represents a functional unit. From a morphological point of view, the visual retina can be divided into 10 layers from the outside to the inside:

1. Retinal pigmented epithelium (RPE)
2. photoreceptor layer (PL), with
   1. Outer segment (OS)
   2. Inner segment (IS)
3. Outer limiting membrane (OLM)
4. Outer nuclear layer (ONL)
5. Outer plexiform layer (OPL)
6. Inner nuclear layer (INL)
7. Inner plexiform layer (IPL)
8. Ganglion cell layer (GCL)
9. Nerve fibre layer (NFL)
10. Inner limiting membrane (ILM)

Layers 2–10 are also summarised as the neural layer of retina (stratum nervosum retinae). Some textbooks regard them as the actual retina and assign only 9 layers to the retina.

The structure of the retina shows that light must first pass through all the inner layers before reaching the photoreceptors. This arrangement is why the retina is often described as being inverted.

2.3. ...by function

In addition to the purely morphological layering, the retina can also be subdivided according to functional aspects. A functional layer combines several morphological layers of the upper table (numbers):

• Pigment epithelial layer: 1
• First retinal neuron - neuroepithelial layer: 2, 3, 4
• Second retinal neuron - retinal ganglion cell layer: 5, 6, 7
• Third retinal neuron - ganglion cell layer of the optic nerve: 8, 9, 10

Action potentials are first triggered in the ganglion cells of the optic nerve. The bipolar cells of the second retinal neuron utilise electrotonic excitation conduction.

The retina is considered to be a superimposed part of the central nervous system, more precisely of the diencephalon. It develops from both leaves of the optic cup – the outer leaf (located towards the sclera) and the inner leaf (located towards the vitreous body). The outer leaf forms the retinal pigmented epithelium, the inner leaf the neuronal retina. This includes all cells used for photodetection, impulse switching and impulse conduction.

The diagnosis and treatment of diseases of the retina are the domain of ophthalmology. As the retina is a relatively small tissue area, even minor pathological changes are usually of great medical importance, as they quickly lead to a loss of vision and thus to considerable disability.

4.1. Diseases of the retina

Diseases of the retina include, among others:

• Diabetic retinopathy
• Hypertensive retinopathy
• Retinal detachment (ablatio retinae)
• Retinitis pigmentosa
• Macular degeneration
• Macular hole
• Retinoblastoma

4.2. Diagnostic procedures for the examination of the retina

• Ophthalmoscopy
• Electroretinography (ERG)
• Electrooculography (EOG)
• Fluorescein angiography (FA)
• Optical coherence tomography (OCT)

• Lüllmann-Rauch et al., Taschenlehrbuch Histologie, Georg Thieme Verlag Stuttgart, 6th edition