The Anatomy of the Retina

Our analysis of vision will pass over the gross anatomy of the eye and begin at the retina. The retina is the most important component of the mammalian eye, because it is photosensitive. This means it's able to process photons of light and produce useful patterns of information. A great deal - scientists still don't know just how much - of information processing occurs within the retina before the code it produces is sent into the depths of the brain.

It will be useful to start our exploration of the retina with its neuroanatomy. This page is meant as a brief overview, and so it doesn't have much explanation on it, listing the locations and orientations of cells without going deeply into the functions they serve. These functions are elucidated as the series progresses.

Should you have the chance to dissect an eye, you'll find that the retina appears rather nondescript; it's a thin strip of what looks like homogenous matter. If you were to aim a microscope at this strip, however, you'd see layers of cells of different shapes and sizes. In the second image below, these layers are illustrated in detail. It's useful to get a sense for the orientation that second image is taken at first:

Note that there's a slightly confusing custom used when describing the retina, in which orientation is with respect to the center of the eyeball. Therefore, when the term 'inner' to describe the position of a layer is used, it means close to the center of the eyeball; 'outer' means close the the periphery.

Also, the retina below has been flipped by 90 degrees, so that the INNER side is at the top of the screen.

The retina is composed of 10 distinct layers. The nuclear layers contain cell bodies. The plexiform layers are where different cell types interface and transmit signals to each other. The limiting membranes form barriers between the retina and external matter. Here is a more in-depth description of each layer that you can always refer back to, going from the periphery of the eye toward the center:

1. The retinal pigment epithelium (RPE): This is a single layer of hexagonal cells to which the photosensitive part of the retina is anchored. The RPE appears dark in color because it uses melanosomes to absorb light, preventing it from scattering and from damaging internal structures. It also nourishes the retina and collects - phagocytoses - debris formed in the layer above it (the photoreceptors). It forms the primary barrier between the retina and blood coming from deeper inside the skull.

2. Photoreceptor layer: In this layer, the outer segments of the photoreceptors are contained. It is where photons are 'collected'.

3. Outer limiting membrane: This layer is a secondary barrier, containing 'junction proteins' which prevent all but necessary substances from entering.

4. Outer nuclear layer: The cell bodies of photoreceptors are contained here.

5. Outer plexiform layer (OPL): The photoreceptors and bipolar cells meet - synapse - in this layer. The branches - processes - of horizontal cells, which connect photoreceptors laterally, are also found here.

6. Inner nuclear layer: Composed of the cell bodies of horizontal cells, bipolar cells, and amacrine cells.

7. Inner plexiform layer (IPL): The processes of cells found in the inner nuclear layer - horizontal, bipolar and amacrine cells - connect with the processes of retinal ganglion cells (RGCs), which are the cells that form the optic nerve and take information out of the brain.

8. Ganglionic layer: The cell bodies of RGCs are contained within this area.

9. Nerve fiber layer (NFL): This layer is formed by the outgoing processes - known as axons - of RGCs collecting together into a fiber, which is technically identified as the optic nerve (Cranial Nerve II) once it leaves the eye.

10. Inner limiting membrane: This is where the retina borders on the middle compartment of the eye, which is called the vitreous humor. It is formed by the footplates of Muller cells - a type of glial, or support, cell - joined tightly together. It prevents unwanted substances from diffusing into the retina.