When I said in the previous blog that we had mostly covered optical parts of the eye by then, I had used the word mostly. Well, there is one more structure further into the eye that the light must pass before reaching the retina. It is the vitreous body. A transparent jelly like structure that fills the interior of the eye ball. As it is the case with all structures, the vitreous too, is subjected to degenerative changes. We perceive those changes as generally harmless floaters (i.e. translucent spots or strands in our visual field). Those may be harmless, nonetheless the sudden onset of a large number of floaters requires medical attention.
Marvellous Complexity of the Retina
Anyway, let us move onto the retina now. So far, the light we are following has travelled through all the transparent structures in the eye (so called optical parts), it has been bent properly by the cornea and the lens before it has finally made it to the retina. Our journey hence continues with the receptive parts of the eye. Arguably one of the most complicated of our tissues, the retina, is also a bit of a marvel. It is where light impulses are translated into the electrical ones and sent deeper into the brain. Quite a few different types of cells comprise the retina, the most important being the photoreceptors, namely cons and rods. We will not go very much into details about the histological and anatomical structure of the retina in today’s blog, but we will discuss more on that in the next one.
Just to get a perspective of how complicated the retina is, let me tell you this. Although it is only 0.5 mm (0.02 inches) thick, it has 10 different layers and millions of each type of 5 different types of neurons. There are also millions of other types of cells that are not directly involved in light perception but are important for metabolism and regulation of photoreceptors. All those millions and millions of cells are required for the retina to function properly. Its complexity is also a main reason why so many different diseases affect the retina. Sadly, retinal diseases are also often progressive, and as of today, only few of them can be addressed effectively. Nonetheless, plenty of research from all around the globe are being done in order to discover new molecules or introduce whole new ways of treatment, so that some of those diseases could be at least treated better, if not yet cured.
Diabetic retinopathy, AMD and other retina-related problems
I bet every single one of you knows someone that suffers from diabetic retinopathy, some elderly with age-related macular degeneration (AMD), perhaps someone very young that developed retinopathy of prematurity, maybe someone with tunnel vision and night-blindness due to retinitis pigmentosa, perhaps someone with some other kind of retinal dystrophy, someone that just lost their vision due to retinal detachment or various retinal vessels occlusions, or simply someone that is slowly but surely losing their vision due to glaucoma. Albeit all the above diseases can eventually lead to extensive loss of vision and legal blindness, the speed and the pattern of vision loss and hence the symptoms that the patients are experiencing vary very much between them. This is due to the fact that very different retinal structures are affected in different conditions.
We will cover some of them and into further details in our next blog.
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Missed few parts of the series? Check them out:
Part I: On the Origin of Light
Part III: Iris, pupil and lens
Part IV: Refraction and the Eye Globe
Part VI: The Retina, continued