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In Tay Sachs disease, a hallmark symptom is a cherry red spot in the macula of the eye surrounded by a halo of white.

I understand that the ganglion cells, which are higher in numbers around the macula rather than in the macula, accumulate gangliosides due to the hexosaminidase deficiency, and thus cause the surroundings of the macula to appear white, but I don’t understand why the macula is red.

It is said that in Tay Sachs disease, the macula is the only normal region of the eye, but isn’t the macula supposed to be yellow, as it is referred to as the yellow spot?

Moreover, I’ve read that the red color comes from the choroid blood vessels, but isn’t the choroid black? It contains blood vessels, yes, but it is pigmented. The hindmost layer of the retina also happens to be a pigmented epithelium. So what is the explanation of a cherry red spot?

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Short answer

  1. The fovea centralis is colored red in both the healthy retina and Tay-Sachs affected retinae. The macular region around the fovea has a yellowish hue. You are confusing fovea and macula in your question.
  2. The reddish hue from the fovea comes from the well-vascularized choroid layer below. The choroid is not black, it is colored red due to the presence of many blood vessels. You are confusing choroid with the retinal pigment epithelium, which is indeed black.

Background
The macula includes the fovea at the center, also called the fovea centralis, which forms the very center of the retina. The macular region around the fovea includes the perifoveal and parafoveal areas. The macular region around the fovea has a slight yellowish appearance to it due to yellowish pigments present in the cone axons (Fig. 1) (Kolb, 2012).

retina
Fig. 1. Healthy retina. The macular region is seen in the center, The parafoveal region has a slight yellow/orange hue to it. Note that the landmark yellow circle at 4 o'clock, with the vessels sprouting from it, is the optic nerve head. source: Webvision.

Below a Tay-Sachs affected retina showing the hallmark cherry red spot (Fig. 2):

Tay Sachs retina
Fig. 2. Tay-Sachs affected eye. The fovea centralis appears red. source: Prezi

The only normal part is the fovea centralis, appearing in its native red color, i.e. the cherry red spot.

The choroid plexus is well-perfused and is, therefore, red. You are confusing it with the pigment epithelium, which is pigmented (Fig. 3).

layers
Fig. 3. Layers of the eye. RPE = Retinal Pigment Epithelium. source: Retina Eye Specialists

Reference
- Kolb (2012), Simple anatomy of the retina. In: Kolb et al. (eds.), Webvision. The organization of the retina and visual system. Salt Lake City, UT, Moran Eye Center

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  • $\begingroup$ But is the RPE is present in the fovea too? I don’t understand why the fovea doesn’t appear black when the rest of the eye’s interior does. $\endgroup$ – lightweaver Mar 8 '16 at 12:18
  • $\begingroup$ The retina does not appear black. See the images above. Note that the black borders are outside the eye. $\endgroup$ – AliceD Mar 8 '16 at 12:19
  • $\begingroup$ But where does the black appear then in the eye? Isn’t it important for the eye’s interior to be black to absorb backscattered light? You mentioned the RPE is black, but this should be visible, should it not? $\endgroup$ – lightweaver Mar 8 '16 at 12:21
  • $\begingroup$ It's a very thin structure. $\endgroup$ – AliceD Mar 8 '16 at 12:23
  • $\begingroup$ So the RPE is so thin that you can’t really see the black? But what about the whole part where the pigmentation helps absorb scattered light? I’m just thinking that if, upon hitting the RPE, light is absorbed and not scattered, then there is no light reflecting back into the observer’s eye and the eye’s interior would appear black. If the eye’s interior doesn’t appear black then that does mean that light is being scattered about in the eye. $\endgroup$ – lightweaver Mar 8 '16 at 12:29
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The black RPE contributes to the "normal" color of the eye fundus in photos of the retina. Albinism is a condition of absence of black pigment, and then you can see the eye fundus without pigment. Compare the two images below left side = no pigment albinism; right side = normal pigmented eye

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  • $\begingroup$ Welcome to Biology.SE! It isn't clear that this answers the original question, which was about Tay Sachs not albinism. In addition, answers are much more likely to receive a favorable response if you include supporting references (primary literature is best). Without that support, your answer is indistinguishable from opinion. ——— Please take the tour and then consult the help pages for additional advice on How to Answer effectively on this site and then edit or delete your answer accordingly. Thanks! 😊 $\endgroup$ – tyersome Jun 24 at 15:44

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