Biology of the human choroid

Published on April 27, 2016   30 min
0:00
Hello, my name is Robert Mullins. I'm a professor at the University of Iowa. And I'm going to be speaking to you today about the choroid.
0:09
Today, we're going to be discussing several points about the choroid, including a general overview of ocular anatomy as it pertains to the choroid, and very briefly we'll discuss retinal signaling and photoreceptor cells, and the fact that that process requires a large amount of oxygen consumption, the source of that oxygen in the form of ocular vasculature. I will then discuss briefly some of the physiological insights into the structure of the choroidal microvasculature. We'll spend some time reviewing the choriocapillaris. We'll discuss the groups of cells that exist in the choroid. And the development of the choroid will be our final point.
0:51
While it is obvious in one sense that the eye has an outside and an inside, the inner-outer axis is very important in describing one's position in the retina. And so what I've drawn here is a cartoon of an eyeball cut in crosssection, and you can see this line that denotes the inner-outer axis. Taking a small piece of the yellow retinal tissue and the brown choroidal tissue in the cartoon, we see the layers of the retina starting from the inside toward the outside where the choroid resides. And these layers have been, clasically, labeled based on this inner-outer access. So the innermost layer is the ganglion cell layer, innermost layer of nuclei. The next layer of nuclei is the inner nuclear layer, and external to that is the outer nuclear layer. And the outer nuclear layer comprises the nuclei of the photoreceptor cells, which also have an inner segment and an outer segment, again, positioned logically along this inner-outer axis as we've discussed. And external to the outer segments of photoreceptor cells, reside the RPE, the choriocapillaris, and the outer choroid. When light enters the eye, it passes through the cornea and the pupil in the lens and is focused on the retina, and a photon of light actually has to penetrate through all of these other retinal neurons before it reaches the outer segments of the photoreceptor cells where the light is converted into electrochemical signal. The photoreceptor cells then signal the bipolar cells in a mechanism that we'll very briefly discuss, the bipolar cells in turn signal the ganglion cells, which send an action potential out through the optic nerve and to the brain.
2:44
The photoreceptors themselves are very interesting kinds of neurons and they have been described since that time of Cajal, as having either a rod-type shape or a cone-type shape. On the left panel of this slide, you can see cone photoreceptor cells that have been labeled green. We can see their nuclei in the outer nuclear layer, and you can see in some cases, their axons which go and send the process to the bipolar cells. The electron micrograph on the right shows again the fine structure of a cone and two rod photoreceptor cells.
3:21
Physiologically, photoreceptor cells are very unusual as neurons. They have this distinct property in which they detect light that's not observed in other kinds of neurons. They have distinct metabolic needs which are met by RPE cells, Muller cells, and other photoreceptor cells in the case of secreted proteins. They actively secrete neurotransmitter in the dark, and when we think of most types of neurons, they secrete neurotransmitter after they're stimulated, and photoreceptor cells in the dark are constantly releasing neurotransmitter. And only when they are stimulated by light, do they interrupt this release which results in triggering of bipolar cells. Finally, photoreceptor cells consume large amounts of oxygen.
4:10
So here, again, we have a cross section of a hematoxylin eosin-stained retina, and you can see the pigmented RPE at the bottom of the panel, the choroid beneath that, and above to the RPE, some cone inner segments that have this distinct teardrop-type shape. If we take this image and rotate it, we now can see, again, all of the layers of the retina. Rob Linsenmeier did some fascinating experiments in which he measured oxygen tension across the depth of the retina. And so in this case, our retina is aligned with the inner cells on the left and the outermost portion of the retina on the right. And what Linsenmeier and his group did was they inserted an oxygen probe into the choroid through all of the layers of the retina, measured the oxygen tension staring in the choroid, and then gradually pulled that probe out through the layers of the retina, staring with the RPE, then the outer segments and the inner segments and all the way out through the ganglion cell layer. And they measured the oxygen tension at each depth of the retina. What was found, that's really quiet amazing, is that oxygen tension is high in the choroid as we would expect. This is the blood supply that we will be spending the remainder of our session discussing. But the use of the oxygen is enormous at the layer of the photoreceptor cells, and there's a plummet in the metabolism of oxygen by photoreceptor cells. So these unusual neurons that are involved in light detection and are the first stage in photo transduction and in vision, require a very large amount of oxygen.
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Biology of the human choroid

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