Transcription
Welcome to the Da Vinci Academy Histology video course. The entire video course is available on YouTube and covers all of the fundamental principles of histology and relevant cell biology. You can find all the videos from the course by clicking the histology playlist link in the description below. Then, you can access the corresponding practice questions and histology lab videos by going to our website, which is also linked in the description below.
This is the first of the four lectures on two important special sensory structures of the body: the eye and the ear. The first three videos are all dedicated to the eye, and the last one is on the ear. So, in this video, we'll overview the anatomy of the eye and look at the histology of the outermost layer of the eye.
Let's start with the macroscopic or anatomical overview of the eye using this simplified diagram. The eye is a spherical globe that's lined by three different layers of tissues, which are also known as the tunics. The outermost layer, indicated by this blue color, is called the fibrous tunic. The fibrous tunic has two distinct parts: this transparent, bulging, anterior-most structure called the cornea and the white of the eye, which is the sclera that we are all familiar with.
So, that's the fibrous tunic, and as the name suggests, it's comprised of fibrous connective tissue. This tissue layer is quite tough and allows the maintenance of the shape of the eye. Underneath, or deep to the fibrous tunic, we have the vascular tunic, which is indicated by this red color in this diagram. This vascular tunic, too, is almost spherical, but it's not a complete sphere because on the anterior-most surface, there is an opening which we all very well know as the pupil. This pupil allows the light to enter through this opening and into the retina.
So, that's the vascular tunic. Another thing that we may note is that the vascular tunic has this kind of hangy bit here in red. This structure contains a lot of melanocytes that impart the particular eye color, depending on how many pigment particles they contain. So, this is what we know very well as the iris, the colored part of our eye.
As you can see, this part continues all the way down, and it contains a lot of vasculature, as the name suggests, and it provides nutrients to the last layer, which is the neural tunic. This neural tunic, indicated by this yellow color, follows very closely the vascular tunic. It lines the backside of the iris as well as this kind of enlarged part, which we'll discuss in the next few slides. Then, it kind of thickens a little bit and becomes the retina, which we very well know.
This neural tunic is interesting in that it develops during embryonic development as a direct outpocketing of the developing brain. So, calling this layer the neural tunic or neural layer is entirely appropriate.
Inside of these three layers, however, we have a lot of spaces. These spaces, where the hollow interior is, are actually subdivided into three separate chambers, and they're all filled with stuff—not air—but filled with more of a liquidy substance. The anterior-most compartment is known as the anterior chamber, surprise surprise. This is the space between the cornea and anterior to the iris, and it's filled with this liquid called aqueous humor, which in its composition is very similar to the plasma of our blood.
Posterior to the anterior chamber, we have this narrow space between the backside of the iris and the anterior surface of this transparent structure called the lens. This narrow space is called the posterior chamber, and it too is filled with aqueous humor. As you can see, the posterior and anterior chambers are communicating with each other via the pupil.
Posterior to the lens, we have this large globular spherical space. This space is called the vitreous chamber. That's because this space is filled with not liquid, but more gelatinous, but also hydrated substance called the vitreous humor. So, this gelatinous substance contributes a little bit to kind of maintaining the spherical shape of the eye.
Another thing we'll discuss later on is the fact that this aqueous humor is constantly being produced by these kind of bulgy structures that encircle the lens. They're constantly producing aqueous humor, which, of course, flows back into the vitreous humor to hydrate this gelatinous substance. But also, it flows more prominently into the posterior chamber and into the anterior chamber.
Because the aqueous humor is constantly being produced, it also needs to be constantly drained. Those drainage systems are indicated by these openings, which are also called the canal of Schlemm. So again, we'll discuss these later a little bit, but it's always good to introduce these types of things early on and then revisit them later on.
Okay, outside of the eye, we have some extraocular structures to be mindful of. Of course, some of these extraocular structures include the eyelids. Now, eyelids are—I mean, we're all familiar with what an eyelid is, right? We have the superior and inferior eyelids, and the exterior of the eyelids are lined by the skin.
So, we would expect to see histologically the keratinized stratified squamous epithelium with all the skin appendages that we would expect to see, such as hair follicles, sebaceous glands, sweat glands, and things like that. At the margins of the eyelids, we have a little bit more substantial hair follicles that give rise to the eyelashes. Their function would be to close the eyes so that our very delicate eyeballs are protected by the more substantial skin and underlying connective tissue.
The important thing about the eyelids is that the underside of the eyelids, like right around here, is actually not lined by the skin but by this delicate mucous membrane structure called conjunctiva. This mucous membrane is typically comprised of stratified columnar epithelium, and as you can see, it coats the underside of the eyelids. Then, it kind of reflects upon itself and coats the exterior of the sclera, ending where the cornea begins.
Of course, we have to remember that this is a three-dimensional structure, so think about these deep pockets that go all the way around in a circle. This pocket-like space, where if you wear contact lenses or something like that, the edges of the contact lens would be sitting in this pocket.
Conjunctiva, as we'll see histologically, is comprised again of the stratified columnar epithelium and the underlying loose connective tissue. This loose connective tissue has a lot of vasculature. So, when you have pink eye or conjunctivitis, then that pink eye is occurring where you have the vasculature that's all dilated within that lamina propria or connective tissue underneath the lining epithelium of the conjunctiva.
Lastly, we have the lacrimal glands. The lacrimal glands are not indicated in this diagram, but they sit on the lateral aspects of the eyeball, and they constantly produce this watery secretion that is the tear. The tear kind of flows on top of the cornea and washes over them in a constant manner to keep the cornea moisturized and basically protect this delicate structure.
It's when we are crying that the lacrimal glands are overproducing the tear, at which point they can actually overflow this tiny little space and come out in between the eyelids or sometimes leak into the nasal cavity.
Now, we're actually looking at the histological section through the eye instead of the diagram. Here, we actually have some portions of the eyelids that have the skin on the exterior and the conjunctiva in the interior that folds upon itself, covering the exterior of the sclera and then transitioning and disappearing here where the cornea is.
Likewise, in this eyelid, we're seeing a similar thing, although this part, the skin, is kind of torn off. But here, we can still see the conjunctiva forming that mucous membrane, folding upon itself and covering the exterior of the sclera, then kind of should be ending right around here.
Here's the fibrous tunic in kind of pinkish color, and it's a lot thinner than that diagram. Inside, we have the vascular tunic, a really dark layer due to a lot of melanocytes that progressively seems to be a little thinner here, then becomes this bulky stuff that we'll discuss later, and then it becomes the iris.
Here, we don't have that little opening within the iris; the pupil is missing. This has to do with the plane of section; we didn't quite cut through the pupil plane, so we have what appears to be a complete sheet of the iris. At any rate, we have this solitary structure in the middle, which would be the lens.
In terms of the space, we have the anterior chamber, the posterior chamber, and behind the lens, we have this large vitreous chamber. As for the neural tunic, it appears to be actually the thickest over here. The neural tunic also seems to have a little dividend, which we'll talk about in later slides.
This is the neural tunic; it continues anteriorly and becomes thinner, thinner, thinner, and thinnest around here, but still abutting all the contours of the vascular tunic and forming the posterior-most aspect of that iris as well.
These are all the structures that we'll look at at higher magnification and explain further in the lab videos. Let's look at each layer of the eye one by one in a little more detail. We'll start with the fibrous tunic. As mentioned earlier, the fibrous tunic has two main parts: the cornea and the sclera.
This thinner-looking dense connective tissue surrounds the rest of the globe. In between the cornea and the sclera, there's a specialized region that I forgot to mention earlier, and that's called the corneal scleral junction. To summarize, we have the cornea, the anterior one-sixth of the fibrous tunic. This area is transparent to allow the light in, and it is convex, meaning it's bulging out. It's the cornea that reflects the light the most as it enters the eye.
The corneal scleral junction, of course, is located on the periphery of the cornea, where the cornea becomes sclera, the opaque white region. Another name for the corneal scleral junction is limbus. This area again marks the transition between the cornea and the sclera, and this is where the canal of Schlemm is also located.
These are vascular network or endothelium-lined channels that drain the aqueous humor that is being constantly produced by these specialized structures in the back. We'll talk about them in later sections. Then we have the sclera. The sclera is the whites of the eye, as mentioned earlier, and the exposed region over here of the sclera is actually lined and protected by the conjunctiva.
The posterior aspect of the sclera, however, does not have the conjunctiva lining it, but it is still protected by a lot of fatty tissues that surround and cushion the exterior of the globe. An interesting bit about the sclera is that on the posterior-most aspect, it actually becomes continuous with the epineurium of the optic nerve.
Looking at the histology of the cornea in a little more detail, we'll recognize as we zoom into this particular area that there are actually five recognizable histological layers that make up the cornea. Anterior-most, we have the corneal epithelium right here. The corneal epithelium is comprised of the non-keratinized stratified squamous epithelium, which we can appreciate by the fact that this tissue is made up of lots of different layers of cells, with the surface-most cells being super flat or squamous in shape.
Just deep to the corneal epithelium, we have the non-cellular basement membrane right here, and this layer is called the Bauman's membrane. Again, it's a modified, slightly thickened basement membrane for the corneal epithelium. Deep to the corneal epithelium, we have the thickest layer of the cornea called the stroma. It is comprised of dense irregular connective tissue with lots and lots of collagen fibers, which are all of these eosinophilic staining thickened lines that are arranged in precise orthogonal fashion from one layer to the next.
This makes them dense irregular connective tissue, but the precise alignment of these collagen fibers from one layer to the next makes the cornea transparent. Of course, true to the dense connective tissue, we have lots of these flattened heterochromatic nuclei that belong to the fibrocytes. Another tidbit here, clinically speaking, this is the layer that gets reshaped when somebody goes through LASIK surgery.
Deep to the stromal layer is another layer of modified and thickened basement membrane right here. It's a really, really thin layer; this is acellular and thickened basement membrane for the last layer of the cornea, which is the corneal endothelium. The corneal endothelium is a simple squamous epithelium, so it's comprised of a single layer of squamous-shaped cells.
We are calling it endothelium because, when we think about it, the aqueous humor that the corneal endothelium abuts is very similar in composition to the blood plasma. So, calling this simple squamous epithelium the endothelium is appropriate.
Now, looking at the histology of the corneal scleral junction or the limbus, which is located right around there in the boxed area and over here as well, we have to remember that our eye is a three-dimensional structure. This means really the corneal scleral junction is a ring around the cornea, so that's just something to be mindful of.
At any rate, in this cross-section, if we were to look at a higher magnification of the boxed area, we would see something like this. Here's the cornea, and where the corneal stroma transitions to this more irregularly arranged collagen fibers, this is where the sclera would begin.
So, the corneal scleral junction or the limbus is located right here, where I'm circling. Histologically, the limbus is where the corneal epithelium kind of ends. It transitions from that nice non-keratinized stratified squamous epithelium to the stratified columnar epithelium of the conjunctiva.
There's a slight thickening of that corneal epithelium at the periphery, but then that transitions to the stratified columnar epithelium. Beneath the corneal epithelium, there's a Bauman's membrane, that thickened specialized basement membrane. This membrane kind of degenerates or becomes thinner and becomes a regular basement membrane underneath the conjunctival epithelium.
The corneal stroma, the thickest region, becomes the stroma of the sclera. As mentioned earlier, we can actually see how the arrangements of the collagen fibers are actually quite regular here, but comparatively in the sclera, the collagen fibers are seemingly irregularly arranged.
So, those precise alignments of collagen fibers are lost in the sclera, which contributes to the opaque nature of the sclera. An important feature about the corneal scleral junction is right here. Deep to the scleral stroma, we have a lot of these trabecular networks, which are lined by endothelium. This is where we'll find the canal of Schlemm.
So, a lot of these endothelial-lined spaces will coalesce and become the canal of Schlemm, which drains the aqueous humor that is constantly being produced by a specialized structure we'll look at in the later section.
A clinical pearl to consider here is glaucoma. Glaucoma is a disease of increased intraocular pressure that can lead to blindness. This has to do with that constant production of aqueous humor that hydrates the vitreous humor and flows into the posterior and anterior chamber. From the anterior chamber, that aqueous humor needs to constantly drain through the canal of Schlemm.
Should that drainage be slowed or blocked for one reason or another, it can cause increased intraocular pressure, which presses down on the retina and reduces the flow of blood into the vascular tunic. Therefore, the retina can get damaged due to lack of blood supply. The most common cause of glaucoma happens to be the reduced drainage of aqueous humor at the corneal scleral junction through the canal of Schlemm.
Thank you for watching this video from the Da Vinci Academy Histology video course, which is completely available on YouTube. To access the corresponding practice questions and histology lab videos, go to our website using the link in the description below. [Music]