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Overview of the anatomy of the lens and corpus ciliare.
Hey everyone! It's Nicole from Kenhub, and welcome to another anatomy tutorial. In this tutorial, we're going to be talking about the lens and the ciliary body.
We're going to begin this tutorial by looking at these two structures seen here and the image on the left shows us a cut section of the eyeball where see the lens highlighted in green. This image on the right, on the other hand, shows the ciliary body highlighted in green as you can see here. The ciliary body is also known as the corpus ciliare in Latin.
So now that you have a general idea of what these two structures look like, I'd like to quickly list the topics that we'll be discussing in this video. This video will be divided into two parts, and we'll first discuss the lens and then we'll discuss the ciliary body. In both parts, I'll briefly introduce the structures and talk about their layers as well as their components in detail, and then we're going to finish off this tutorial by mentioning some of the clinical correlations that can be made. So, let's begin.
As we can see in this image, the lens is an encapsulated biconvex body located anterior to the iris and posterior to the vitreous humor of the vitreous body. For those of you who don't know, the vitreous body is the clear gel that fills up the main cavity of the eye between the lens and the retina. Here in this illustration, you can see the vitreous body depicted and highlighted in green.
The lens is bathed in aqueous humor and is encircled and anchored to the ciliary processes by zonular fibers issued from the ciliary body. Collectively, the zonular fibers, seen here highlighted in green in this illustration, transmit the forces needed to stretch the lens. From here, we come to the lens's main function which is accommodation. Accommodation is the active process of changing the lens's convexity to adjust the focus of the eye. This is done by the contraction of the ciliary muscles.
Now let's move on to discuss the lens surfaces and some of the important terminologies associated with them. The lens has two surfaces – an anterior surface and a posterior surface. It should be noted that the convexity of the anterior surface is less steep than that of the posterior surface. Now, both of these surfaces have a central point each – the anterior and the posterior pole. The imaginary line that connects between these two poles is termed the lens's axis and the marginal circumference of the lens is called the equator.
Now, let's discuss the layers of the lens in detail.
The lens can be divided into three layers – the lens capsule, the lens epithelium, and the lens fibers. Let's begin by talking about the first layer – the lens capsule. Now, I want you to look at the image presented here. The part highlighted in green is the lens capsule. As you might notice, the lens capsule covers the entirety of the lens surface. Now, what is that made up of, I hear you ask. The lens capsule is made up of various classes of collagen – I, III and IV. This gives the lens its elastic quality which plays a central role in the process of accommodation that we mentioned prior. It is also made up of a wide range of glycosaminoglycans and glycoproteins. It's good to mention that the capsule thickness depends on the position and the age.
In the second illustration here, we see a close-up look at the lens capsule. By the way, before we proceed, I'd like to mention that the lens capsule is mainly derived from the epithelial cells of the anterior lens. Now moving on to the attachments real quick – anteriorly, the lens capsule is attached to the lens epithelium; posteriorly, it is attached to the lens fibers.
The second layer of the lens is the lens epithelium. The lens epithelium covers the anterior surface of the lens between the outer capsule and the underlying lens fibers. It's made up of simple cuboidal epithelial cells that differentiate into lens fibers by mitosis where they are then displaced towards the equator.
Alright, now, we're ready to move on to the third and final layer of the lens – the lens fibers.
Here we see another image of the lens from a different angle. The lens fibers, as mentioned earlier, are inwardly displaced elongated epithelial cells that are flattened and tightly packed together. They are attached to each other by a variety of mechanical junctions which includes desmosomes and numerous gap junctions. It should be noted that the fibers at the equator and near the lens surface are nucleated; however, as we go deeper, the lens fibers will lose their nuclei as well as most of their organelles. This variation in structure will help distinguish between the soft cortical zone representing younger fibers and the firm central part representing the older ones.
Now all lens fibers cross the equator to terminate on both the anterior and the posterior surfaces at what we call the lens sutures. These sutures radiate out from the anterior and the posterior poles that we mentioned at the beginning of the tutorial towards the equator. The lens sutures represent the lines of interlocking junctions between the terminating lens fibers.
Alright, we finished our discussion regarding the lens but before moving on to our next topic, I'd like to mention something important to you. Notice that we haven't mentioned any blood supply or innervations. That's because the lens is an avascular structure devoid of nerve fibers or any other structure that might affect its transparency. Its nutritional requirements are supplied by the aqueous humor in which it bathes.
Now, let's move on to part two of our tutorial – the ciliary body. Let's begin with the small introduction. The ciliary body, also known as the corpus ciliare, is a ring-like thickening of the layer posterior to the corneoscleral junction. Here you can see an image of the ciliary body highlighted in green.
The ciliary body has four main functions. The first one, like we mentioned early on, is anchoring the lens through its suspensory ligaments – the zonular fibers. Now, we also mentioned that the ciliary body is involved in the accommodation reflex of the lens by the contraction of its smooth muscle. Moreover, the ciliary body's anterior internal surface is the source of aqueous humor while its posterior internal surface is continuous with the vitreous humor and secretes several of its contents.
Now, let's move on to the ciliary body attachments.
The ciliary body projects posteriorly from the scleral spur where it is attached. For those of you who don’t know, the scleral spur is a protrusion of the sclera into the anterior chamber. Anteriorly, the ciliary body merges with the periphery of the iris.
Now, let's talk about an important landmark present on the internal surfaces of the ciliary body - the ora serrata. The ciliary body has a scalloped periphery called the ora serrata seen here highlighted in green. As you can see, the ora serrata separates the ciliary body from the light sensitive retina.
Next, we're going to talk about the different parts of the ciliary body.
The ciliary body can be divided into two parts. Anteriorly, we have the rigid pars plicata, also known as the corona ciliaris, and it's highlighted here in green. The second part of the ciliary body is known as the pars plana. We'll look at the second part of the ciliary body shortly.
As we can see from the image, the pars plicata accounts for one third of the ciliary body and also surrounds the base of the iris. Posteriorly, we have the smooth and thin pars plana, also known as the orbicularis ciliaris which is we just saw is the second part of the ciliary body. We can see that the pars plana is adjacent to the ora serrata of the ciliary body.
Just like the lens, a cross-section of the ciliary body reveals different layers. The ciliary body has four layers. From internal to external, these are a double layer of epithelial cells, the stroma, ciliary muscle, and the superciliary layer.
Let's start with the first layer. The ciliary body is covered by a double layer of epithelium. The inner layer is non-pigmented while the outer layer is pigmented, which means that it's packed with melanin. Now, these two layers of epithelium are normally united but can be pathologically separated due to fluid accumulation.
Alright, so let's get into a bit more detail.
First of all, the inner epithelial layer consists of two types of cells. The inner epithelial layer is made up of columnar cells while over the ciliary processes of the pars plicata, seen here highlighted in green, it's made up of cuboidal cells. This is not the case with the outer epithelial layer, which consists of cuboidal cells only. It should be noted here that the outer pigmented epithelium is involved in something called the black box effect. It absorbs stray light thus enhancing the quality of an image.
The outer epithelial layer is attached to the stroma of the ciliary body, which brings us to the second layer. The ciliary stroma is mainly made up of loose bundles of collagen, which extends to the ciliary muscle and ciliary processes forming a considerable mass between them. One of the important characteristics of the ciliary stroma is that it contains numerous larger branches of ciliary vessels. This facilitates the passage of substances from the blood plasma during the production of aqueous humor.
Proceeding to the third layer of the ciliary body, let's talk a bit about the ciliary muscle. The ciliary muscle is composed of smooth muscle cells arranged in three different orientations. The outermost fibers are longitudinal. They cross posteriorly into the stroma and the choroid. The innermost fibers are circumferential and are present near the periphery of the lens. And, finally, in between these two muscular layers, we find the radical fibers which are oblique.
Now moving on to the fourth and last layer of the ciliary body – the superciliary layer. The superciliary layer is the outermost layer of the ciliary body that separates the sclera from the ciliary muscle. The superciliary layer is largely comprised of collagen fibers and is sometimes used as a less frequent alternative route for the drainage of aqueous humor – the main route being the canal of Schlemm.
Alright, we finished discussing all four layers of the ciliary body so let's briefly mention its blood supply and innervation before moving on.
Unlike the lens, the ciliary body is both avascular and innervated structure. It receives its blood supply from the ciliary arteries originating in the ophthalmic artery. These arteries are divided into three groups – the long posterior ciliary artery, the short posterior ciliary artery, and the anterior ciliary artery. The long posterior and anterior ciliary arteries meet in the ciliary body along with all the major nerves of the anterior tissues of the eyeball.
As for innervation, the ciliary body is innervated by long ciliary nerves, which are branches of the nasociliary nerve, and the short ciliary nerve, which comes from the parasympathetic ciliary ganglion.
Before wrapping up this tutorial, I'd like to share an important clinical correlation associated with the ciliary body.
Glaucoma refers to a group of eye diseases that can affect the optic nerve leading to limited and/or loss of vision. It is traditionally characterized by higher intraocular pressure due to problems in the production and/or resorption of the aqueous humor. Risk factors of glaucoma include old age, family history, race, hypertension, and diabetes as well as other factors. People with glaucoma can either be asymptomatic or experience symptoms such as decreased vision, headache, severe eye pain and seeing halos around lights.
As we mentioned earlier, one of the ciliary body's functions is the production of aqueous humor, and that's why the ciliary body is a target for many of the treatments against glaucoma which include medication, laser therapy and surgery. It should be noted that the type of treatment recommended will depend on the type of glaucoma.
Alright, so we've come to the end of our tutorial now and we're going to go over what we discussed today.
So, in summary, we discussed two structures in today's tutorial – the lens and the ciliary body. The lens is an encapsulated biconvex body whose main function is accommodation. It has two surfaces – the anterior and the posterior surfaces – and consists of three layers – the lens capsule, the lens epithelium, and the lens fibers. As for blood supply and innervation, the lens is an avascular structure that's devoid of nerve fibers as well.
The ciliary body is a ring-like thickening with four main functions – lens anchoring, accommodation reflex, and aqueous and vitreous humor secretion. The ciliary body is divided into two parts – the pars plicata and the pars plana – and consists of four layers – a double layer of epithelial cells, the stroma, the ciliary muscle, and the superciliary layer. The ciliary body's blood supply and innervation include the ciliary arteries and the long and short ciliary nerves.
We also looked at a disease that affects the ciliary body known as glaucoma.
And that's it for today, happy studying, and thanks for watching!