Epithelial Cells

00:01 My name is Geoff Meyer.

00:03 In a previous lecture, I explained a typical mammalian cell.

00:09 I explained the structural characteristics and components of a typical mammalian cell and the functions each of those components performed in the cell.

00:24 In this lecture, I'd like to concentrate on some specialized cells and explain how the unique morphologies they have make them so specialized.

00:39 I'm going to describe briefly epithelial cells.

00:43 Although there is a separate lecture on epithelial cells, I want to explain here some of the unique characteristics epithelial cells possess I'm going to show you some conducting cells and what specialized structures they have as well as cells that contract.

01:05 There are cells in the body that are involved in transporting water and various electrolytes.

01:13 I too want to explain some of the unique characteristics those cells possess and then I want to look at cells that predominantly produce proteins that predominantly produce mucus and that predominantly produce steroids and just briefly point out some of their unique structural characteristics.

01:41 So let's look at epithelial cells first.

01:44 On the right-hand side is a histological section of an epithelium.

01:51 It is a whole group of cells stacked together to form the surface of a tube.

01:59 This happens to be in the nasal cavity of the body, and what I'd like you to focus on is the idea that epithelial cells form an epithelium which is a tissue.

02:19 An epithelium lines cavities within the body's tubes in the body such as our gut, our blood vessels.

02:28 It's part of our skin.

02:31 The epidermis of our skin is an epithelium and in your histology course, you will come across other places where epithelia dominate and perform very specific functions and because of that, they're structured very uniquely.

02:49 On the left-hand side is a diagram pointing out some of the unique characteristics of epithelial cells.

02:56 Firstly, there is a central cell shown there and it has two adjacent cells.

03:04 Epithelia are tightly bound together as a unit. The cells lie very close together.

03:11 There's no interstitial fluid or connective tissue between them and they're bound together by tight junctions at the top or the apical surface of the cell near the lumen of the tube in which they line, and those junctional complexes, the tight junctions you see pointed out there, enable the epithelium to decide what passes through from the lumen to the interior of the tissue or from the cells into the lumen.

03:43 It doesn't allow any components to leak between the cells, so those tight junctions are very important in epithelium.

03:53 Epithelia also have a basolateral domain or the side surface and apical surface and a very basal surface.

04:04 And those domains are very different in terms of what the cell membrane has within its makeup.

04:14 All epithelia sit on a connective tissue scaffolding called a basement membrane. It supports the epithelium.

04:25 And on the right-hand side, if you look very carefully at those stacks of nuclei that make up this epithelium, the epithelial cells all sit on a very clear homogeneous pink line you see running across the image.

04:42 That pink line is the basement membrane, and it consists of connective tissue components.

04:50 Now, rather than talk about the details of the cells of epithelium, I just want to focus on really the surface specializations which are a characteristic feature of these very specialized cells.

05:07 On the diagram, you can see the word microvilli.

05:12 Microvilli are finger-like projections from the cell surface of the cell membrane, and those finger-like projections or microvilli are there to increase the surface area of the apical part of the cell.

05:30 Filaments within those microvilli can make those finger-like projections open up and create a very large surface area, and that's important for absorption.

05:44 For instance, if you're absorbing nutrients or material that we ingest and pass through our digestive system, when we want to absorb into the body all the broken-down products, to have a massive surface area makes that absorption very efficient, and that's what microvilli do.

06:06 On the right-hand slide, you can see in the image some rather projections from the surface of this epithelium.

06:15 They're not microvilli. They're cilia, another surface specialization and those cilia are attached to basal bodies at the very apical surface of the cell and the basal bodies appear as a very fine rouge-colored line across the surface of the cells.

06:37 Before I move on, have a look at the length of the cilia on the surface of this epithelium and just compare the length of these cilia with the size or diameter of the nuclei, those round purpley-stained structures in the cells, because that's a good index or a good way of determining whether you're looking at cilia which are far longer projections than microvilli.

07:08 Cilia are greater in size relative to the nuclei in the cells and we'll see that in this slide.

07:18 This slide shows you two epithelial surfaces separated by a very thin clear line which is the lumen, and if you look very carefully at the surface right near that clear line, you can just make out a rather pink-stained surface area called the brush border and that brush border, that bright pink-stained surface, is a lot smaller relative to the size of the nucleus as cilia are in the previous slide.

07:56 The common fold is often to mistake microvilli and cilia, but always remember to compare it with the size of the nucleus and that helps your identification.

08:09 Here's an electron micrograph of microvilli, and you can see that they are extensions of the cell membrane.

08:18 They have long dark-stained material going into the apex of the cell and these are filaments that help to move these microvilli, to spread them apart, to shorten them, to lengthen them particularly at a time where very efficient and rapid absorption across the cell surface is wanted.

08:41 And the fuzzy coating material on the surface these microvilli are the glycocalyx.

08:49 The glycocalyx is a very important component of the surface of epithelial cells for both containing digestive enzymes but also in cell recognition.

09:04 And here on the right on this image, you can see a typical epithelium and cilia on the surface, totally different to microvilli.

09:14 This happens to be in the nasal cavity again.

09:20 Cilia are embedded in the apex of the cell via basal bodies. You only see partial components of the basal bodies shown here, but the cilia are highly specialized structures.

09:35 They consist of lots of microtubular structures that anchor these cilia to the apex of the cell and the cilia beat along. They move in certain directions and because of that, they push fluid along through the lumen in which they line.

09:57 Here is another example of a surface specialization of epithelial cells.

10:02 They're not cilia, they're not microvilli. What they are is stereocilia.

10:11 Stereocilia is a term we don't often use now.

10:14 We tend to refer to them as being branched microvilli.

10:18 They're very long branched structures just like micro villi but much longer.

10:25 They're not cilia because they don't beat in unison and move fluid along.

10:31 What these stereocilia do here or very specialized microvilli is to absorb all the fluid from the lumen of the epididymis.

10:42 The epididymis is an organ in the male reproductive system which receives all the spermatozoa produced by the testes and undergoes a maturity program for them before they're capable of fertilization, and they move into the epididymis with an enormous amount of fluid produced by the testes.

11:06 All that fluid needs to be reabsorbed back into the system, and it occurs via the stereocilia.

11:14 Again, it's a method for increasing surface area.

11:19 Here is another specialized epithelium.

11:23 These cells you see here sitting on a very pale-stained dermis are the epidermal cells of skin.

11:32 They go through a process of active mitosis of the basal layers and then as they move towards the surface, they begin to accumulate keratin and finally, they die and flake off the keratin which you see at the surface of this particular group of epithelial cells.

11:53 That keratin is very important in making sure that our skin waterproofs our body and protects our body.

12:02 In the bladder or the urinary tract tubes, cells at the surface of this epithelium contain, you can just make out these reddish-pink plaques, protein plaques.

12:20 These cells are also binucleate—they have two nuclei in them, and what happens when the bladder or the urinary tract fills up as urine passes down through these tubes or into the bladder? This epithelium starts to distend or stretch out and those surface cells contain those very special protein plaques that stains that reddy color in the top that I just mentioned.

12:50 They stretch out also and they form a seal.

12:55 They seal the epithelium from the very toxic lumen of urine, and they prevent urine leaking back into the system through those protein plaque barriers.

13:08 It's a very important function of the bladder, and the urinary tract duct system do not allow us to reabsorb all that toxic component in the urine that we spent so much trouble in our kidney trying to excrete.