Cell Sizes

00:00 Let's have a look at cell sizes.

00:05 On this very simple image, you see a number of small circles indicating the relative sizes of various cells in the body.

00:15 They're not just all the one size as you can see. There is a variety of cell sizes.

00:21 Cells are also different shapes.

00:25 Normally, during the early development of ourselves and our body, they're roughly spherical as you see here, but later on when they get very specialized for their functions, they can be very flat or fusiform or squamous, which is another description of a flat cell.

00:46 They can be cuboidal. They can be very tall or columnar. They can be what we call stellate, having lots of processes. So they have this variety of shapes to suit their function, and later on in these histology lectures, you will be introduced to these different cell shapes and you'll know that those shapes are designed to make the function of that cell very, very efficient.

01:17 Here is an image on the right hand side of blood.

01:21 You can see those small circular pink structures are red blood cells or erythrocytes.

01:30 You can see a lymphocyte in the middle. That's that dark stained circular structure that I'll mention later on, but have a look at those erythrocytes. They're about 10 microns in diameter.

01:44 They've got a little white clear center in the middle, and I'm going to explain what that is in a moment, but it really means that when you study histological sections, you've got to learn to be very, very observant and notice things like that little white space or clear area in the central region of each of those erythrocytes or red blood cells So they're about 10 microns. They're very small cells.

02:13 I've mentioned the word micron or micrometer (µm).

02:17 What is a micrometer or a micron? You know, when I look at my index finger, the very bottom of my fingernail is about 1 cm in length, and that equals 10 mm.

02:35 and I can roughly visualize my fingernail or 1 cm being divided into 10 mm.

02:43 In fact, you can get a ruler and you can look, and you can see, you can actually identify what 1 mm means in terms of distance.

02:54 1 mm is equal to 1000 µm or 1000 microns. Now, if I visualize a millimeter in length with my eyes, I certainly cannot visualize dividing that 1 mm into 1000 µm nor could I do it using a ruler, so these micrometers are very, very small distances.

03:23 So 1 cm is 10000 µm or microns. Again, if I look at my fingernail and I look at that 1 cm, there's no way in the world I can visually divide that into that 10000 microns.

03:41 They're too small and 1 micron itself consists of 1000 nm.

03:48 Nanometer is the measurement or distance we refer to sometimes when we look at structures within the cell.

03:56 Tiny, tiny structures. That cell membrane, for instance, I mentioned earlier is about 8 to 10 nm in thickness Very, very tiny.

04:09 The structures we're going to see in the light microscope, again as I said earlier, we only see structures that are really separated by about 0.2 of a µm or micron.

04:23 So it is important to understand that range of dimension when we go from the real world looking at our fingernail, and then visualizing very tiny structures that have distances in microns.

04:39 Back to the red blood cell. That's why there is this clear structure or light stained area in the middle of the cell.

04:47 They're designed to be shaped like a biconcave disk and therefore, the central part of the cell is going to stain less intensely as the outside part where it's mostly full of hemoglobin.

05:02 Now, here is another series of slides that I'm going to show you.

05:07 First of all, you have a medium-sized cell shown here on the top left hand side.

05:14 At the top left hand side, you can see a large neutrophil.

05:19 It's got a nucleus stained very purple, and that nucleus consists of a number of different segments.

05:26 This is called a segmented nucleus because of that.

05:30 You can see 1, 2, 3, 4 lobes or segments to this nucleus whereas down the bottom, where you see a tiny little lymphocyte about the size of red blood cell, you only see the nucleus that's very dark staining and it's just a single structure. It's not segmented.

05:52 If you remember, these erythrocytes are about 10 microns in diameter, then if you then superimpose a red blood cell across the diameter of that large neutrophil, I reckon you'll fit it across about 3 times.

06:08 In other words, the distance or the diameter of that neutrophil is about 30 microns.

06:19 Here's a very dark stained structure or structures in this slide.

06:24 The very dark black, the three black stained structures are large Purkinje cells.

06:34 They're in the cerebellar cortex. They're in our brain, and that dark stained area is the cytoplasm of these large neurons.

06:44 In the middle of each of those dark stained areas, you can see a very pale brown colored nucleus.

06:52 And within that nucleus, you can just make out a circular structure which is going to be the nucleolus.

06:59 Now, they're very large cells. They're about anywhere between 80 and 120 microns in diameter.

07:07 In the background, particularly in the top right hand side of this image, you can just make out some other light brown-stained circular structures. They're the nuclei of some of the smaller cells in the body.

07:23 The granule layer of the cerebellum, and those cells are only about 5 microns in diameter compared to the huge diameter of the Purkinje cells right next door to them.

07:36 And if you compare the size of the nucleolus in these Purkinje cells to the nuclei in the granular layer I described earlier, they're about the same size.

07:47 So the nucleolus can be up to 5 microns in diameter.

07:54 Another example of a big cell again is a neuron. This large neuron you see towards the bottom left-hand side of this image is a ventral horn cell or a motor neuron that's housed in our spinal cord.

08:10 It's got a very dark blue to purple-stained cytoplasm, but you can see roughly a very faint circular structure which is the nucleus and within that nucleus again is the rather dark stained, very prominent nucleolus and around the periphery of that cell, you see some other little nuclei belonging to supporting cells and that light bluey-stained area happens to be the myelin that surrounds cell processes in the nervous system.

08:44 We'll learn about that in more detail when we look at nerves in later lectures.

08:52 One of the largest cells in the body is the megakaryocyte.

08:56 On the bottom right hand side of this image, you can see a megakaryocyte. It's multinucleated.

09:03 On the top left hand side, you can see another one. Very large cells. They live in the bone marrow and they provide platelets or thrombocytes into the circulation.

09:16 Platelets or thrombocytes are involved with clotting and other processes in the blood vessels, and they're cytoplasmic fragments that break off these megakaryocytes and are released into the bloodstream to circulate through the blood.

09:35 Some cells such as these large neurons that I've described before in an area of the brain here have very, very long processes.

09:47 This is a motor neuron, very dark brown stained cell body that's living in the spinal cord or it can live in the brain, but this long process you see extending across the left-hand side of the image all the way across is the cell process or the axon that is going to transmit information or an action potential to bring about the contraction of a skeletal muscle cell somewhere in the body.

10:21 And if these neurons are housed in the spinal cord or the brain and they have to innervate skeletal muscles in my fingertips or in my toes, you can imagine how long these axons are going to be, and these axons collectively make up what we call a peripheral nerve, so they're extremely long cell processes.

10:47 Imagine the size or the length of the axons of a big animal such as the giraffes shown in this image.

10:56 Imagine the motor neuron living in the spinal cord of this giraffe and sending a cell process all the way down the leg of that giraffe.

11:07 They're extremely long structures, these peripheral nerves and they're all consisting of cell processes from a cell body that's located some distance away.

11:19 So some cells, as I've stressed a couple of times now, have very, very long processes rather than a simple circular cell such as the blood cell you saw earlier.

11:33 And now getting towards the largest cell of the body is the human egg or the oocyte.

11:42 It's about 120 to 150 microns up to the time of ovulation.

11:48 If you look at the image here, we can actually try and work out which is the oocyte. It's in the center.

11:56 It's surrounded by a layer of two or three nuclei belonging to supporting granulosa cells.

12:03 We don't need to know the details in this lecture, but the oocyte is that central pale stained structure which has an outer rim of very dark pink, which is the zona pellucida.

12:17 It's like a shell around the egg. It's a protective layer.

12:21 And then you can see the cytoplasm within, pale staining, and then another circular structure within that, that is the nucleus of the oocyte, and that little dark spot is going to be the nucleolus.

12:35 That's a huge cell when you compare it to the neighboring cells supporting it, and other cells you see in this image.

12:42 That's the largest cell in our human body, the oocyte.

12:49 Of course, the largest cell in the animal kingdom is the ostrich egg shown here.

12:56 It's got a very tough hard shell to protect the inner egg, but it weighs about 1.4 kg.

13:04 It's roughly ovoid in shape, about 15-16 cm by about 13 or 14 cm, a little bit of variety.

13:17 So what I've tried to show in this lecture is just the concept of what a cell is, what a mammalian cell is, that the mammalian cell contains a nucleus and cytoplasm.

13:31 You can see in the central part of this image running down from the top right to the bottom left corner, a chain of nuclei, rather, a condensed lot of little circular profiles. They're nuclei of cells.

13:48 There's other nuclei and cells in the periphery, but I want you to focus roughly on that structure running down the middle.

13:56 And now, hopefully you can identify a nucleus.

14:00 And around that nucleus is a very, very pale-stained pink region. That's the cell cytoplasm.

14:10 Then if you look very, very carefully towards the center of that structure I brought to your attention to, I'm sure you'll be able to see very, very fine little pink lines. That's the cell boundary, the boundary between one cell and its neighbor. And if you look at those little lines, roughly, you can work out that the cell shapes are almost cuboidal. They're cuboidal-shaped cells, and that's also indicated by the fact that the nuclei are circular.

14:42 If they are columnar, tall cells, the nuclei tend to be elongated.

14:48 And lastly in this slide, you can see some white spaces. That is the interstitial compartment.

14:58 Sometimes in parts of the body between the cells, there is this fluid interstitial space where the cell can exchange nutrients through that interstitial space to the blood and vice versa.

15:14 It's a bit exaggerated in this slide because in normal H&E processing, there is a little bit of tissue shrinkage, and therefore, those white spaces tend to be rather exaggerated.

15:29 And lastly on the left hand side, you can see some very bright pink or red stained red blood cells.

15:37 As I said earlier, they're some of the smaller cells in the body and they can be used as a very good ruler to determine the size of other structures in that section based on the knowledge that those red blood cells are only 10 microns in diameter.

15:57 So now you have enough background to hopefully go a little bit further and look at cell structure in more detail in subsequent lectures.