Histological Distinction and Extracellular Constituents

00:01 Here is a number of images of basically the same part of the body, a wall through a blood vessel except for the lymph node down below showing reticular fibres.

00:15 Using H&E, it is very difficult to tell the different between smooth muscle, collagen and elastic tissue. But if you use the Weigert's stain, you see here, the elastic tissue is now differentiated, and you can see the elastic tissue from the smooth muscle and the collagen.

00:36 If you use the Masson stain, the green stains collagen and the browny reddy component stains smooth muscle. The Gomori stain down below, the collagen stains blue and smooth muscles stains the reddish colour. And you can see a very shiny white line on the very edge of the blood vessel that is elastic tissue. It is the elastic lamina, we will talk about when we talk about blood vessels later on. So we can use these stains in certain tissues to differentiate the sorts of fibres in the tissue from smooth muscle and from each other. And the reticular fibres are shown here using a silver stain.

01:29 Well, let's have a look at the extracellular constituents of connective tissue, the matrix. The extracellular matrix is really a combination of three components, collagen we know that from what we have seen so far but also non-collagenous glycoproteins. Remember I said collagen is a glycoprotein.

01:57 Well, there are a lot of other glycoproteins in the extracellular matrix and they have a very important function or a numbers of functions. They actually combine the cells to the fibres.

02:13 They can regulate a lot of the activity of the cells, signalling of these cells.

02:21 They have a role in dictating lots and lots of functions to the cells. I am not going to talk about it in great detail, but the cell biologists and the physiologists will certainly emphasize the role of these special glycoproteins when you talk about the physiology and the cell biology of the extracellular matrix. The major component of the extracellular matrix that has a real important function are the proteoglycan aggregates. They are called glycosaminoglycans and here is a diagram illustrating one of these highly organised, highly complex proteoglycan aggregates. And I am not going to go through all the detail of these again. I emphasized the cell biologists will talk to you about these in cell biology courses, but have a look at the diagram and the main features is to understand that you have these huge molecules running through the extracellular matrix. In this case, hyaluronic acid, the hyaluronan molecule, the blue one running through this diagram. And attached to this hyaluronic acid molecule, this huge molecules are different sorts of core proteins shown in green. And on those core proteins, you see glycosaminoglycans. These are the most important functional components or molecules of the extracellular matrix. Here is a diagram shown on the left-hand side and here in the middle is an Australian Bottlebrush. You know when you talk about Australian wild flowers, everyone knows the bottlebrush. But why am I showing you is bottlebrush in this lecture? Well it really looks like a glycoaminoglycans because if you look at the core protein in the diagram, it is just like the stem of the bottlebrush, and the glycoaminoglycans projecting off that core protein are like those red fine lines making up the bottlebrush flower.

04:42 Now, the most important role that these glycoaminoglycans have, is that they are all negatively charged.

04:51 So all those little fine fibres you see making up the bottlebrush, if they are negatively charged, they repel each other as they do in the extracellular matrix. And that creates an enormous space for water. It attracts water and therefore the connective tissue extracellular matrix is very turgent. It is a gel like viscous strong structure because of this ability to attract and hold lots of water in. And on the far right-hand side, that strange looking structure illustrates the importance of the extracellular matrix. We don't seem to appreciate the importance much when we think about it in the lamina propria for instance, but yet it is very resistant to compressive forces. Well, this structure on the right-hand side is an umbilical cord consists of two arteries and one vein. It is not important.

05:53 We identified these components here, but they're surrounded by extracellular matrix, a special mucous connective tissue that is called Wharton’s jelly. Now that extracellular matrix has lots and lots of these glycosaminoglycans within it, attracting water, holding the water in and making this umbilical cord not at all compressive. And that is important because the umbilical cord is a lifeline when we are developing in the womb. It twists and turns and we do not want it to be compressed. Otherwise, blood flow to and from the developing foetus is retarded. So this extracellular matrix has a very important role in making sure the umbilical cord does not become compressed. Again it illustrates the important role of these very special molecules in normal connective tissue matrix.