4. Connective Tissues. Connective tissue in real life. The term ‘connective tissue’ makes us think of something that ‘connects’. The term is convenient for its brevity, but this type of tissue does a lot more than just connect structures. Besides functioning as ‘ties’ between structures, it can do wrapping, filling and packing, act as soft cushions and also form strong supporting structures like bones. Even this list is not complete, because it just describes some of the mechanical functions of this versatile tissue. Connective tissue can act as a medium of interaction between other tissues and cells and can also take part in defence mechanisms of the body. Such a wide variety of function is expressed through an equally wide variety of microscopic structure. Indeed, it may be difficult to think of soft fat and hard bone as two varieties of connective tissue. Yet, all connective tissue has a common histological basis. Before we embark upon the histological aspects, let us learn to recognise some varieties of connective tissue in the gross form. The best way to do this is to examine a few cuts of meat of a mammal – lamb, beef or pork. In a formalin fixed specimen in the anatomy laboratory, natural colour and form is altered to some extent. Moreover, in most prosections in the dissecting room, a lot of connective tissue has already been removed to enable us to see other structures. Examination of fresh meat gives us a realistic picture of the tissues. In the following account, lamb cuts are described as examples because of their manageable size. Comparable pork or beef cuts are likely to be too large; and smaller cuts may not show the details we wish to see. Fig. 1 shows two photographs of a lamb forequarter chop. Note the circular outline of bone. Most of the reddish brown mass is muscle. This picture does not show the skin, a pork cut may. Notice how white layers (1) wrap around muscles in A. These wrappers also separate masses of muscle. In A the entire cut is also wrapped in a thick sheet-like layer (2). In B, muscles have been pulled apart to show delicate, almost transparent tissue (3). In an embalmed

cadaver, such delicate areas appear like fluffy cotton wool. Gaps of various sizes are filled with masses of fat (4). The white mass in the centre of the bone (better

seen in B) is also fat, so is the thick mass shown by the asterisk. If you examine a whole lamb leg or a lamb shank, you will also see shiny, white, rope-like structures extending from muscles to bones. These are tendons in anatomical terms, sinew in plain English. If the cut includes a joint (like a shoulder in a forequarter chop), the slippery surface of bone is seen to be covered with a firm tissue which can be poked with a knife. This is cartilage, gristle in plain English. All these are varieties of connective tissue. This description was meant to introduce us to the huge variety of this tissue. We shall return to these appearances in more precise anatomical and histological terms soon. At this stage we are concerned with a fundamental question : What is the common pattern among all these varieties? The answers are at the histological level.

The common feature of all these connective tissue types is abundant non-cellular substance called matrix.

The matrix is produced by cells of the tissue.

Fig. 1 A : Photograph of a lamb forequarter chop. B : Enlarged portion of a similar photograph.

A B3

4

1 4 2 *

Fig. 2. Collagen bundles are ‘wavy’. A tissuecontaining them can be ‘stretched’ until the bundles are straight, but not beyond this because the bundles themselves cannot be stretched.

Our task is to gain some knowledge of the matrix and the cells and learn to recognise the varieties of connective tissue in histological images. We should never lose sight of the functions and gross appearances of these tissues as we do this. Connective tissue matrix. The non-cellular matrix determines the properties and appearance of a connective tissue. Matrix has two major components – fibres and ‘ground substance’. Fibres are threadlike, as the name suggests. Since they have this distinct form, they are also called the ‘formed elements’ of the matrix. The ground substance is ‘formless’ or amorphous, jelly-like, and fills all the space which is not occupied by cells and fibres in a connective tissue. Thin fibres usually form a loose network; thicker bundles may be woven into a dense network. In some cases the fibres running parallel or at least in a common direction, may form compact, strong bundles. In others, well-defined layers of spiralling fibres form an almost geometrical arrangement. Types of fibres. Collagen fibres. Collagen is a protein. Historically it is famous, because it was one of the first large biological molecules to be studied by modern physical and chemical methods. Very fine threads (fibrils) of collagen are organized to form visible fibres. In the fresh state, collagen fibres are white in colour and appear as bundles of various sizes. Indeed, it is collagen fibres that give most connective tissues their white colour.

Microscopically, relatively small bundles of collagen left to themselves have a wavy appearance. If the wavy bundles are straightened, they appear to increase in length, but this is not stretching. Once straightened, they resist stretching. This is illustrated in fig. 2. Biochemically, there are many types of collagen, designated by capital Roman numerals (I, IV etc). Different tissues have different kinds of collagen. The biochemical variants are beyond the scope of this unit.

Elastic fibres. Elastic fibres are thin, single, straight and branched. They are made of elastin, a protein again, but there is a carbohydrate component as well. In the fresh state they have a yellowish colour. This colour is appreciable only in a few locations where there is a preponderance of elastin. Elastic fibres, if stretched, spring back to their original length when the stretching force stops acting. Important : Mere stretchability is not elasticity! Elasticity is indicated by the restoring force generated. Both collagen and elastic fibres are visible in histological sections with routine stains. They stain pink due to affinity to eosin. Collagen fibres are usually seen as bundles, elastic fibres are rather bright, and seen as a loose network. Reticular fibres. The third variety of fibres is called reticular fibres. In reality this is a type of collagen, but the fibres are too fine and do not take any of the routine stains. They are therefore invisible in H&E sections. Special methods, involving deposition of heavy metals (like silver or mercury) can make them visible as a very fine network around cells of delicate tissue.

Fibres are largely responsible for the mechanical properties of a connective tissue.

Fig. 3. Collagen (wavy bundles), elastic (single) and reticular (thin, dark) fibres in a loose network.

Ground substance. The ground substance of connective tissues is a mixture of many components, most of them are complexes of proteins and carbohydrates. The detailed biochemistry of these is beyond the scope of this unit. The interested student can find excellent accounts in a histology textbook. Ground substance contains a lot of water. Its jelly-like consistency can vary tremendously, from almost watery to a firm gel. Ground substance allows easy passage of substances through it.

Ground substance is mainly responsible for exchange of substances through a connective tissue. In some types, it also gives additional mechanical properties (explained later).

Classification of connective tissues. Before we take a look at the cells of connective tissues, it is helpful to know some basic features of the types of connective tissue. Connective tissues with an obvious predominance of fibres are called fibrous tissues or connective tissue proper. The arrangement of fibres which may be loose, dense but irregular and dense and regular. Special varieties of connective tissue include fat (also known as adipose tissue), cartilage and bone. Fatty tissue has cells which produce and store fat. Cartilage and bone owe their mechanical properties to special features of their matrix, which in turn is influenced by their cells. Embryonic connective tissue is also a special variety. It has relatively less matrix, but a high proportion of unspecialised cells. Some highly specialised connective tissues have quite a different appearance and microscopic structure. These include lymphoid tissue (a type of tissue involved in defence mechanisms); and even blood is considered to be a special connective tissue. We shall learn about them in a later chapter. Cells of connective tissue. In most connective tissues there is one principal cell type, which is responsible for producing all elements of the matrix. During the period of growth of the body, connective tissues grow as well. Even when the body stops growing, connective tissues may require constant maintenance and repair. This means that at any time, some of the principal cells are active, while some may be resting.

The names given to the principal cells are derived from the appearances of the tissues described above. In connective tissue proper or fibrous tissues, the principal cell is

called fibroblast (= fibre-producing). When resting, these cells are called fibrocytes.

Cartilage is indicated by the root word chondro- an active cell of cartilage is thus a chondroblast, a resting cell is chondrocyte.

The root word for bone is osteo-. Thus the corresponding cell names are osteoblast and osteocyte.

In most of the tissues, other cell types also exist, and we shall consider these as we come across them. Connective Tissue Proper (fibrous connective tissue). Loose connective tissue. Seen with the unaided eye, as during dissection, loose connective tissue looks like cotton wool. (Take a facial cotton pad and stretch it gently in random directions!) This type of tissue has a loose network of collagen, elastic and reticular fibres with plenty of ground substance. The principal cell type is the fibroblast. Fibroblasts make all fibres and ground substance. Because of its loose nature, it is an excellent medium for carrying blood vessels. Larger blood vessels travel through layers of loose connective tissue. At the microscopic level, fine blood capillaries are also seen in loose connective tissue. In this sense loose connective tissue also gives ‘nutritional support’ to neighbouring tissues.

Fig. 4. A fibroblast produces all types of fibres and ground substance in a

fibrous tissue.

This function of loose connective tissue is best understood in the context of epithelium. Epithelium has no blood vessels within itself. It is always supported by loose connective tissue. Nutrients and oxygen in capillaries in loose connective tissue diffuse through the ground substance and are taken up by epithelial cells. Waste products from epithelial cells similarly diffuse back into capillaries. Moreover, much as an epithelium is a barrier tissue, some chemicals or micro-organisms can get past the epithelium and land up in this loose connective tissue. This tissue is also equipped with defence mechanisms. The following description in this box is included here in order complete this line of thought. Defence reactions and the cells involved in them will be discussed again in the appropriate chapter. At this stage, recognise the fact that loose connective tissue has cells other than fibroblasts and fibrocytes. The first line of defence is in blood. The ‘weapons’ in blood include white blood cells and proteins in the liquid plasma. In an infection, blood vessels dilate (increase in diameter). Their walls allow increased passage of cells and molecules. These reactions are brought about by a special cell type (‘mast cell’) in loose connective tissue. Blood cells which come out the blood vessels for this purpose are often seen in loose connective tissue. The fight between blood cells and bacteria results in a considerable amount of debris. This is cleared by cells called macrophages. Macrophages may be temporary or permanent residents in connective tissue. Yet another type of cell frequently seen in loose connective tissue is called a plasma cell. It produces a defence weapon in form of proteins (gamma globulin or antibodies). Loose connective tissue is thus a versatile tissue : It has mechanical properties – it can be stretched to an extent and yet is elastic. It also supports other types of tissue (epithelium, in the example above) by way of nutrition. It has important defence-related functions.

The mechanical properties of loose connective tissue can easily be demonstrated on the back of the hand. Pinch the hairy skin on the back of your hand. You can even lift the skin for a short distance. This is because the collagen bundles are being straightened out. You cannot pull it beyond a certain limit, when the collagen fibres are fully straightened. At the same time elastic fibres are being stretched. When you let go of the skin, it falls back to its original position. This is because of the “recoil” of the elastic fibres. In gross anatomical specimens (prosections), loose connective tissue is often difficult to see. It is quite evident while dissecting. Since we do not do dissections in this unit, your best chance to see it is in fresh meat! Watch out for mentions of loose connective tissue in our study in the following weeks. Dense irregular connective tissue. Collagen fibre bundles are thicker and more densely arranged in this type of tissue. Such tissue is often found as sheets. This type offers a greater degree of support and strength. A good example is dermis, the deeper layer of the skin. In some animals it is so thick and strong that it can be processed as leather. The principal cell type is again the fibroblast. It can have other cell types like loose connective tissue as well. In the matrix, collagen fibres are the main ingredient, but elastic fibres are also present in good numbers, along with some reticular fibres. The amount of ground substance is relatively less. Dense regular connective tissue. Here the collagen bundles form fairly thick bundles, are densely packed, but they are arranged in a more-or-less parallel manner. If the bundles are not parallel, they are still organised in line with the forces they are subjected to. Tendons, which connect muscles to bones, are an excellent example. Since collagen bundles resist stretch, they can effectively transmit forces generated by muscles to small areas of bone. Another example is ligaments, which connect bones to each other. In this tissue, ground substance is scanty. In a mature tendon, the fibroblast have done their job and are resting, thus they are better called fibrocytes. They are seen as flat nuclei in rows among the collagen bundles. The cytoplasm is squeezed between fibre bundles, and is difficult to see. In a growing tendon or a tendon under repair, fibroblasts are seen.

It is extremely important to realise that the classification of fibrous connective tissues is not rigid. Adjacent tissue layers of different grades of density make a gradual transition from one to another. It is interesting to take the example of a section of a limb to see these arrangements and also correlate anatomical and histological entities. See fig. 6.

The dermis is largely dense connective tissue, functionally intimately related to the skin. We shall study this along with the skin. Compare A with B and note that there are layers of connective tissue which completely surround the deeper structures, like a bandage. Indeed, the word ‘fascia’ means a bandage. The deep fascia (dense CT) is functionally associated with the deeper structures, especially muscles. The superficial fascia connects the skin to the deeper structures. Part of the loose CT of superficial fascia has fat, described on the next page. Extensions of deep fascia separate muscle masses as intermuscular septa (intermuscular = between muscles, septum = partition; ‘Septa’ is the plural form of septum). Within a muscle mass, thinner partitions further wrap and separate parts of a muscle (more about this with the histology of muscle). Imagine this picture without the muscles – we see that there is an elaborate framework of connective tissue which determines the arrangement of these structures.

Fig. 5. Histological appearances of fibrous connective tissue. Left : Loose connective tissue. The upper part of the photograph shows an epithelium (double-headed arrow). In the lower part, note the light pink collagen bundles (thin arrows). The apparent clear spaces are areas of ground substance. Centre : Dense irregular connective tissue. The thick pink bands are collagen bundles. The red dots show the directions of some of these bundles. The asterisks indicate collagen bundles cut across. This arrangement shows that the bundles run in different directions. The flat nuclei belong to fibrocytes. The round nucleus probably belongs to an active fibroblast. Right : This section is stained by the van Gieson stain, not H&E. The collagen bundles are red. Note that they are parallel and the nuclei (therefore, the cells) are flattened between collagen bundles.

Fig. 6. A: A diagrammatic cross section through the arm. B : Part of ‘A’ in the rectangle, enlarged . See explanation below.

Muscle

Bone

Epidermis

Dermis (Mostly dense CT)

Loose CT with fat

Loose CT, fibrous

Dense CT

Skin Superficial fascia Deep fascia Intermuscular septum

A B

Specialised connective tissues. Adipose Tissue (fat). Adipose tissue or fat is generally regarded as an unwanted tissue. In some locations adipose tissue is normal and it does have a function to perform. Connective tissue under the skin has a variable amount of fat. Masses of fat act as cushion in certain moving parts of the body. Fat also acts as excellent packing material and in some sites as an insulator. However, excess fat is a useless tissue, and is a burden on the circulatory system because blood has to be pumped into it as it is a living tissue. Normal amount of fat under the skin has a characteristic pattern of distribution in males and females. The principal cells of fat are called adipocytes. They synthesize fats which are stored in the cytoplasm as droplets. As the number and size of these cells increase, groups of such cells form ‘lobules’ of various sizes. This growth compresses the fibrous loose connective tissue among them. The compressed tissue is seen as partitions between fat lobules and also carries blood vessels and nerves for the lobules. Many such lobules form visible masses or layers of fat. Histologically two types of fatty tissue are seen. The commoner variety is white fat. In this type, small fat (lipid) droplets in the cytoplasm fuse to form a larger one which grows to a large size, almost filling up the cytoplasm. The rest of the cytoplasm and the nucleus are pushed to one side of the cell. During preparation with routine methods the fat is dissolved and the cell appears like an empty shell with a slender ring of cytoplasm, thickened at one point by the blue dot of the nucleus. This is described as the signet ring appearance (fig. 7).

The other type of adipose tissue is called brown fat. This type of fatty tissue has plenty of blood vessels. The white colour of the fat is mixed with the red of the abundant blood vessels, giving it a brown hue. The cells of this type have numerous small droplets of fat in the cytoplasm. Dissolution of these gives a cell a ‘foamy’ appearance. The nucleus is not displaced to one side; it is more or less central in location. (Fig. 8). In humans, brown fat is seen only in the foetus and in the newborn in specific locations. Such locations include the area between the shoulder blades and lower parts of the abdomen. A newborn baby, exposed to the external atmosphere after the warm surroundings of the womb, metabolises (‘burns’) this fat to produce warmth.

Fig. 7. Stages in the formation of a white fat cell (diagrammatic).

Fig. 8. Formation of a brown fat cell.

Embryonic / foetal connective tissue. Fibres in connective tissue develop along the lines of forces exerted on it. In the protected environment of the uterus, floating in amniotic fluid, the embryo/foetus is not exposed to many stresses of the mechanical kind. A lot of connective tissue in the embryo / foetus has a scant proportion of fibres and a large amount of ground substance. The matrix appears more jellylike, the cells are numerous and not being compressed by fibres, spread out in a starlike manner. Connective tissue in the umbilical cord is a good example. A lot of connective tissue in the embryo is undifferentiated connective tissue or mesodermal tissue. All connective tissue originates from the mesoderm. Undifferentiated masses of mesoderm found among tissues that it supports, are also referred to as mesenchyme. Mesenchyme is a highly cellular mass. Cells of mesenchyme can specialise (differentiate) to form fibroblasts, adipocytes, chondroblasts or osteoblasts. At the same time, we find a huge amount of connective tissue specialising as fibrous tissue, cartilage or bone in preparation for life after birth. Cartilage Fibrous connective tissue depends on its fibres for most of its mechanical properties. Although collagen resists stretching and elastin restores tissue, none of these can resist compression. In situations where compressing forces act on tissues, something that resists or withstands compression is required.

Cartilage is a connective tissue that is capable of resisting compression. Cartilage owes this property largely to some components of ground substance in its matrix. Large molecules, shaped like bottlebrushes, have negatively charged sulphate ions attached to them. The repelling charges and the amount of water the ground substance holds makes the matrix resistant to compression to a significant extent. Besides these, cartilage matrix has the usual collagen and elastic fibres. The tough (not ‘hard’) nature of the matrix leads to a characteristic appearance of cartilage in histological sections. The cells are surrounded by the matrix they produce; and the cell membrane is in contact with the surrounding matrix. During processing for microscopy the soft cells shrink to some extent, the tough matrix shrinks very little, if at all. The shrunken cells thus seem to ‘fall off’ from the surrounding matrix, leaving a small clear space. It appears as if the matrix has a hole or a gap containing a cell that is smaller than the gap.

The gap, called lacuna, is an artefact. It is helpful in recognising cartilage as a tissue in histological sections.

Cartilage has one more distinct feature : the main mass of a cartilage has no blood vessels – it is avascular. Cartilage receives its nutrition from the surrounding fibrous connective tissue called perichondrium (= ‘around cartilage’). Important exceptions to this pattern will be mentioned soon. Cartilage is an excellent intermediate between fibrous tissue and bone – it is strong and light, requires less blood supply and has a good potential for growth. Histologically, three main types of cartilage are recognised. The first one, called hyaline cartilage due to its glassy appearance, is the best example to study properties of cartilage as a tissue.

Fig. 9. Brown and white adipose tissue in the same section (a ‘lucky find’). The upper left portion shows brown fat. Note the ‘foamy’ appearance of the cells, due to discrete fat droplets. The cell in the oval is characteristic, with its nucleus in the centre. White fat on the lower right hand side. Note the empty appearance of the cells due to dissolution of the single large fat droplet. The nucleus, pushed to one side, is shown by the arrow.

Fig. 10. Hyaline cartilage. Higher magnification : next page

Trachea : diagrammatic outline.

Note C-shaped cartilage. Portion in the rectangle magnified on the right.

Fig. 11. Hyaline cartilage : A portion close to the surface, with perichondrium. The line drawn across the picture shows the junction between perichondrium and the cartilage. The dense fibrous nature of the perichondrium is obvious, the nuclei of the cells are flat. In contrast, the cells of the cartilage proper are rounded, with outlines of the lacunae. Smaller, flatter cells in the cartilage (arrows) are those which have recently become chondrocytes. The ‘connective tissue nature’ of cartilage is evident in the fact that the lacunae are scattered within the matrix.

Hyaline cartilage. In the fresh state this variety has an appearance like frosted glass, hence the name hyaline (= glassy). Its functional importance is easily seen if we consider sites where it is found. The trachea (windpipe) is an excellent example – it must be kept open for the passage of air at all times, and this is achieved by C-shaped ‘rings’ of hyaline cartilage. The ends of ribs are joined to the sternum in the thoracic wall by means of hyaline cartilage. Hyaline cartilage is also found in almost all movable (synovial) joints, covering the joint-forming surfaces of bones. Hard bony surfaces can undergo tremendous wear-and-tear as they move against each other. In this location it is called articular cartilage, meaning cartilage of or in a joint (articulation). Finally, as we shall see later, hyaline cartilage also provides a mechanism for growth between parts of a bone. With the exception of articular cartilage, all hyaline cartilage is surrounded by a covering of dense fibrous tissue called perichondrium (= ‘around a cartilage’). Perichondrium carries blood vessels for the cartilage which, as mentioned earlier, is avascular. In the deep part of the perichondrium there are cells which can specialise into chondroblasts. When they specialise as chondroblasts, they produce cartilage matrix. They are therefore called chondrogenic ( cartilage forming) cells.

Fig. 10 illustrates the basic features. In the photomicrograph on the right, recognise the lacunae scattered in the matrix. Cartilage is a tough tissue – some of the cells fall off the section during processing, showing some ‘empty’ lacunae. The dark blue vertical lines are wrinkles in the sections, an artefact. The perichondrium is better seen at the lower border. Note that some of the matrix is almost colourless, while some of it has a blue tinge (explained below).

The wall of the trachea has other tissues as well, these are not shown in this illustration for clarity. Cartilage matrix has ground substance and all three types of fibres. Fibres are not visible in H&E sections. Under the microscope the matrix appears uniformly pale. Thus, even microscopically hyaline cartilage appears almost transparent, except for cells. Matrix that is of recent origin does have a blue colour. Such matrix is seen immediately surrounding the cells that produce it. Such bluish matrix is called territorial matrix. Growth of cartilage. Cartilage grows by two methods. Chondrogenic cells in the perichondrium become chondroblasts, produce new matrix under the perichondrium, that is, close to the surface of the cartilage mass. The growth of these cells and the new matrix add to the size of the cartilage. This is called growth by apposition. Deeper in the cartilage, some of the resting cells (chondrocytes) can resume division and growth. When such a cell divides, initially we see two cells in a single lacuna. Addition of new matrix by these cells, all around themselves, separates the cells over time. Such growth occurs deep within the cartilage and is called interstitial growth. Often, two cells in a single lacuna divide again before being separated. Such groups of cells, either in one lacuna or just separated by territorial matrix are called isogenous cells or cell nests.

Fig. 12. Deeper part of a hyaline cartilage. The diagram on the left illustrates two cells from the photomicrograph, explaining the appearance of lacunae. Even if the cytoplasm is not clearly visible, the shapes of the nuclei indicate the shapes of the cells they belong to. Compare this with the flat nuclei in the fibrous tissue of the perichondrium. The picture also shows two empty lacunae (explained in fig. 9.

Fig. 14. Elastic cartilage : Low magnification (left) and high magnification (right). These sections are stained with a special stain for elastic material, which appears as blue or blue-black. Note that except for the elastic fibres, the structure is the same as hyaline cartilage.

Elastic cartilage. Feel the flap of your ear anywhere except the lower soft part. It feels firm. It retains its shape because of a thin plate of cartilage. Bend the flap and release it – it springs back to its original position – the cartilage is elastic! As the name suggests, this type of cartilage must have a high proportion of elastic fibres in its matrix. In the fresh state when seen with the unaided eye, elastic cartilage has a pale yellowish colour due to the preponderance of elastic material. Histologically the cartilage looks like hyaline cartilage – it has perichondrium, chondrocytes in lacunae, isogenous groups – the works. The matrix is however, quite characteristic – it has a large amount of fine pink fibres. In the laboratory, if you change the fine focus of the microscope, you will see these fibres appearing alternately bright and dull. These are elastic fibres. With specific special staining methods, elastic fibres can be seen dramatically as dark brown or almost black structures. The flap of the ear (‘pinna’) is an excellent example of this cartilage.

Fig. 13. Interstitial growth : Isogenous cells (cell nests) and territorial matrix. The two cells in the single lacuna on the left are the products of a recent cell division. On the right, two cells have begun to be separated by the matrix they have produced. Note the colour of the territorial matrix in contrast with matrix elsewhere. Remember that a histological section represents the state of the tissue at the time the tissue is collected and fixed.

Fig. 15. White fibrocartilage. The vivid pink background suggests matrix with abundant collagen. The clear spaces (asterisks) are tears in the section. The shapes of nuclei suggest that the cells are oval or round, not flat as in dense connective tissue. Two cells indicated by arrows are enlarged in the inset to show their features.

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White fibrocartilage. White fibrocartilage is a remarkable tissue. It combines the flexibility of dense fibrous tissue with the firmness of cartilage. In the fresh state this variety appears white (as against the “glassy” hyaline cartilage). This is due to a predominance of collagen fibres in the matrix. The thick bundles of collagen appear similar to those in dense fibrous tissue. The distinguishing feature is the rounded cells in lacunae. White fibrocartilage merges imperceptibly with the surrounding dense fibrous tissue. In this sense, it does not have a recognisable perichondrium. The chondrocytes in the true cartilage part are arranged in ones, twos or in rows roughly parallel to the collagen bundles around them. In the overall picture (not easily seen in sections of small portions), the collagen bundles are arranged along the lines of force the mass is subjected to. With such large proportions of collagen, white fibrocartilage is deformable to some extent. It is often seen in tendons where they run over a bone, especially if there is a bend in the tendon. The cartilage makes a tendon stronger in such locations, at the same time retaining some degree of flexibility required for movement of the tendon against the bone. White fibrocartilage is also seen as fairly large masses joining two bones where its deformability allows a very small degree of movement. A good example is an intervertebral disc. Complete or incomplete discs of white fibrocartilage are found in synovial joints where the bony surfaces are mismatched – it forms a cushion to optimise movement, yet is tough enough to withstand forces within a joint. We shall learn more about such sites when we study joints. Bone as a connective tissue is an extensive topic, and is discussed in a separate chapter.

Summary : Key concepts

Connective tissues serve many functions. They join structures, resist or transmit forces, form wrapping for muscles/nerves/blood vessels or other organs, separate structures, form packing material etc.

Each type of connective tissue has a principal cell type, which produces large amounts of intercellular matrix.

Connective tissue matrix comprises fibres (collagen, elastic and reticular) and ground substance.

Ground substance is usually not visible in routine histological preparations.

Connective tissues can be regarded as fibrous (with easily visible fibres) or special connective tissues.

Fibrous connective tissues are further described by the density and arrangement of fibres – loose, dense irregular, dense regular.

Loose connective tissue has plenty of ground substance, and serves as a medium of exchange between other tissues in addition to its mechanical functions. It also has other types of cells besides fibroblasts and fibrocytes.

Dense connective tissue can take on a variety of forms like sheets, bundles, bands etc, depending on its functional requirements.

Cartilage is a special connective tissue, able to resist compression. This ability is dependent on the biochemical nature of its ground substance. All varieties of cartilage have fibres and ground substance in the matrix.

Fibres are not visible in hyaline cartilage.

Cartilage can grow from the surface (appositional) and by division of chondrocytes deeper inside (interstitial).

Elastic cartilage resembles hyaline cartilage histologically, except for the network of elastic fibres.

White fibrocartilage is like an intermediate stage between dense fibrous tissue and cartilage – it is slightly deformable.

Most connective tissues model themselves in response to the forces that act on them. At the present stage this concept may be difficult to grasp – it will be revisited when we study muscle tissue and bone, as also along with defence mechanisms of the body.

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