Connective tissue
Structure
Connective tissues function primarily to support the body and to bind or connect together all types of
tissue. This tissue also provide a mechanical framework (the skeleton) which plays an important role in
locomotion. Unlike epithelial tissue, connective tissue is characterised by the large amounts of
intercellular substance (also called ground substance or the matrix) that it contains.
Connective tissue are relatively few cells which are widely seperated from each other. These living cells
are responsible for secreting the large amounts of intercellular ground substance (matrix). The matrix is a
non-living material which may be liquid (eg. blood), semi-solid (eg. connective tissue) or solid (eg. bone).
Embedded in the matrix are a variety of connecting and supporting fibres, eg. collagen fibres and elastic
fibres.
Classification of the basic connective tissue depends on the predominant fibre type present in each.
Connective tissue can be divided into four main types.
Cartilage
Cartilage is usually found in close association with bone in the body. It is a type of connective tissue
which is tough, semi-transparent, elastic and flexible. The matrix or ground substance of cartilage
consists mainly of glyco-protein material, chondroitin. The cartilage cells (chondrocytes) lie scattered in
the matrix. Cartilage is covered by a dense fibrous membrane, the perichondrium. No nerves or blood
vessels occur in cartilage.
In some vertebrates, such as sharks, the entire skeleton is made up of cartilage. In mammal embryos,
the skeleton first forms as cartilage tissue. Cartilage acts as a model and is gradually replaced by bone
as the embryo grows. Such cartilage is known as temporary cartilage. The process by which bone tissue
follows the cartilage model and slowly replaces it is known as ossification. Permanent cartilage (cartilage
which does not become ossified) is found in the tip of the nose, in the external ear and in the walls of the
trachea (windpipe) and the larynx (voice-box).
Hyaline cartilage
Hyaline cartilage is semi-transparent and appears bluish-white in colour. It is extremely strong, but very
flexible and elastic. Hyaline cartilage consists of living cells, chondrocytes, which are situated far apart in
fluid-filled spaces, the lacunae. There is an extensive amount of rubbery matrix between the cells and the
matrix contains a number of collagenous fibres. Hyaline cartilage occurs in trachea, the larynx, the tip of
the nose, in the connection between the ribs and the breastbone and also the ends of bone where they
form joints. Temporary cartilage in mammalian embryos also consists of hyaline cartilage.
Functions:
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Reduces friction at joints. By virtue of the smooth surface of hyaline cartilage, it provides a sliding
area which reduces friction, thus facilitating bone movement.
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Movement. Hyaline cartilage joins bones firmly together in such a way that a certain amount of
movement is still possible between them.
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Support. The c-shaped cartilagenous rings in the windpipes (trachea and bronchi) assist in keeping
those tubes open.
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Growth. Hyaline cartilage is responsible for the longitudinal growth of bone in the neck regions of
the long bones.
White Fibrocartilage
White fibrocartilage is an extremely tough tissue. The orientation of the bundles depends upon the
stresses acting on the cartilage. The collagenous bundles take up a direction parallel to the cartilage.
Fibrocartilage is found as discs between the vertebrae between the pubic bones in front of the pelvic
girdle and around the edges of the articular cavities such as the glenoid cavity in the shoulder joint.
Functions:
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Shock absorbers. The cartilage between the adjacent vertebrae absorbs the shocks that will
otherwise damage and jar the bones while we run or walk.
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Provides sturdiness without impeding movement. The white fibrocartilage forms a firm joint between
bones but still allows for a reasonable degree of movement.
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Deepens sockets. In articular cavities (such as the ball-and-socket joints in the hip and shoulder
regions) white fibrocartilage deepens the sockets to make dislocation less possible.
Elastic cartilage
Basically elastic cartilage is similar to hyaline cartilage, but in addition to the collagenous fibres, the
matrix of the elastic also contains an abundant network of branched yellow elastic fibres. They run
through the matrix in all directions. This type of cartilage is found in the lobe of the ear, the epiglottis and
in parts of the larynx.
Functions:
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Maintain shape. In the ear, for example, elastic cartilage helps to maintain the shape and flexibility
of the organ.
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Support. Elastic cartilage also strengthens and supports these structures.
Types of Cartilage
Bone
Bone tissue occurs in the different bones of the skeleton. Bone is a hard and rigid tissue. Like cartilage,
bone consists of living cells with large amounts of ground substance or matrix. It is impregnated with
organic salts such as calcium carbonate (7%) and calcium phosphate (85%). Small amounts of sodium
and magnesium is also present. In addition to this, the matrix contains numerous collagenous fibres and
a large amount of water. Collagen fibres together with the bone cells constitute the organic (living) matter
in bone tissue. There are different groups of bone in the skeleton, inter alia long bones such as the
humerus and femur.
Structure of a Bone
A long bone such as the femur, consists of a centre piece, the shaft (diaphysis) and a thickened head
(epiphysis) at each end. The heads articulate with other bones in the joints and are covered with a thin
layer of hyaline cartilage. The remainder of the bone is covered with a tough, strong membrane, the
periosteum which is richly supplied with blood vessels. There is a small artery which penetrates the shaft
near the centre to supply the bone tissue with blood. Beneath the periosteum is a layer of compact bone
which is thicker in the shaft than in the two heads. The shaft encloses a hollow, the marrow cavity, which
is lined with a thin soft membrane known as the endosteum. The marrow cavity contains a soft tissue
richly supplied with fat cells and blood corpuscles, the yellow marrow. The epiphysis of a long bone
consist of spongy (or cancellous) bone covered with a thin layer of compact bone. This is made up of
bony bars (or trabeculae) arranged in such a way that they are able to resist any force which a applied
upon the bone. Between the bars are many tiny cavities filled with a red marrow which contains
numerous red blood corpuscles in different stages of development.
Microscopic Structure of Compact Bone
Under the microscope dense, compact bone shows a definite and a characteristic pattern of arrangement.
The ground substance of bone is arranged in concentrated layers (lamellae) round the small canals
which run parallel to the long axis (shaft) of the bone. These canals, called Haversian canals, are
interconnected with one another via Volkmann's canals and contain a blood vessel, a nerve and a lymph
vessel. Each Haversian canal is surrounded by concentric layers of bone matrix (called lamallae) and
concentric rings of bone forming cells (osteoblasts). Bone cells remain alive and once they have
completely surrounded by the hard bone matrix, they are called osteocytes. The osteocytes are
embedded in fluid-filled cavities within the concentric lamellae. These cavities are known as lacunae and
occur at regular intervals in these concentric layers of bone tissue. The lacunae are connected to one
another and to the Haversian canals by a system of interconnecting canals known as canaliculi. Each
Haversian canal, its concentric lamellae, lacunae with osteocytes and canaliculi forms a long cylinder and
is called a Haversian system. Separate Haversian systems are joined to each other by means of
interstitial lamellae.
Growth of Bone Tissue
In a child a long bone has a layer of cartilage between the head (epiphysis) and the shaft (diaphysis). The
cartilage grows actively which causes an increase in the length of the bone. The layer does not thicken
since the edges (on both sides) are constantly replaced by bone (become ossified). The bone grows in
the length until the child reaches its adult size. The cartilage then also ossifies and disappears. At the
same time the bone increases in thickness as a result of the formation of bone tissue immediately
beneath the periosteum. The innermost layer, nearest to the marrow cavity, are constantly absorbed,
which enlarges the size of the marrow cavity.
Functions of Bone Tissue
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Support. The skeleton, which consists mainly of bone tissue, forms a supportive framework, giving
shape and rigidity to the body.
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Locomotion. The bone tissue forms a system of levers to which the voluntary muscles are attached.
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Protection. It serves to protect the soft and delicate organs of the body such as the skull protects the
brain.
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Manufacturing of Blood Cells. Red blood cells are manufactured in the red bone marrow, which is
situated in the spongy tissue at the ends of long bones.
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Homeostasis. Bone plays a part in homeostasis because it helps to maintain a constant level of
calcium in the blood.
Structure of long bone with enlargement of a section of compact bone
Blood
Although blood is a fluid it must be seen as a connective tissue which consists of a ground substance,
blood plasma, and cell elements, blood corpuscles. The only difference with the other connective tissues
is that it does not contain fibre elements .
Blood is a sticky fluid with a slightly salty taste. It has a bright red or scarlet colour when it flows from the
arteries but a dark red or purple colour when it flows from the veins. It is slightly alkaline (pH 7.4).
Blood Plasma
Plasma is a yellowish, straw-coloured liquid which consists mainly of water (>90%). The other 10% of the
blood plasma consists of dissolved substances of which the following are the most important: Organic
constituents (2%) which include nutrients such as glucose, fats, amino acids and vitamins. Inorganic salts
and ions which include ions such as bicarbonates, phosphates, sulphates, chlorides, calcium potassium,
sodium and magnesium. Secretions such as enzymes and hormones. Dissolved gases such as oxygen
and carbon (IV) oxide, i.e. gases involved with respiration. Antibodies which are protective protein
compounds. Plasma proteins (7%), the most important of which are fibrinogen, albumen and globulin.
Functions:
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Plasma transport the various blood types throughout the body.
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It transports food and nutrients from the digestive system to the various tissues in the body.
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It transports waste products from the tissues to the excretory organs.
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Fibrinogen plays an important role in the clotting of blood.
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Blood plasma plays an important role in regulating the body temperature.
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Hormones are transported by the plasma to their target organs where they bring about a specific
function.
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Albumen and globulin regulate the water content of cells and extracellular body fluids .
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Globulin also gives rise to antibodies that provide immunity against various diseases.
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The proper balance of ions allows for the normal functioning of nerves, muscles, etc.
Erythrocytes (Red Blood Corpuscles)
In humans there are about 5 million erythrocytes per cubic millimetre of blood. Erythrocytes are small,
round, bi-concave discs which float in the blood plasma. They are actually yellowish in colour but when
present in large numbers they are red. Each adult red blood cell represents a cell without a nucleus,
which is surrounded by a thin, elastic membrane. They are soft, flexible and elastic and therefore move
easily through the narrow blood capillaries. Approximately 90% of the content of each erythrocyte is
haemoglobin which supplies the characteristic colour of the red corpuscles. Red blood cells are formed in
the red marrow of long and flat bones, especially in the spongy regions in the heads of the long bones.
The life span of an erythrocyte is approximately 4 months.
Functions:
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The erythrocytes transport oxygen in the blood from the lungs to all the cells and tissues of the
body.
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Red blood corpuscles also assist with the transport of carbon (IV) oxide from the tissues to the
lungs.
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They play an important role in regulating the acid-base balance of the blood, thus preventing large
changes in pH.
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Erythrocytes also assist when a blood clot is formed.
Leucocytes (White Blood Cells)
Leucocytes are far less numerous than red blood corpuscles. Leucocytes are larger than red blood
corpuscles and have a definite nucleus. They are irregular in shape, slightly translucent and nearly
colourless.They are able to change their shape because of the fact that they move by means of
pseudopodia (false feet). Many are phagocytic, i.e. they are able to engulf micro-organisms and foreign
intruders into their cytoplasm by flowing around them. There are five types of leucocytes which can be
divided into two groups, namely granular white blood cells where the cytoplasm is granular, and
non-granular white cells where the cytoplasm does not contain granules. The two principal types of white
blood cells are neutrophils and lymphocytes. Neutrophils are the most abundant and are produced in the
red bone marrow. Their nuclei are divided to form 3 to 5 lobes, connected by thin threads of nuclear
material. They all have conspicuous granules in their cytoplasm. Eosinophils and basophils also have
granules in their cytoplasm and irregular-shaped nuclei. Lymphocytes are produced in the spleen, tonsils
and lymph nodes and are the smallest of the white blood cells. There are no granules in the cytoplasm
but a large spherical nucleus is present.
Functions:
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Neutrophils are active in phagocytosis and defend us against harmful viruses, bacteria and other
foreign intruders.
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Neutrophils also play a role in the healing of wounds and repairing worn out and damaged tissues.
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Neutrophils prevent infections from spreading to other tissues of the body.
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Lymphocytes are involved in the synthesis and distribution of antibodies in the blood. The B-cells
are responsible for humoral or antibody immunity. The T-cells are responsible for cellular immunity.
Blood Platelets
These are small, colourless, plate-like discs. No nucleus is visible. They are not true cells but are
cytoplasmic fragments of large cells found in red bone marrow. When tissue is damaged and the platelets
leave the blood vessels, they release a substance which transforms soluble fibrinogen in the plasma to a
network of fibrin threads.
Functions:
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Blood platelets play an important role in initiating the process of blood-clotting and in the plugging
up and sealing of damaged blood vessels and form tissues.
Composition of Blood
(Provided by Prof. Li Jicheng)