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
| Hyaline cartilage| White fibrocartilage| Elastic 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
· 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.
· Movement
Hyaline cartilage joins bones firmly together in such a way that a certain amount of movement is still possible between them.
· Support
The c-shaped cartilagenous rings in the windpipes (trachea and bronchi) assist in keeping those tubes open.
· 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
· Shock absorbers.
The cartilage between the adjacent vertebrae absorbs the shocks that will otherwise damage and jar the bones while we run or walk.
· Provides sturdiness without impeding movement.
The white fibrocartilage forms a firm joint between bones but still allows for a reasonable degree of movement.
· 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
· Maintain shape.
In the ear, for example, elastic cartilage helps to maintain the shape and flexibility of the organ.
· 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
· Support.
The skeleton, which consists mainly of bone tissue, forms a supportive framework, giving shape and rigidity to the body.
· Locomotion.
The bone tissue forms a system of levers to which the voluntary muscles are attached.
· Protection.
It serves to protect the soft and delicate organs of the body such as the skull protects the brain.
· 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.
· 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.