Tissue|Medical World

What is Tissue?

Tissues consists of large numbers of the same type of cells and are classified according to the size, shape and functions of their constituent cells. There are four main types of tissue each with subtype

Types of Tissue

It consists of the following terms-

• Epithelial tissue
• Connective tissue
• Muscle tissue
• Nervous tissue

Epithelial tissue

This tissue type covers the body and lines cavities, hollow organs and tubes. It is also found in glands. The structure of epithelial is closely related to its functions, which include: Protection of underlying structure from,

Example-dehydration, chemical and mechanical damage

• Secretion
• Absorption

The cells are very closely packed and the intracellular substance, the matrix, is minimal. The cells usually lie on a basement membrane, which is an inert connective tissue made by the epithelial cells themselves. Epithelial tissue may be:

• Simple: a single layer of cells
• Stratified : several layers of cells

Simple epithelial

Simple epithelial consists of a single layer of identical cells and is divided into three main types. It is usually found on absorptive or secretary surfaces, where the single layer enhances these processes, and seldom on surfaces subject to stress. This type are named according to the shape of the cells, which differs according to their functions. The more active the tissue, the taller the cells.

Squamous epithelial

This is composed of a single layer of flattened cells. The cells fit closely together like flat stones, forming a thin and very smooth across which diffusion occurs easily. It forms the lining of the following structures;

• Heart - where it is known as endocardium
• Blood vessels - where it is also known as endothelium
• Lymph vessels - endocardium
• Alveoli of the lungs
• Lining the collecting ducts of nephrons in the kidneys

Cuboidal epithelium

This consists of cube-shaped cells fitting closely together lying on a basement membrane. It forms the kidney tubules and is found in some glands such as the thyroid. Cuboidal epithelium is activity involved in secretion, absorption and/or excretion.

Columnar epithelium

This is formed by a single layer of cells, rectangular in shape, on a basement membrane. It lines many organs and often has adaptations that make it well suited to a specific function. The lining of the stomach is formed from simple columnar epithelium without surface structures. The free surface of the columnar epithelium lining the small intestine is covered with microvilli. Microvilli provide a very large surface area for absorption of nutrients from the small intestine. In the trachea, columnar epithelium is ciliated and also contains a goblet cells that secrete mucus . This means that inhaled particles that stick to the mucus layer are moved towards the throat by cilia in the respiratory tract. In the uterine tubes, ova are propelled along by ciliary action towards the uterus.

Stratified epithelia

Stratified epithelia consists of several layers of cells of various shapes. Continual cell devision in the lower layers pushes cells above nearer and nearer to the surface, where they are shed . Basement membranes are usually absent. The main function of stratified epithelium is to protect underlying structures from mechanical squamous and transitional .

Stratified squamous epithelium

This is composed of several layers of cells. In the deepest layers the cells are mainly columnar and, as they grow towards the surface, they become flattened and are then shed.

Keratinised stratified epithelium

This found on dry surfaces subjected to wear and tear, i.e. skin, hair and nails. The surface layer consists of dead epithelial cells that have lost their nuclei and contain the protein keratin. This forms a tough, relatively waterproof protective layer that prevents drying of the live cells underneath. The surface layer of skin is rubbed off and is replaced.

Non-keratinised stratified epithelium

This protects moist surface subjected to wear and tear, and prevents them from drying out, e.g. the conductive of eyes, the lining of the mouth, the pharynx, the oesophagus and the vagina.

Transitional epithelium

This is composed of several layers of pear-shaped cells. It lines several parts of the urinary tract including the bladder and allows for stretching as the bladder fills.

Connective tissue

Connective tissue is the most abundant tissue in the body. The connective tissue cells are more widely separated from each other than in epithelial issues, and intracellular substance is present in considerably larger amounts. There are usually fibres present in the matrix, which may be of a semisolid jelly-like consistency or dense and dense and rigid, depending upon the position and function of the tissue. The fibres from a supporting network for the cells attach to. Most types of connective tissue have a good blood supply. Major functions of connective tissue are:

• Binding and structural supporter
• Protection
• Transport
• Insulation

Cells in connective tissue

Connective tissue, excluding blood , is found in all organs supporting the specialised tissue. The different type of cells involved include: fibroblast, fat cells, macrophages, leukocytes and mast cells

Fibroblast

fibroblast are large cells within irregular processes. They manufacture collagen and elastic fibres and a matrix of extracellular material. Very fine collagen fibres sometimes called reticulin fibres, are found in highly active tissue, such as the liver and reticular tissue. Fibroblast are particularly active in tissue repair where they may bind together the cut surfaces of wound or form granulation tissue following tissue destruction.

Fat cells

it is also known as adipocytes, these cells occur singly or in groups in many types of connective tissue and are especially abundant in adipose tissue . They vary in size and shape according to the amount of fat they contain.

Macrophages

these are large irregular-shaped cells with granules in the cytoplasm. Some are fixed , i.e. attached to connective tissue fibres, and others are motile. They are an important part of the body's defence mechanisms because they are actively phagocytic, engulfing and digestive cell debris, bacteria and other foreign bodies.

Leukocytes

white blood cells are normally found in small numbers in healthy connective tissue but neutrophils migrate in significant numbers during infection when they play an important part in tissue defence plasma cell. They synthesise and secrete specific defensive antibodies into the blood and tissues.

Mast cells

these are similar to basophil leukocytes . They are found in loose connective tissue, under the fibres capsule of some organs, e.g. liver and spleen, and in considerable numbers round blood vessels. Their cytoplasm is packed with granules containing heparin, histamine and other substances, which are released when the cells are damaged by disease or injury.

Loose (areolar) connective tissue

This is the most generalised type of connective tissue. The matrix is semisolid with many fibroblast and some fat cells , mast cells and macrophages widely separated by elastic and collagen fibres. It is found in almost every part of the body, providing elasticity and tensile strength. It connects and supports other tissues,

For example ;

• under the skin
• Between muscle
• Supporting blood vessels and nerves
• In the alimentary canal
• In glands supporting secretary cells

Adipose tissue

Adipose tissue consists of fat cells, containing large fat globules, in a matrix of areolar tissue . There are two types -

• White
• Brown

White adipose tissue

This makes up 20-25% of body weight in adults with a normal body mass index; more is present in obesity and less in those who are underweight. Adipose tissue secrete the hormone leptin. The kidneys and eyeballs are supported by adipose tissue, which is also found between muscle fibres and under the skin, where it acts as a thermal insulator and energy store.

Brown adipose tissue

This is present in the newborn. It has a more extensive capillary network than white adipose tissue. When brown tissue is metabolised, it produces less energy and considerably more heat than other fat, contributing to the maintenance of body temperature . Sometimes small amount are present in adults.

Reticular tissue

Reticular tissue has a semisolid matrix with fine branching reticulin fibres. It contains reticular cells and white blood cells . Reticular tissue is found in lymph nodes and all organs of the lymphatic system.

Dense connective tissue

This tissue is made up mainly of closely packed bundles of collagen fibres with very little matrix. Fibrocytes are few in numbers and lie in rows between the bundles of fibres. Fibrous tissue are found;

• Forming ligaments , which bind bones together
• As an outer protective covering for bone, called periosteum.
• As an outer protective covering of some organs , e.g. kidneys, lymph nodes and the brain
• Forming muscle sheaths, called muscle fascia

Elastic tissue

Elastic tissue is capable of considerable extension and recoil. There are few cells and the matrix consists mainly of masses of elastic fibres secreted by fibroblasts. It is found in organs where stretching or alteration of shape is required, e.g. in large blood vessels walls, the trachea and bronchi and the lungs.