The structure of epithelial tissue is related to the function it carries out which include; protection; secretion and absorption. Simple epithelium consists of a single layer of identical cells, which are divided into three main types. The types are named depending on their shape which alters according to the functions they carry out. The more the tissue gets used, the taller the cells. Simple epithelium is usually found on absorptive or secretory surfaces, where it helps to speed up these processes due to the single layer of cells. Simple Epithelium can be squamous, cuboidal, columnar or ciliated. Squamous cells are a single layer of flat cells which contain a nucleus that forms a lump in the centre. They fit closely together forming a thin and smooth layer of membrane across which allows materials to pass through easily via diffusion and osmosis. Squamous cells are found lining the walls of lung alveoli, the heart and blood vessels, where it is known as endocardium. Cuboidal epithelium is made up of cube shaped cells that fit closely together, which have spherical nuclei in the centre of the cell.
They are found lying on the basement membrane for support. Cuboidal epithelium is actively involved in secretion, absorption and excretion. They can be found in glands such as the thyroid gland and sweat ducts as well as forming kidney tubules. Columnar epithelium is a single layer of tall and rectangular cells with slightly oval nuclei which are attached on a basement membrane. Sometimes they can contain microscopic filaments known as cilia, which look like little hairs on top of the epithelium. These cells are then called Ciliated Epithelia. Ciliated epithelium cells often contain goblet cells which secrete mucus. The cilia move in a wave like motion movement, this direction of movement causes the mucus to move in that direction. This means that inhaled particles that stick to the mucus are moved to the throat by the cilia. Cilia provide a large surface area for absorption of nutrients. This usually happens in the small intestines where nutrients are passed through.
Stratified epithelia are constructed of several layers of cells in various shapes. Unlike simple epithelium there is no basement membrane, the continual cell division in the lower layers pushes cells nearer and nearer the surface. This is where they are shredded. The main function of stratified epithelia is to protect deeper structures from wear and tear. There are two main types, these are stratified squamous and transitional. Stratified squamous epithelia are composed of different layer of cells. The deepest layers of the tissue are mainly formed from columnar but as they grow towards the surface they become flattened and shred to make way for newer cells deeper down. They are usually found on dry surfaces due to wear and tear such as the skin, hair and even nails.
The surface layers of flattened cells are dead epithelial cells which have lost their nuclei. Transitional epithelium is also composed of several layers but is constructive of pear shaped cells instead. This type of tissue is stretchy, meaning it can contract and expand. It is usually found lining the urinary bladder as it allows for stretching as the bladder fills up. The second type of tissue found in the body is connective tissue. They lie beneath the epithelial tissue helping to connect different part of the internal structure, the cells are more widely separated from each other then in epithelial tissue. The intercellular substance known as the matrix is found in considerably large amounts.
Within the matrix there are usually fibres which may be a jelly like consistency or dense and rigid depending on the type, function and positioning of the tissue. Theses fibres form a supporting system for cells to attach to. The major functions of connective tissue are to transport materials, give structural support and protection. The types of connective tissue that will be explained are blood, bone, cartilage, bone, areolar tissue and adipose tissue. Blood is made up of straw coloured plasma, the matrix, in which various types of blood are carried. Plasma is mainly water where substances are carried such as oxygen and carbon dioxide, nutrients such as glucose and amino acids, salts, enzymes and hormones. Also there is a combination of important proteins which help with blood clotting, transport, defence against invading organisms and osmotic regulation.
Cartilage is a smooth, translucent substance which is firmer than other connective tissue. It protects the ends of bones from friction during movement. It also forms the major part of the nose and the external ear flaps which are known as pinnae. The cells that make up cartilage tissue are called chondrocytes which are a firm but flexible glass like material. They become embedded within the matrix which is reinforced by collagen and elastic fibres. There are three types of cartilage: hyaline cartilage, fibrocartilage, and elastic fibrocartilage. Bone is a much harder substance then cartilage but unlike cartilage it can be worn away by friction. The rigid matrix of collagen fibres that surround bone cells are strengthened by calcium and phosphate salts which form around the collagen fibres and gives bone its hardness. The collagen fibres offer the ability for the bone to bend under strain and pressure. It also prevents bone from being too brittle which means its less likely to fracture. Also bone has considerable capacity for growth in the first two decades of life and redevelopment throughout life.
There are two types of bone that can be identified by the naked eye which are: compact bone that appears solid or dense and spongy or cancellous bone which has a honeycomb appearance. Bone is created to bear weight by limbs bones being hollow. The bone is also used to protect weaker and more delicate tissues such as the brain and lungs. Areolar tissue is the most common type of connective tissue in the body. It is a sticky white material that binds muscle groups, blood vessels and nerves together. The matrix is semi fluid which contains many fat cells, mast cells and macrophages separated by elastic and collagen fibres secreted by the cells found in this loose connective tissue. It is found in almost every part of the body with the elastic fibres providing elasticity and tensile strength. Areolar tissue offers support and helps to connect other tissues it surrounds such as under the skin, between muscles and blood vessels and nerves. Adipose tissue is a technical term used for fatty tissue. It is a variation of areolar tissue, in which it consists of fat cells, which contain large fat globules in a matrix of areolar tissue.
There are two types of adipose tissue; white and brown. White adipose tissue makes up 20 to 25% of body weight in well nourished adults. Brown adipose tissue is present in the new born; it has a more extensive capillary than the white adipose tissue. It is also common to find it under the skin and around organs such as the heart, kidneys and parts of the digestive tract. Muscle tissue is able to contract and relax, helping to provide movement within and for the body. Muscle contraction needs a good blood supply in order to provide sufficient oxygen, nutrients and calcium and to remove waste products. There are three different types of muscle found in the human body: striated, non striated and cardiac. Each type of muscle is composed of muscle fibres that are capable of contracting or shortening and relaxation. Contraction causes movement of the skeleton, soft tissue, blood or specific material such as urine or food. Muscle has both blood and nerve supplies.
Striated muscle also known as voluntary or skeletal muscle is found attached to bones of the skeleton even though some facial muscles are attached to the skin. It forms the muscles that move the bones of the skeleton. This type of muscle contracts when receiving nerve impulses that are controlled by conscious thought from the central nervous system. The name striated means striped, each single fibre shows alternate dark and light banding which occurs from the muscle filaments from which it is made. Each fibre is cylindrical and multinucleate, lying parallel to neighbouring fibres. Some fibres can be 30 centimetres long and one hundredth of a millimetre wide. The muscle fibres contain thousands of mitochondria which supplies ATP for energy that is used in muscular contraction. Non-Striated muscle may also been known as involuntary or smooth still contains protein filaments they do not lie in an ordered pattern meaning it does not produce the banding that is characteristic of striated muscle.
The muscle fibres of this type of muscle tissue are spindle or cigar shaped which have a single central nuclei and dovetail with each other. Although non-striated muscle still requires nervous stimulation to effect contraction, just like striated this is not under conscious thought but is supplied by the automatic nervous system. It is found around hollow internal organs such as the stomach, intestines, iris of the eye and bladder. It is not attached to bones. Cardiac muscle tissue is found only in the four chambers of the heart, atria and ventricles. It can rhythmically contract without receiving any nervous stimulation, it is not under conscious control. Therefore it is different from other muscles. Each fibre of cardiac tissue has a nucleus and one or more branches to form a network. The ends of the cells along with the branches are in very close contact with the ends and branches of neighbouring cells. The end to end flow of cardiac muscle cells has a significance in relation the way the heart contracts.
A wave of contraction spreads from one cell to another across intercalated discs. This means the cells don not have to be stimulated one by one. Under normal circumstances, where the cardiac tissue is healthy, it is not allowed to contract myogenically. The next tissue group to be found in the human body is Nervous tissue. Nervous tissue is only found in the nervous system. It consists of the brain, spinal cord and nerves. It creates consistency, co-ordination, and communication between different parts of the body while still receiving stimuli from both external and internal sources. There are two types of nervous tissue: excitable and non excitable. Excitable cells are called neurones which initiate, receive, conduct and transmit information from one part of the body to the other. Non-excitable cells are also known as glial cells give support to the neurons.
The brain is the centre of the nervous system, made up of mainly nervous tissue. It controls and regulates all the functions in the body, messages are sent to and from the brain. When a message comes into the brain, the brain then tells the body how to react. The brain consists of billions of neurons, which are each connected by synapses to other neurons. The neurons communicate with one another by fibres called axons, these carry signal pulses to distant parts of the brain or body targeting recipient cells. For example if a person touches a hot surface, the nerves in the skin shoot a message of pain to the brain as it monitors and regulates the bodys actions. The brain also consists of connective tissue, it is made up of 3 covering layers called meninges. Meninges are the 3 surrounding membranes that line and enclose the brain. The outer layer of the meninges, which is directly underneath the skull, is called the dura master. The dura is tough and thick, it can restrict the movement of the brain within the skull.
This protects brain from movement which may break brain blood vessels. The middle layer is called the arachnoid which is attached to the inside of the dura. The third layer which is closest to the brain is called the pia mater. The pia is a thin, mesh like meningeal envelope that is directly attached to the brain. It allows blood vessels to pass through and nourish the brain. The meninges are known as the blood brain barrier. The blood vessels are made up of smooth tissue. The heart is part of the cardiovascular system, it pumps blood around the body. It also carries vital materials around our body to help it function as well as removing wastes such as carbon dioxide. The heart is mainly made up of cardiac tissue, which is found in the four chambers of the heart: right and left atrium and right and left ventricle. These tissues perform the pumping of the heart, they are self contracting, automatically regulated and contract in a rhythmic fashion.
This pumps blood around the entire body helping to oxygenate muscles and delivers the vital materials. The heart also consists of smooth muscle tissue, which is found in the walls of large blood vessels. Smooth muscle tissue provides stability and flexibility so the large arteries can contract and expand. This helps to maintain pressure of blood volume. Smooth muscle also allows the flow of blood to the organs change for example during the exercise, the arteries to the muscles that are being used relax to allow more blood through so more oxygen is supplied. It also contains epithelial cells, which line the chambers and vessels. They stop parts of blood from moving to the muscle as well as helping to prevent blood clots.
Beryl Stretch (2010), Health and Social Care, Level three, BTEC National, Book one. Anne Waugh and Alison Grant (2010), Ross and Wilson Anatomy and Physiology in health and illness, 11th edition. Dr, Kenneth R. Chien (2012) Working tissue inside the heart. Available at: http://www.boston.com/yourlife/health/diseases/articles/2006/11/23/Working_tissues_inside_the_heart/ [Accessed 15th December 2012]