A tissue is a group of cells that perform a specific function, the basic
types of tissues in the human body include: epithelial, muscle, nervous
and connective.
Epithelial Tissue
This type of tissue covers the outer body surfaces, forms glands, lines
internal body cavities and surfaces of the internal organs.
The cells have a simple shape and fit closely together and may be a single layer as in simple epithelium or several layers as with compound or stratified epithelium. Their functions include movement of materials in/out or around the body, protection and secretion.
There are six classifications of epithelial tissue which are classified
according to their shape:
Squamous epithelium – flat, scale like cells that form the walls of the blood capillaries, epidermis, lining of the mouth cavity and air sacs of the lungs.
- Cuboidal epithelium – cube shaped cells that are found in the lining of the sweat glands, kidney tubules and thyroid gland.
- Columnar epithelium – elongated cells that line the stomach,
intestines and urinary ducts. - Ciliated columnar epithelium – elongated cells that cover wet
surfaces and are interspersed with goblet cells that secrete mucus.
The hair like cilia help to move particles along, and it is found in the air passages where it helps to filter the air breathed in. - Compound transitional epithelium – several layers of cells when relaxed but only one/two layers when stretched. Therefore found in areas where the tissue needs to stretch such as the bladder wall.
- Glandular epithelium – simple or compound cells that secrete
necessary chemicals and are found in the glands such as sweat
glands, sebaceous glands, mammary glands and salivary glands.
Connective Tissue
This type of tissue connects, fills in space, supports and binds all other
types of tissue.
Inside the connective tissues are living cells, as well as the matrix, collagen and elastin fibres (which are all non-living structures). The collagen fibres are produced by fibroblasts and are required to provide a supporting fibrous framework. The elastin fibres allow areas of the body to stretch as they have elastic qualities. The connective tissue usually has a rich blood supply and cells that are not tightly packed together.
There are five types of connective tissue:
- Loose connective tissue or areolar tissue – smooth and moist tissue that has a loose network of collagen and elastin fibres, few blood/nerve cells, however some fat cells. It forms a tough transparent lining in between the organs and attaches the skin to
the underlying muscles. - Adipose tissue – loose connective tissue that contains a network of fat cells that provide energy reserves and insulation for the body. It also protects the delicate organs. It can be found in the subcutaneous layer of the skin, around the heart and kidneys.
- Dense connective tissue – contains bundles of strong collagen fibres and fibroblasts, it is very tough and looks silvery to white in colour. It can be found in the tendons, ligaments and fascia surrounding the muscles.
- Cartilage – this is a tough, elastic tissue that contains cells called
chondrocytes and is separated by fibres. This tissue does not have a
blood supply and is located in-between the vertebrae where it forms
the intervertebral discs. - Bone tissue – basically a specialised type of cartilage which has
undergone ossification. It has collagen fibres to provide strength;
mineral salts provide rigidity and a rich blood supply to provide
nutrients. It is the tissue that makes up the human skeleton.
Muscle Tissue
There are three types of muscle tissue: voluntary, cardiac and smooth.
Voluntary muscle is attached to the skeleton and is controlled
voluntarily by the brain and nervous system. When viewed under a
microscope it has visible stripes and is therefore called striated. Each
muscle tissue consists of bundles of parallel muscle fibres. Each fibre
has an outer membrane that binds together smaller fibrils, many of
which have nuclei. It is responsible for allowing movement and
maintaining posture. It has the power to perform a great deal of work
but tires easily.
Cardiac muscle is found only in the walls of the heart. It is made from
short, striated muscle fibres and is structured so it only has a single
nuclei per fibre. This muscle contracts automatically and rhythmically
without nervous stimulation and is developed to allow communication
between the cells enabling sequential contraction of the cells from the
bottom of the ventricle to the top, facilitating maximal ejection of blood
during contraction. It is regulated by the autonomic nervous system and
adrenalin. It doesn’t tire readily – it works continuously throughout life to
make the heart beat.
Smooth muscle is found in the walls of the food canal, blood vessels and
urinary system. Each cell has one nucleus, are spindle shaped and
interlock with other cells. It is made up of contractile fibres which are
not striated and is regulated by the autonomic nervous system and
adrenalin. This tissue is capable of slow contraction and does not tire
easily.
Nervous Tissue
This type of tissue consists if nerve cells called neurones which transmit
messages (nerve impulses). These messages pass from inside and
outside the body to the tissues within the body.
Each neuron has:
- A cell body – containing the nucleus, mitochondria and other
organelles. - Dendrites – several short projections that receive information from other cells and transmit the message on to the cell body.
- Axon – one long projection that conducts messages away from the
cell body. Axons have a myelin sheath which insulates them to
prevent against loss of electrical impulses and therefore increase the
speed at which the impulse is conducted.
