The tissues of the body consist of large numbers of cells and they are classified according to the size, shape and functions of these cells.
There are four main types of tissue They are-
1.Epithelial tissue or epithelium
This group of tissues is found covering the body and lining cavities and tubes. It is also found in glands. The structure of epithelium is closely related to its functions which include:
- Protection of underlying structures from, for example, dehydration, chemical and mechanical damage
The Epithelial tissue may be: -
- Simple: a single layer of cells
- Stratified: several layers of cells.
Simple epithelium consists of a single layer of identical cells and is divided into four types. It is usually found on absorptive or secretory surfaces, where the single layer enhances these processes, and not usually on surfaces subject to stress. The types are named according to the shape of the cells.
Its divided in four types-
- Squamous (pavement) Epithelium
- Cuboidal (cubical) Epithelium
- Cuboidal (cubical) Epithelium
- Ciliated epithelium
- Squamous Epithelium
This is composed of a single layer of flattened cells. The cells fit closely together like flat stones, forming a thin and very smooth membrane. Diffusion takes place freely through this thin, smooth, inactive lining of the following structures: -
- Blood vessels
- Lymph vessels
- Alveoli of the lungs.
It is also known as endothelium.
2.Cuboidal (cubical) epithelium-
- This consists of cube-shaped cells fitting closely together lying on a basement membrane.
- It forms the tubules of the kidneys and is found in some glands.
- Cuboidal epithelium is actively involved in secretion absorption and excretion.
This is formed by a single layer of cells, rectangular in shape, on a basement membrane. It is found lining the organs of the alimentary tract and consists of a mixture of cells; some absorb the products of digestion and others secrete mucus.
- This is formed by columnar cells each of which has many fine, hair-like processes, called cilia.
- The cilia consist of microtubules inside the plasma membrane that extends from the free border (luminal border) of the columnar cells.
- The wave-like movement of many cilia propels the contents of the tubes, which they line in one direction only.
- Ciliated epithelium is found lining the uterine tubes and most of the respiratory passages.
Stratified epithelia consist of several layers of cells of various shapes. The superficial layers grow up from below. Basement membranes are usually absent. The main function of stratified epithelium is to protect underlying structures from mechanical wear and tear.
There are two main types:
- Stratified squamous
1.Stratified squamous epithelium
This is composed of a number of layers of cells of different shapes representing newly formed and mature cells. In the deepest layers the cells are mainly columnar and, as they grow towards the surface, they become flattened and are then shed.
Non-keratinised stratified epithelium. –
This is found on wet surfaces that may be subjected to wear and tear but are protected from drying, e.g. the conjunctiva of the eyes, the lining of the mouth, the pharynx, the oesophagus and the vagina.
Keratinised stratified epithelium-
This is found on dry surfaces that are subjected to wear and tear, i.e. skin, hair and nails. The surface layer consists of dead epithelial cells to which the protein keratin has been added. This forms a tough, relatively waterproof protective layer that prevents drying of the underlying live cells.
This is composed of several layers of pear-shaped cells and is found lining the urinary bladder. It allows for stretching as the bladder fills.