Tissues

There are more than 200 different types of cells in the human body. These types are highly specialised (differentiated) to carry out specific functions. Groups of cells carrying out a similar function and forming a structure are called tissue. Different tissues combine to form organs. There are four main types of tissue.

• Epithelial tissue is found at the boundaries of structures in the body. The basal (basolateral) side faces the underlying tissue, which provides nutrients and support, while the apical side is exposed to a different environment. For instance, the apical side of intestinal epithelium faces the inside of the intestine and is exposed to constituents from (digested) food; the apical side of stomach epithelium is exposed to the acidic environment inside the stomach; lung and skin are exposed to air. Secreting glands are also formed by epithelial tissue. There are distinct types of epithelial structures:
  
  
  
  • stratified epithelium: formed by layers of epithelial cells;
    
  • simple epithelium: formed by a single layer of cells;
    
  • squamous epithelium: formed by cells which are wider than tall;
    
  • columnar epithelium: formed by cells which are taller than wide.
  
  
• Stratified squamous epithelium can be keratinised, forming a hard and dry layer as found in the skin, nails and hair, or non-keratinised, which is found in soft tissue such as the inside of the mouth. An example of simple columnar epithelium is the lining of the stomach.
  
• Connective tissue provides structure and rigidity. It is characterised by a large space between cells, which is filled with fibrous material that is part of the extracellular matrix. Fibroblasts are the most common cell type in connective tissue. Adipose tissue stores energy in the form of fat, but is also important for the protection and insulation of organs and is now recognised as having an endocrine role. Other types of connective tissue include blood, cartilage and bone.
  
• Muscle cells (myocytes) are characterised by their ability to contract when they receive appropriate signals. There are three different types of muscle tissue: skeletal muscle is directly attached to bones; cardiac muscle is the muscle of the heart; smooth muscle lines blood vessels and organs of the body.
  
• Nerve cells can sense stimuli and transfer electrical signals to chemical signals, which are sensed by surrounding cells. Nerve cells can have long extensions that are involved in the sensing and transmittance of signals (Chapters 18 and 19).