The cell is the simplest component of a living organism. Cells are organized with a three-dimensional perspective in order to create a tissue. On their turn, tissues constitute organs.

Tissues are divided into four categories:

• The epithelial tissue: in this tissue, cells are very close among them, and adhere to one another. Consequently, they present a very reduced intercellular space.
• The connective tissue: as opposed to the epithelial tissue, here cells present a large intercellular space and this type of organization is a typical feature of the connective tissue itself. It presents many different variants and is, therefore, the most varied tissue.
• The muscle tissue: its cells have a specific feature: they can contract.
• The nervous tissue: nerve cells can receive, process and transmit impulses.

From a proliferative perspective, cells composing the various tissues are classified in:

• Labile cells (constitutive regenerative): these cells can be replaced very quickly, and their proliferation allows a constant tissue regeneration. For example, epidermis cells and intestinal mucosa cells are labile.
• Stable cells (conditional regenerative): usually, these cells do not proliferate, but they can in case the tissue they belong to is damaged. An example of stable cells are the hepatocytes.
• Permanent cells (non-regenerative): these are highly differentiated cells which have lost their ability to proliferate, for example the auditory hair cells present in the inner ear.