The cells of multicellular organisms may differentiate and become adapted for specific functions. Tissues as aggregations of similar cells, and organs as aggregations of tissues performing specific physiological functions. Organs are organised into systems.

Each cell in the body starts off the same but then they specialise by expressing certain genes.

This is beneficial as different cells are better off at carrying out certain jobs.

Aggregation is grouping together.

A groups of similar cells function together as a tissue (are aggregated).

Tissues are then aggregated into organs. These have several different types of tissue which function together as a system.