Multicellular organism

Multicellular organisms are those organisms consisting of more than one cell, and having differentiated cells that perform specialized functions. Most life that can be seen with the naked eye is multicellular, as are all animals (i.e. members of the kingdom Animalia) and plants (i.e. members of the kingdom Plantae).

A group of similarly differentiated cells performing a function in a multicellular organism is known as a tissue. Although some single-celled organisms, such as myxobacteria, have differentiated cells, the differentiation is less dramatic than that typically found in multicellular organisms.

A multicellular organism can work on a number of levels. For instance, a sponge works on a cellular level as the different cells are almost like many seperate organisms co-operating together for a common goal. This is illustrated by the fact you can put a sponge in a blender and the cells of the sponge will aggregate again to form another sponge. They have no tissues, organs or organ systems which require a higher level of organisation. More advanced organisms such as jellyfish, coral and sea anemones work on a tissue level of organisation, as their cells can aggregate to form different tissues. For instance, a jellyfish has an outer epidermis and an inner gastrodermis, an example of different tissues to serve a different purpose.

More advanced still are organisms with a organ level of development, whereby groups of tissues form organs such as a primitive brain in platyhelminthes (merely a grouping of ganglion cells). Finally, the most advanced organisms (like mammals) have organ system's whereby groups of organs act together for a common purpose with each organ focusing on a particuler task. An example would be the digestive system. for instance the small intestine focuses on absorbtion of food into the blood stream, the stomach on crushing food and killing bacteria with its acidic enviroment, the pancreas for creating enzymes that catalyse the digestion of food, etc.

Multicellular organisms must solve the problem of regenerating the whole from germ cells (i.e. sperm and egg cells), an issue that is studied in developmental biology. The overall spatial organization of differentiated cells is a topic of study in anatomy.

Multicellular organisms can suffer from cancer, when cells fail to regulate their growth within the normal program of development.

See also