Multicellular organisms are those organisms containing more than one cell, and having differentiated cells that perform specialized functions. Most life that can be seen with the naked eye is multicellular; all animals (i.e. members of the kingdom Animalia) and plants (i.e. members of the kingdom Plantae) are.
Although some single-celled organisms have differentiated cells (e.g. myxobacteria), the differentiation is less dramatic than that typically found in multicellular organisms. A group of similarly differentiated cells performing a function in a multicellular organism is known as a tissue.
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.
Traditionally in medicine, a vector is an organism that does not cause disease itself but which spreads infection by conveying pathogens from one host to another.
A tumor suppressor gene is a gene that reduces the probability that a cell in a multicellularorganism will turn into a tumor cell.
A genetically modified organism (GMO) is an organism whose genetic material has been altered using techniques in genetics generally known as recombinant DNA technology.
Multicellularity enables a plant, for example, to become physically large; to have roots in the ground, where one set of cells can take up water and nutrients; and to have leaves in the air, where another set of cells can efficiently capture radiant energy from the sun.
To show how it is possible to generate multicellularorganisms of such size, precision, and complexity as a tree, a fly, or a mammal, however, it is necessary to consider more closely the sequence of events in development.
The evolution of large multicellularorganisms depended on the ability of eucaryotic cells to express their hereditary information in many different ways and to function cooperatively in a single collective.
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