Overview

A multicellular organism is any living being composed of more than one cell that cooperate to form a single functional individual. Multicellularity is the dominant organizational mode for animals, plants, most fungi and many algae. In these organisms cells commonly become specialized for particular roles, enabling larger size, more complex body plans and new ecological opportunities than are typical of single-celled life.

Organization and parts

Individual cells (cell) remain the basic unit, but they are arranged into higher levels of organization. Groups of similar cells form a tissue, tissues combine into organs, and organs assemble into organ systems in more complex animals and plants. Special features of multicellular organization include:

  • division of labor among cell types (e.g., photosynthetic, structural, nervous, contractile);
  • extracellular matrices and junctions that hold cells together and allow communication;
  • developmental programs that guide cells into particular shapes and arrangements.

Some multicellular organisms also show a germ–soma separation, where reproductive cells are distinct from non‑reproductive somatic cells.

Origin and evolution

Multicellularity evolved from unicellular ancestors in multiple, independent lineages. Simple forms of cellular cooperation and adhesion can give rise to colonial life that, over evolutionary time, may lead to integrated multicellular bodies. Advantages of this transition include improved resource exploitation, predator avoidance, and ability to develop specialized tissues, but it also introduced new costs such as the need for internal transport systems and developmental regulation.

Reproduction and life cycles

Multicellular species use a range of reproductive strategies. Many reproduce sexually, producing specialized gametes such as sperm and eggs; others reproduce asexually by budding, fragmentation, or spore formation. Some species are gonochoric (separate sexes) while others are hermaphroditic — carrying both male and female reproductive cells (hermaphrodite). Life cycles can alternate between unicellular and multicellular stages in certain algae and fungi.

Examples, distinctions, and notable cases

Typical multicellular groups include animals, land plants and most fungi, but the boundary between colonial and truly multicellular is a continuum. Notable model systems that illuminate multicellularity include the volvocine algae (a range from single cells to spherical colonies) and social amoebae that aggregate into multicellular fruiting bodies. Understanding multicellularity connects cell biology, development and evolutionary theory and remains an active area of research.

For further reading on definitions, cellular structure, tissues and reproductive terms see related entries: definition of multicellular organism, cell biology, tissue, and hermaphrodite.