Definition

A multicellular organism is a living thing made of many cells that have different jobs and depend on each other to keep the whole organism alive. Its cells are specialized and organized into levels: cells form tissues, tissues form organs, organs form organ systems, and organ systems form the organism, which allows large size and complex tasks but requires systems for transport, communication, and coordination. Plants, animals, and most fungi are multicellular. At the cellular level, multicellularity depends on stable cell adhesion, intercellular communication, and differential gene expression that produces distinct cell types from a common genome, and development from a single zygote relies on regulated division, signaling gradients, and gene regulatory networks that assign cell fates. Multicellularity has evolved independently many times, in animals, land plants, fungi, and several algal lineages, and because cells sharing a genome must cooperate, mechanisms such as apoptosis and immune surveillance police cells that divide selfishly; cancer is a breakdown of that cooperation.