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The mechanism by which cells in a multicellular organism become specialized to perform specific functions in a variety of tissues and organs. Specialized cells are the product of differentiation. The process can be understood only from a historical perspective, and the best place to start is the fertilized egg. Different kinds of cell behavior can be observed during embryogenesis: cells double, change in shape, and attach at and migrate to various sites within the embryo without any obvious signs of differentiation. Cleavage is a rapid series of cell cycles during which the large egg cell is divided into a ball of small cells that line the primitive body cavity as a single layer of embryonic cells. This blastula stage is followed by gastrulation, a complex coordinated cellular migration which not only shapes the embryo but segregates the single-cell layer of the blastula into the three germ layers: endoderm, mesoderm, ectoderm. They give rise to specific cell types; for example, skin and nerves from the ectoderm, the digestive tract from the endoderm, and muscle and connective tissue from the mesoderm. See Blastulation, Cell cycle, Cleavage (embryology), Embryogenesis, Gastrulation
The stable differentiated state is a consequence of multicellularity. A complex organism maintains its characteristic form and identity because populations of specialized cell types remain assembled in a certain pattern. Thus several kinds of cells make up a tissue, and different tissues build organs. The variable assortment of about 200 cell types allows for an almost infinite variety of distinct organisms.
Epithelia, sheets of cells of specific structure and function, cover the outer surface of the vertebrate body and line the lungs, gut, and vascular system. The stable form of a vertebrate is due to its rigid skeleton built from bone and cartilage, forming cells to which the skeletal muscles adhere. All other organs, such as liver and pancreas, are embedded in connective tissue that is derived from fibroblast cells which secrete large amounts of soft matrix material.
Some cells, like nerve cells, are so specialized that they need divide no longer in order to maintain a complex network. Their finite number decreases even during embryonic development. Other cell types are constantly worn out and must be replaced; for example, fibroblasts and pancreas cells simply divide as needed, proving that the differentiated state of cells is heritable, as the daughter cells remember and carry out the same special functions. The renewal of terminally differentiated cells that are unable to divide anymore, such as skin and blood cells, is carried out by stem cells. They are immortal and choose, as they double, whether to remain a stem cell or to embark on a path of terminal differentiation. Most stem cells are unipotent because they give rise to a single differentiated cell type. However, all cell types of the blood are derived from a single blood-forming stem cell, a pluripotent stem cell. A fertilized egg is a totipotent stem cell giving rise to all other cell types that make up an individual organism. See Embryonic differentiation, Embryonic induction, Oogenesis