A cell type is a classification used to distinguish between morphologically or phenotypically distinct cell forms within a species. A multicellular organism may contain a number of widely differing and specialised cell types, such as muscle cells and skin cells in humans, that differ both in appearance and function yet are genetically identical. Cells are able to be of the same genotype, but different cell type due to the differential regulation of the genes they contain. Classification of a specific cell types is often done through the use of microscopy and the analysis of molecules on the cell surface (such as those from the cluster of differentiation family that are commonly used for this purpose in immunology).
Many species of single-celled organisms exhibit different morphological forms. Individual cells may even exhibit different morphologies during the course of their life cycle. In times of environmental stress, some bacteria form endospores which are distinct from their normal free-living state. Some amoeba may differentiate into different cell types upon conglomeration with other amoeba of the same species, as in Dictyostelium discoideum.
All higher multicellular organisms contain cells specialised for different functions. Most distinct cell types arise from a single totipotent cell that differentiates into hundreds of different cell types during the course of development. Differentiation of cells is driven by different environmental cues (such as cell–cell interaction) and intrinsic differences (such as those caused by the uneven distribution of molecules during division). Multicellular organisms are composed of cells that fall into two fundamental types: germ cells and somatic cells. During development, somatic cells will become more specialized and form the three primary germ layers: ectoderm, mesoderm, and endoderm. After formation of the three germ layers, cells will continue to specialize until they reach a terminally differentiated state that is much more resistant to changes in cell type than its progenitors.
A list of cell types in the human body may include several hundred distinct types depending on the source.
- ^ Molecular Biology of the Cell, Fourth Edition, Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, Peter Walter
- ^ ines.de/cope.cgi COPE database