<P> The process controls the organized spatial distribution of cells during the embryonic development of an organism . Morphogenesis can take place also in a mature organism, in cell culture or inside tumor cell masses . Morphogenesis also describes the development of unicellular life forms that do not have an embryonic stage in their life cycle, or describes the evolution of a body structure within a taxonomic group . </P> <P> Morphogenetic responses may be induced in organisms by hormones, by environmental chemicals ranging from substances produced by other organisms to toxic chemicals or radionuclides released as pollutants, and other plants, or by mechanical stresses induced by spatial patterning of the cells . </P> <P> Some of the earliest ideas and mathematical descriptions on how physical processes and constraints affect biological growth, and hence natural patterns such as the spirals of phyllotaxis, were written by D'Arcy Wentworth Thompson in his 1917 book On Growth and Form and Alan Turing in his The Chemical Basis of Morphogenesis (1952). Where Thompson explained animal body shapes as being created by varying rates of growth in different directions, for instance to create the spiral shell of a snail, Turing correctly predicted a mechanism of morphogenesis, the diffusion of two different chemical signals, one activating and one deactivating growth, to set up patterns of development, decades before the formation of such patterns was observed . The fuller understanding of the mechanisms involved in actual organisms required the discovery of the structure of DNA in 1953, and the development of molecular biology and biochemistry . </P> <P> Several types of molecules are important in morphogenesis . Morphogens are soluble molecules that can diffuse and carry signals that control cell differentiation via concentration gradients . Morphogens typically act through binding to specific protein receptors . An important class of molecules involved in morphogenesis are transcription factor proteins that determine the fate of cells by interacting with DNA . These can be coded for by master regulatory genes, and either activate or deactivate the transcription of other genes; in turn, these secondary gene products can regulate the expression of still other genes in a regulatory cascade of gene regulatory networks . At the end of this cascade are classes of molecules that control cellular behaviors such as cell migration, or, more generally, their properties, such as cell adhesion or cell contractility . For example, during gastrulation, clumps of stem cells switch off their cell - to - cell adhesion, become migratory, and take up new positions within an embryo where they again activate specific cell adhesion proteins and form new tissues and organs . Developmental signaling pathways implicated in morphogenesis include Wnt, Hedgehog, and ephrins . </P>

Cellular basis of growth and morphogenesis in plants