Chreod

Creode or chreod is a neologistic portmanteau term coined by the English 20th century biologist C.H. Waddington to represent the developmental pathway followed by a cell as it grows to form part of a specialized organ. Combining the Greek roots for "necessary" and "path," the term was inspired by the property of regulation. When development is disturbed by external forces, the embryo attempts to regulate its growth and differentiation by returning to its normal developmental trajectory.

Waddington used the term along with homeorhesis, which describes a system that returns to a steady trajectory, in contrast to homeostasis, which describes a system which returns to a steady state. Waddington explains development with the metaphor of a ball rolling down a hillside, where the hill's contours channel the ball in a particular direction. In the case of a pathway or creode which is deeply carved in the hillside, external disturbance is unlikely to prevent normal development. He notes that creodes tend to have steeper sides earlier in development, when external disturbance rarely suffices to alter the developmental trajectory. Small differences in placement atop the hill can lead to dramatically different results by the time the ball reaches the bottom. This represents the tendency of neighboring regions of the early embryo to develop into different organs with radically different structures. Since intermediate structures rarely exist between organs, each ball that rolls down the hill is "canalised" to a region distinct from other regions, just as an eye, for instance, is distinct from an ear.

Waddington refers to the network of creodes carved into the hillside as an "epigenetic landscape," meaning the formation of the body depends on not only its genetic makeup but the different ways genes are expressed in different regions of the embryo. He expands his metaphor by describing the underside of the epigenetic landscape. Here we see that the "landscape" is really more like a giant sheet that would blow away except that a series of tension-bearing cables holds it down. The pegs that connect the cables to the ground are the genes. The cables themselves are the epigenetic factors that influence gene expression in various regions of the embryo. The depth and direction of the channels is thus determined by a combination of genetic makeup and the epigenetic feedback loops by which genes are regulated.

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