Creode or chreod is a
neologistic
A neologism Greek νέο- ''néo''(="new") and λόγος /''lógos'' meaning "speech, utterance"] is a relatively recent or isolated term, word, or phrase that may be in the process of entering common use, but that has not been fully accepted int ...
portmanteau
A portmanteau word, or portmanteau (, ) is a blend of words[C. H. Waddington
Conrad Hal Waddington (8 November 1905 – 26 September 1975) was a British developmental biologist, paleontologist, geneticist, embryologist and philosopher who laid the foundations for systems biology, epigenetics, and evolutionary devel ...](_blank)
to represent the developmental pathway followed by a
cell
Cell most often refers to:
* Cell (biology), the functional basic unit of life
Cell may also refer to:
Locations
* Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...
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
Regulation is the management of complex systems according to a set of rules and trends. In systems theory, these types of rules exist in various fields of biology and society, but the term has slightly different meanings according to context. Fo ...
. When
development
Development or developing may refer to:
Arts
*Development hell, when a project is stuck in development
*Filmmaking, development phase, including finance and budgeting
*Development (music), the process thematic material is reshaped
* Photograph ...
is disturbed by external forces, the embryo attempts to regulate its growth and differentiation by returning to its normal developmental trajectory.
Developmental biology
Waddington used the term along with
canalisation and
homeorhesis
Homeorhesis, derived from the Greek for "similar flow", is a concept encompassing dynamical systems which return to a trajectory, as opposed to systems which return to a particular state, which is termed homeostasis.
Biology
Homeorhesis is steady ...
, which describes a system that returns to a steady trajectory, in contrast to
homeostasis
In biology, homeostasis (British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism and ...
, 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
River engineering is a discipline of civil engineering which studies human intervention in the course, characteristics, or flow of a river with the intention of producing some defined benefit. People have intervened in the natural course and be ...
" 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 that 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
Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...
by which genes are regulated.
While Waddington does assert that the process of development is genetically driven, he makes no attempt to explain how this works and even offers evidence to the contrary. He observes, for instance, that genes ordinarily determine peripheral traits, such as eye color, rather than "focal" traits, such as the structure of the eye itself. Moreover, when genetic mutation influences basic structures, the result tends to be the complete transformation of a structure into another rather than piecemeal change, which Waddington illustrates with the developmental ball rolling out of one creode into another. Thus his account gives the impression that genes influence development, perhaps altering the course of a region of cells, without determining the endpoints toward which the embryo develops.
This interpretation is further reinforced by Waddington's discussion of the organization of the gene pool, where he points out that "the epigenetic process occurring during the development of the organism might be so buffered or canalized that the optimum end-result is produced irrespective of the genes which the individual contains." The more deeply creodes are carved into the epigenetic landscape, the weaker the influence of genes over development. He also argues that deep creodes will resist not only genetic but environmental pressures to change course. This phenomenon, which he calls "stabilizing selection," puts genes and environment on a par in secondary importance compared to the epigenetic system.
Waddington's emphasis on epigenetics over genes prefigured the current interest in
evolutionary developmental biology. As
Sean B. Carroll
Sean B. Carroll (born September 17, 1960) is an American Evolutionary developmental biology, evolutionary developmental biologist, author, educator and executive producer. He is a distinguished university professor at the University of Marylan ...
and others have explained, genes involved in development are roughly the same in all animal species, from insect to primate. Instead of mutations in developmental genes, evolution has been driven by changes in gene expression, namely which genes are expressed at which times and locations in the developing organism.
Architecture
Architectural theorist
Sanford Kwinter
Sanford Kwinter is a Canadian-born, New York-based writer and architectural theorist, and a co-founder of Zone Books publishers. Kwinter currently serves as Professor of Theory and Criticism at the Pratt Institute. He formerly served as an associ ...
described the concept of the chreod as "the most important concept of the 20th century." The word "chreod" also closely describes paths of decision within what
Christopher Alexander has called
configuration space, his term for what he notes that Stuart Kaufmann calls "fitness landscape." By Alexander's theory, because conscious human design decisions do not need to follow these chreods, conscious human design can lead to mixed results. Therefore, he proposes that discovering ways to allow architecture to follow these paths is the best way to get good results in the built environment. Alexander sees his theories of "The Fundamental Process", "structure preserving transformations" and "15 fundamental properties" which he outlines in his work ''
The Nature of Order
''The Nature of Order: An Essay on the Art of Building and the Nature of the Universe'' () is a four-volume work by the architect Christopher Alexander published in 2002–2004. In his earlier work, Alexander attempted to formulate the principles ...
'' as instrumentally shaping paths through configuration space.
[ Christopher Alexander]
''New Concepts in Complexity Theory Arising from Studies in the Field of Architecture''
p 17.
See also
*
Cell biology
*
Systems biology
References
{{reflist, 30em
Sources
* C.H. Waddington, ''The Strategy of the Genes'', George Allen & Unwin, 1957
Cell biology
1950s neologisms
Developmental biology