Chreode
Encyclopedia
Creode is a neologism coined by the 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 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 channeled or "canalized" 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.
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.
This leaves open the question of the ultimate source of creodes and the epigenetic system that defines them. In the 1980s Waddington's concept reappeared in the work of Rupert Sheldrake
, who suggested that networks of creodes are shaped by the embryo's inherent memory of the developmental pathway of previous, similar embryos, generally those belonging to the same species. Sheldrake's top-down or "holistic"
approach, which he expressed in the terminology of "morphogenetic fields,"
contrasts with the bottom-up or "mechanistic" approach dominant in biology.
Waddington's emphasis on epigenetics over genes prefigured the current interest in Evo Devo. As Sean B. Carroll
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.
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
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
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
. When development
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
is disturbed by external forces, the embryo attempts to regulate its growth and differentiation by returning to its normal developmental trajectory.
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 channeled or "canalized" 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
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
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.
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.
This leaves open the question of the ultimate source of creodes and the epigenetic system that defines them. In the 1980s Waddington's concept reappeared in the work of Rupert Sheldrake
Rupert Sheldrake
Rupert Sheldrake is an English scientist. He is known for having proposed an unorthodox account of morphogenesis and for his research into parapsychology. His books and papers stem from his theory of morphic resonance, and cover topics such as animal and plant development and behaviour, memory,...
, who suggested that networks of creodes are shaped by the embryo's inherent memory of the developmental pathway of previous, similar embryos, generally those belonging to the same species. Sheldrake's top-down or "holistic"
Holism
Holism is the idea that all the properties of a given system cannot be determined or explained by its component parts alone...
approach, which he expressed in the terminology of "morphogenetic fields,"
Morphogenetic field
In developmental biology, a morphogenetic field is a group of cells able to respond to discrete, localized biochemical signals leading to the development of specific morphological structures or organs. The spatial and temporal extent of the embryonic fields are dynamic, and within the field is a...
contrasts with the bottom-up or "mechanistic" approach dominant in biology.
Waddington's emphasis on epigenetics over genes prefigured the current interest in Evo Devo. As Sean B. Carroll
Sean B. Carroll
Sean B. Carroll is a Professor of Molecular Biology, Genetics, and Medical Genetics at the University of Wisconsin–Madison. He studies the evolution of cis-regulation in the context of biological development, using Drosophila as a model system...
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.