Heterotopia (medicine)
Encyclopedia
In medicine
, "heterotopia" refers to normal tissue present at an abnormal site or the displacement of an organ or part of an organ from its normal position. For example, gray matter heterotopia
, the displacement of gray matter
into the cerebral white matter
or ventricle
s. Heterotopia is often divided into three groups: subependymal heterotopia, focal cortical heterotopia and band heterotopia.
In biology
specifically, heterotopy refers to an altered location of trait expression. In her landmark book,Developmental Plasticity and Evolution Mary-Jane West Eberhard has a provocative cover art of the Sulphur Crested Cockatoo and comments on the back cover "Did it's long crest[head] feathers evolve by gradual modification of ancestral head feathers? Or are they descendants of wing feathers, developmentally transplanted onto the head" This idea sets the tone for the rest of her book which goes into depth about developmental novelties and their relation to evolution. Heterotopy is a somewhat obscure but well demonstrated example of how developmental change can lead to novel forms. The central concept is that a feature seen in one area of an organism has have its location changed in evolutionary lineages.
examples of heterotopy include the transfer of bright flower pigments from ancestral petals to leaves that curl and form to mimic petals. In other cases experiments have yielded plants with mature leaves present on the highest shoots. Normal leaf development progresses from the base of the plant to the top: as the plant grows upwards it produces new leaves and lower leaves mature.
, is the experimental induction of legs in place of antennae in fruit flies,Drosophila. The name for this specific induction is 'antennapedia'. Surprisingly and elegantly, the transfer takes place in the experiment with no other strange pleiotropic
consequences. The leg is transplanted and still is able to rotate on the turret-like complex on the fruit fly's head. The leg simply replaced the Antennae. Before this experiment it was thought that anatomical structures were somehow constrained into certain not well understood and undefined domains. Yet the relatively simple modification took place and caused a dramatic change in phenotype
.
This further demonstrated that structures that were thought to be homologous
at one time and were later modified still retained some modularity
, or were interchangeable even millions of years after evolution had sent antennae down a separate path than the other appendages. This is due to the common origin of homeotic genes. Another well-known example is the environmentally induced heterotopic change seen in the melanin
of the Himalayan rabbit and the Siamese cat and related breeds. In the Himalayan rabbit pigments in fur and skin are only expressed in the most distal portions, the very ends of limbs. This is similar to the case Siamese cats. In both the placement of fur pigmentation is induced by temperature. The regions furthest from core body heat and with the lowest circulation develop darker as an induced result. Individuals raised at a uniform external temperature above 30 °C do not express melanin in the extremities and as a result the fur on their paws is left white. The specific gene complex determined to be responsible is in the melanin expression series that is also responsible for albinism
. This change is not heritable because it is a flexible or Plastic phenotypic change. The heterotopy demonstrated is that colder body regions are marked by expression of melanin.
The Himalayan rabbit and the Siamese cat are examples of artificial selection
on heterotopy, developed by breeders incidentally long before the concept was understood. The current theory is that people selected for stereotypical phenotypic patterns (dark extremities) that happened to be repeatedly produced given a typical temperature. This is perhaps the only known example of convergent mechanisms in artificial selection. The common human breeding cultures that breed the rabbits and cats tended to themselves favor the pattern, in a way closely mimicking the way that the underlying genetics that form flexible adaptations can be selected for based on the phenotype they typically produce in an assumed environment in natural selection
.
where a gene product used for one underlying purpose in a diverse group of organisms can re-emerge repeatedly to produce seemingly paraphyletic distributions of traits. But actual phylogenetic analysis supports a monophyletic model as does evolutionary theory. Heterotopy is used to explain this and there are so commonly cited examples.
An example is chitin
a very durable structural protein used in surgical sutures as well as durable varnishes but is common to many animals especially crustaceans and insects. But is also found in the African crawled frogXenopus laevis
Wagner et al., suggest that chitin might have a microscopic function observed in cell to cell signaling and the manufacture of insect cuticle for example might represent a recurrent change in the location of expression chitin
Speculative, but however Chitin synthase is maintained in many lineages where it does not have an obvious macroscopic function.
It is thought that because so many organisms share such a profound degree of genetic and molecular similarity that shifts in the location of expression might be a regular occurrence throughout time.
Molecular analysis shows that proteins that seem to have a single specific function are instead found in many different tissue types. One example of this phenomenon is crystallin, a clear protein that makes up the lens of the eye; it is also has structural functions in the heart.
Medicine
Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....
, "heterotopia" refers to normal tissue present at an abnormal site or the displacement of an organ or part of an organ from its normal position. For example, gray matter heterotopia
Gray Matter Heterotopia
Gray matter heterotopia is a neurological disorder caused by clumps of grey matter being located in the wrong part of the brain. It is characterized as a type of cortical dysplasia. The neurons in heterotopia appear to be normal, except for their mislocation; nuclear studies have shown glucose...
, the displacement of gray matter
Gray Matter
"Gray Matter" is a short story by Stephen King, first published in the October 1973 issue of Cavalier magazine, and later collected in King's 1978 collection Night Shift. It is set in the same area as King's novel Dreamcatcher.-Setting:...
into the cerebral white matter
White matter
White matter is one of the two components of the central nervous system and consists mostly of myelinated axons. White matter tissue of the freshly cut brain appears pinkish white to the naked eye because myelin is composed largely of lipid tissue veined with capillaries. Its white color is due to...
or ventricle
Ventricular system
The ventricular system is a set of structures containing cerebrospinal fluid in the brain. It is continuous with the central canal of the spinal cord.-Components:The system comprises four ventricles:* right and left lateral ventricles* third ventricle...
s. Heterotopia is often divided into three groups: subependymal heterotopia, focal cortical heterotopia and band heterotopia.
In biology
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...
specifically, heterotopy refers to an altered location of trait expression. In her landmark book,Developmental Plasticity and Evolution Mary-Jane West Eberhard has a provocative cover art of the Sulphur Crested Cockatoo and comments on the back cover "Did it's long crest[head] feathers evolve by gradual modification of ancestral head feathers? Or are they descendants of wing feathers, developmentally transplanted onto the head" This idea sets the tone for the rest of her book which goes into depth about developmental novelties and their relation to evolution. Heterotopy is a somewhat obscure but well demonstrated example of how developmental change can lead to novel forms. The central concept is that a feature seen in one area of an organism has have its location changed in evolutionary lineages.
Heterotopy in Botany
In botanyBotany
Botany, plant science, or plant biology is a branch of biology that involves the scientific study of plant life. Traditionally, botany also included the study of fungi, algae and viruses...
examples of heterotopy include the transfer of bright flower pigments from ancestral petals to leaves that curl and form to mimic petals. In other cases experiments have yielded plants with mature leaves present on the highest shoots. Normal leaf development progresses from the base of the plant to the top: as the plant grows upwards it produces new leaves and lower leaves mature.
Heterotopy in Zoology
One textbook example of heterotopy in animals, a classic in genetics and developmental biologyDevelopmental biology
Developmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, differentiation and "morphogenesis", which is the process that gives rise to tissues, organs and anatomy.- Related fields of study...
, is the experimental induction of legs in place of antennae in fruit flies,Drosophila. The name for this specific induction is 'antennapedia'. Surprisingly and elegantly, the transfer takes place in the experiment with no other strange pleiotropic
Pleiotropy
Pleiotropy occurs when one gene influences multiple phenotypic traits. Consequently, a mutation in a pleiotropic gene may have an effect on some or all traits simultaneously...
consequences. The leg is transplanted and still is able to rotate on the turret-like complex on the fruit fly's head. The leg simply replaced the Antennae. Before this experiment it was thought that anatomical structures were somehow constrained into certain not well understood and undefined domains. Yet the relatively simple modification took place and caused a dramatic change in phenotype
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
.
This further demonstrated that structures that were thought to be homologous
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
at one time and were later modified still retained some modularity
Modularity
Modularity is a general systems concept, typically defined as a continuum describing the degree to which a system’s components may be separated and recombined. It refers to both the tightness of coupling between components, and the degree to which the “rules” of the system architecture enable the...
, or were interchangeable even millions of years after evolution had sent antennae down a separate path than the other appendages. This is due to the common origin of homeotic genes. Another well-known example is the environmentally induced heterotopic change seen in the melanin
Melanin
Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...
of the Himalayan rabbit and the Siamese cat and related breeds. In the Himalayan rabbit pigments in fur and skin are only expressed in the most distal portions, the very ends of limbs. This is similar to the case Siamese cats. In both the placement of fur pigmentation is induced by temperature. The regions furthest from core body heat and with the lowest circulation develop darker as an induced result. Individuals raised at a uniform external temperature above 30 °C do not express melanin in the extremities and as a result the fur on their paws is left white. The specific gene complex determined to be responsible is in the melanin expression series that is also responsible for albinism
Albinism
Albinism is a congenital disorder characterized by the complete or partial absence of pigment in the skin, hair and eyes due to absence or defect of an enzyme involved in the production of melanin...
. This change is not heritable because it is a flexible or Plastic phenotypic change. The heterotopy demonstrated is that colder body regions are marked by expression of melanin.
The Himalayan rabbit and the Siamese cat are examples of artificial selection
Artificial selection
Artificial selection describes intentional breeding for certain traits, or combination of traits. The term was utilized by Charles Darwin in contrast to natural selection, in which the differential reproduction of organisms with certain traits is attributed to improved survival or reproductive...
on heterotopy, developed by breeders incidentally long before the concept was understood. The current theory is that people selected for stereotypical phenotypic patterns (dark extremities) that happened to be repeatedly produced given a typical temperature. This is perhaps the only known example of convergent mechanisms in artificial selection. The common human breeding cultures that breed the rabbits and cats tended to themselves favor the pattern, in a way closely mimicking the way that the underlying genetics that form flexible adaptations can be selected for based on the phenotype they typically produce in an assumed environment in natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
.
Heterotopy in Molecular Biology
Heterotopy in molecular biology is the name given to the expression or placement of a gene product from what is typically found in one area to another area. It can also be further expanded to a subtle form of exaptationExaptation
Exaptation, cooption, and preadaptation are related terms referring to shifts in the function of a trait during evolution. For example, a trait can evolve because it served one particular function, but subsequently it may come to serve another. Exaptations are common in both anatomy and behaviour...
where a gene product used for one underlying purpose in a diverse group of organisms can re-emerge repeatedly to produce seemingly paraphyletic distributions of traits. But actual phylogenetic analysis supports a monophyletic model as does evolutionary theory. Heterotopy is used to explain this and there are so commonly cited examples.
An example is chitin
Chitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
a very durable structural protein used in surgical sutures as well as durable varnishes but is common to many animals especially crustaceans and insects. But is also found in the African crawled frogXenopus laevis
Wagner et al., suggest that chitin might have a microscopic function observed in cell to cell signaling and the manufacture of insect cuticle for example might represent a recurrent change in the location of expression chitin
Chitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
Speculative, but however Chitin synthase is maintained in many lineages where it does not have an obvious macroscopic function.
It is thought that because so many organisms share such a profound degree of genetic and molecular similarity that shifts in the location of expression might be a regular occurrence throughout time.
Molecular analysis shows that proteins that seem to have a single specific function are instead found in many different tissue types. One example of this phenomenon is crystallin, a clear protein that makes up the lens of the eye; it is also has structural functions in the heart.