Heterotopia (medicine)

In medicine, "heterotopia" refers to presence of a particular tissue type at a non-physiological site, but usually co-existing with original tissue in its correct anatomical location. In other words, it implies ectopic tissue, in addition to retention of the original tissue type. In neuropathology, for example, gray matter heterotopia, is the presence of gray matter within the cerebral white matter or ventricles. Heterotopia within the brain is often divided into three groups: subependymal heterotopia, focal cortical heterotopia and band heterotopia. Another example is a Meckel's diverticulum which may contain heterotopic gastric or pancreatic tissue.

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 had its location changed in evolutionary lineages.

In botany 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.

One textbook example of heterotopy in animals, a classic in genetics and developmental biology, 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.

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