Imaginal disc
An imaginal disc is one of the parts of a holometabolous
Holometabolism, also called complete metamorphism, is a term applied to insect groups to describe the specific kind of insect development which includes four life stages - as an embryo or egg, a larva, a pupa and an imago or adult. Holometabolism is a monophyletic trait that all insects in the...

Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...

 that will become a portion of the outside of the adult insect
In biology, the imago is the last stage of development of an insect, after the last ecdysis of an incomplete metamorphosis, or after emergence from the pupa where the metamorphosis is complete...

 during the pupal transformation. Contained within the body of the larva, there are pairs of discs that will form, for instance, the wing
Insect wing
Insects are the only group of invertebrates known to have evolved flight. Insects possess some remarkable flight characteristics and abilities, still far superior to attempts by humans to replicate their capabilities. Even our understanding of the aerodynamics of flexible, flapping wings and how...

s or leg
Arthropod leg
The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments are of Latin origin, and may be confused with terms for bones: coxa , trochanter , femur, tibia, tarsus, ischium, metatarsus, carpus, dactylus ,...

s or antennae
Antenna (biology)
Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....

 or other structures in the adult. The role of the imaginal disc in insect development was first elucidated by Jan Swammerdam.

During the pupal stage, many larval structures are broken down, and adult structures, including the discs, undergo rapid development. Each disc everts and elongates, with the central portion of the disc becoming the distal part of whichever appendage, wing, leg, antenna, etc., it is forming. During the larval stage, the cells in the growing disc appear undifferentiated, but their developmental fate in the adult is already determined.

The experiment that demonstrates this developmental commitment is to take an imaginal disc from a third instar
An instar is a developmental stage of arthropods, such as insects, between each molt , until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or assume a new form. Differences between instars can often be seen in altered body proportions, colors, patterns, or...

 larva, about to undergo pupation, and subdivide it and culture it in the body of a younger larva. Discs can be continuously cultured this way for many larval generations. When such a cultured disc is eventually implanted in the body of a larva that is allowed to pupate, the disc will develop into the structure it was originally determined to become. That is, an antenna disc can be cultured this way and will, almost always, become an antenna (out of place, of course) when final development is triggered by pupation.

The study of imaginal discs in the fruit fly Drosophila melanogaster
Drosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...

led to the discovery of homeotic mutations such as antennapedia
Antennapedia is a HOM-C gene first discovered in Drosophila which controls the formation of legs during development. Loss-of-function mutations in the regulatory region of this gene result in the development of the second leg pair into ectopic antennae...

, where the developmental fate of a disc could sometimes change. It is of great interest that the kinds of developmental switches that occur are very specific, leg to antenna for instance. Study of this phenomenon led to the discovery of the homeobox
A homeobox is a DNA sequence found within genes that are involved in the regulation of patterns of anatomical development in animals, fungi and plants.- Discovery :...

 genes, and started a revolution in the understanding of development in multi-celled animals.

Imaginal Cells

Imaginal cells are tissue-specific progenitors allocated in embryogenesis
Embryogenesis is the process by which the embryo is formed and develops, until it develops into a fetus.Embryogenesis starts with the fertilization of the ovum by sperm. The fertilized ovum is referred to as a zygote...

that remain quiescent during embryonic and larval life. During Drosophila metamorphosis, most larval cells die. Pupal and adult tissues form from imaginal cells. Clonal analysis and fate mapping of single, identified cells show that tracheal system remodeling at metamorphosis involves a classical imaginal cell population and a population of differentiated, functional larval tracheal cells that reenter the cell cycle and regain developmental potency. In late larvae, both populations are activated and proliferate, spread over and replace old branches, and diversify into various stalk and coiled tracheolar cells under control of fibroblast growth factor signaling. Thus, Drosophila pupal/adult tissue progenitors can arise both by early allocation of multipotent cells and late return of differentiated cells to a multipotent state, even within a single tissue.

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