Developmental Biology Terms Starting With I
Developmental Biology Glossary: I
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Imaginal Disc
/ ih-MAJ-ih-nul DISK / · Latin imago, adult insect form; discus, disc
Imaginal disc is a cluster of undifferentiated cells set aside in an insect larva that remains quiescent during larval life and then rapidly differentiates into a specific adult structure during metamorphosis.
Imaginal discs form during embryogenesis in holometabolous insects such as the fruit fly (Drosophila melanogaster) and persist through all three larval instars without differentiating, even as surrounding larval tissues grow and function. A single third-instar fruit fly larva carries approximately 19 imaginal discs, including three pairs of leg discs, a pair of wing discs, a pair of haltere discs, and discs for the eyes, antennae, and genitalia. When the larva pupates, rising ecdysone titers trigger disc cells to activate developmental programs, and each disc completes differentiation into its adult structure within roughly 100 hours.
Each disc is divided into anterior and posterior compartments whose cells never intermix, a lineage restriction established by the selector gene engrailed that allows precise positional patterning within the disc.
Transplanting a leg imaginal disc into the abdomen of an adult fruit fly, where it cannot receive metamorphic ecdysone signals, causes the disc to remain undifferentiated indefinitely. When researchers then transplanted those same cells back into a pupating larva, the disc differentiated normally into leg tissue, showing that the cells had retained their identity through dozens of cell divisions in an ectopic location.
An adult insect forms from complete reorganization of larval tissue. Imaginal discs preserve the cellular blueprint for adult organs throughout larval life, and most adult external structures derive from these pre-specified disc populations rather than from dedifferentiated larval cells.
A single fruit fly eye imaginal disc contains roughly 2,000 cells at the start of the third larval instar and differentiates into a compound eye bearing approximately 800 ommatidia, each composed of 8 photoreceptor cells and 12 accessory cells. This roughly 400-fold increase in cell number and structural complexity occurs within about four days of pupal development.
Incubation
/ in-kyuh-BAY-shun / · Latin incubare (to lie upon)
Incubation is keeping eggs warm so embryos can develop. It lasts from laying until the eggs hatch. Most birds supply heat with their own bodies. They press eggs against a bare brood patch. Some reptiles and birds use sun, soil, or rotting plants. The length of this stage is the incubation period.
An embryo inside an egg needs steady warmth to grow. Too much heat or cold can stop development. Parents also turn eggs to keep membranes from sticking.
Birds are endotherms, so body heat can warm the eggs. A brood patch loses feathers and gains extra blood vessels. This bare skin transfers heat to the eggs efficiently.
In many species, both parents share incubation duties. In others, only one parent sits on the eggs. The yolk sac feeds the embryo throughout this time.
The Malleefowl (Leipoa ocellata) never sits on its eggs. It buries them in a huge mound. The mound can reach 8 meters across. Rotting leaves and sunlight supply the heat. The male keeps the inside near 33°C.
Many people think warmth alone makes an egg hatch. Eggs also need proper humidity and regular turning. Unturned embryos can stick to the shell membrane. Chicken (Gallus gallus domesticus) eggs hatch in about 21 days.
A male Emperor Penguin (Aptenodytes forsteri) incubates on his feet. A fold of feathered skin covers the egg. Hatching takes 65 to 75 days. By then he has fasted about 120 days. He can lose up to 20 kilograms.
Types of Penguins →Indeterminate Cleavage
/ in-deh-TER-mih-nut KLEE-vij / · Latin indeterminatus, unlimited
Indeterminate cleavage is an early embryonic cell division pattern in which individual blastomeres retain the developmental potential to form a complete organism if separated from the rest of the embryo.
Indeterminate cleavage occurs when early blastomeres remain pluripotent, meaning their fates are not yet fixed by asymmetrically distributed cytoplasmic determinants, in contrast to determinate cleavage where cell fate is sealed at the first division. In humans and mice, blastomeres through the eight-cell stage each express pluripotency factors Oct4, Sox2, and Nanog at comparable levels, with no cell-type-specific gene silencing yet imposed. This regulative capacity underlies the formation of identical twins, which arise when the embryo splits spontaneously at the two-cell stage or, less commonly, at the blastocyst stage.
Splitting at the blastocyst stage produces monochorionic twins who share a placenta, while earlier splits produce dichorionic twins with separate placentas, a difference that carries distinct clinical risks during pregnancy.
Hans Driesch demonstrated in 1891 that separating the first two blastomeres of a sea urchin (Strongylocentrotus purpuratus) embryo produced two complete, normally proportioned larvae rather than two half-embryos, providing the first experimental evidence that early animal cells could regulate their own development.
Identical twins always result from a single embryo splitting in two. Identical multiples can also arise from successive splitting events, and rare cases of identical triplets trace to two sequential splits of the same original embryo.
When mouse blastomeres are separated at the two-cell stage, each cell develops into a complete, fertile mouse, demonstrating that neither blastomere carries an irreversible fate at that point. Researchers have used this technique routinely since the 1970s to produce genetically identical mouse pairs for experimental controls.
Induction
/ in-DUK-shun / · Latin inducere, to lead into
Induction is a developmental interaction in which a signaling tissue directs the fate of an adjacent responding tissue by secreting molecules that activate new gene expression programs in the receiving cells.
Induction occurs when cells in one tissue produce and secrete signaling molecules that bind receptors on nearby cells, activating gene expression programs that commit the receiving cells to a specific developmental fate. During vertebrate eye development, the optic vesicle secretes FGF and BMP4, which activate Pax6 and Sox2 in the overlying surface ectoderm and commit it to lens formation. Inductive signals operate over short distances through diffusible factors such as Nodal, Wnt, and Hedgehog proteins, or through membrane-bound interactions like Notch-Delta signaling, providing spatial precision in tissue patterning.
The responding tissue’s developmental competence, meaning its current gene expression state and receptor repertoire, determines whether and how it responds, so identical signals produce different outcomes in different tissues at different times.
Hans Spemann and Hilde Mangold demonstrated embryonic induction experimentally in 1924 by transplanting the dorsal lip of the blastopore from one newt embryo into a host embryo, inducing a complete secondary body axis. Mangold performed the grafts as a doctoral student, but died before the work was published; Spemann alone received the 1935 Nobel Prize in Physiology or Medicine for the discovery.
Any tissue can be induced to form any structure given the right signal. Competence restricts which fates a tissue can adopt in response to inductive signals, and a tissue that lacks the appropriate transcription factor context will not respond even when the correct inducing molecule is present.
In the African clawed frog (Xenopus laevis), the Spemann organizer produces Chordin and Noggin, proteins that block BMP signaling in the adjacent ectoderm and induce it to form neural tissue rather than epidermis. Microinjection experiments showed that a single Chordin-expressing cell transplanted into ventral ectoderm is sufficient to redirect roughly 200 neighboring cells toward a neural fate.