Developmental Biology Terms Starting With A
Developmental Biology Glossary: A
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Anterior-Posterior Axis
/ an-TEER-ee-or pos-TEER-ee-or AK-sis / · Latin anterior (front) + posterior (back) + axis (axle)
Anterior-Posterior Axis is the body direction running from the head end to the tail end of an embryo or animal, established by molecular gradients before any visible body structures appear.
In fruit fly (Drosophila melanogaster) embryos, maternal bicoid mRNA localizes at the anterior end and translates into Bicoid protein, which forms a concentration gradient highest at the head end. Nanos protein, localized at the posterior end, represses translation of hunchback mRNA in posterior regions. These opposing morphogen gradients provide positional information that instructs cells about their location along the axis and their resulting developmental fate.
Vertebrates use a different but analogous system, with Wnt and fibroblast growth factor signaling elevated at the posterior end and retinoic acid signaling elevated anteriorly.
Christiane Nüsslein-Volhard and Eric Wieschaus won the 1995 Nobel Prize in Physiology or Medicine for identifying the genes that establish the anterior-posterior axis in Drosophila, including bicoid, nanos, and the gap and pair-rule genes that subdivide the embryo into segments.
The head-to-tail direction appears only after the body is visible. Early embryos contain molecular differences that mark this direction before any body parts form.
In zebrafish (Danio rerio) embryos, Wnt signaling is elevated at the future posterior end as early as the 16-cell stage, roughly 1.5 hours after fertilization. Experimental reduction of posterior Wnt signaling at this stage causes embryos to develop duplicated anterior structures in place of a tail.
Apical Ectodermal Ridge
/ AY-pih-kul ek-toh-DER-mul rij / · Latin apicalis + Greek ektos (outside) + derma (skin) + ridge
Apical Ectodermal Ridge is a thickened band of ectoderm cells at the distal tip of a developing vertebrate limb bud that secretes fibroblast growth factors to sustain limb outgrowth and patterning.
The apical ectodermal ridge forms as a line of surface ectoderm roughly one to two cells thick running along the distal edge of the limb bud. It produces fibroblast growth factors, particularly FGF4 and FGF8, that diffuse into the underlying mesenchyme and keep progress zone cells in a proliferative, undifferentiated state. Without this outgrowth signal, mesenchymal cells differentiate prematurely into cartilage or bone and limb elongation stops.
In chickens (Gallus gallus domesticus), the ridge persists for approximately five to six days before regressing as the limb reaches its final proportions.
John Saunders and Mary Gasseling demonstrated in 1948 that surgically removing the apical ectodermal ridge from a chick wing bud at progressively later stages truncates the limb at progressively more distal levels, establishing that proximal-to-distal limb elements are specified in sequence during outgrowth.
Limb bones form by stretching a tiny pre-made limb template. Each limb segment is specified sequentially as new mesenchymal cells are added at the distal tip during active outgrowth.
When researchers grafted an extra apical ectodermal ridge onto the posterior edge of a developing chicken wing bud, a duplicate set of wing structures grew from that location. This experiment by John Saunders showed that the ridge directs outgrowth wherever it is placed, not just at the natural distal tip.
Asymmetric Cell Division
/ ay-sih-MET-rik sel dih-VIH-zhun / · Greek asymmetros (without measure) + Latin cellula + divisio
Asymmetric Cell Division is a type of cell division in which a parent cell produces two daughter cells that differ in size, molecular contents, or developmental fate through the unequal segregation of fate-determining proteins or organelles.
During asymmetric division, fate-determining proteins such as Numb and Prospero localize to one pole of the dividing cell and segregate entirely into one daughter. In the nematode Caenorhabditis elegans, the first division of the fertilized egg is asymmetric, producing a larger somatic precursor cell and a smaller germline precursor cell with distinct developmental potentials. PAR polarity proteins establish the asymmetric cortical domains that position the cleavage spindle and direct differential inheritance of these determinants.
Disrupting PAR protein function in C. elegans causes both daughters to inherit identical molecular contents, converting what should be an asymmetric division into a symmetric one.
In the fruit fly (Drosophila melanogaster) nervous system, asymmetric division of neuroblasts generates the entire central nervous system from a relatively small pool of progenitors. A single neuroblast can produce more than 100 neurons through repeated asymmetric divisions over roughly 24 hours of larval development.
Every cell division produces identical daughters. Some divisions are intentionally unequal, with specific proteins partitioned to only one daughter to direct different cell fates.
In Drosophila neuroblasts, the protein Numb concentrates at the basal cortex before division and segregates into the basal daughter cell. That basal daughter becomes a ganglion mother cell that differentiates, while the apical daughter retains neuroblast identity and continues dividing.