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#segmentation

3 public questions tagged with this topic.

Paracrine Factor

Which family of paracrine factors plays a key role in limb development, lens induction, and segmentation?

Fibroblast Growth Factor family comprises 22 ligands signaling via four RTK FGFR1-4 requiring heparan sulfate cofactor for high-affinity binding and dimerization. During development, FGF10 from lateral plate mesoderm induces FGF8 in apical ectodermal ridge driving limb outgrowth via positive feedback loop with Sonic Hedgehog from zone of polarizing activity maintaining proliferation and patterning. FGFs from optic vesicle induce lens placode in surface ectoderm via Pax6 upregulation, and FGF8 from isthmus organizer patterns midbrain-hindbrain boundary. Segmentation clock regulated by FGF gradient opposing retinoic acid. Mutations cause skeletal dysplasias highlighting broad patterning.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 18: FGF family roles in limb bud outgrowth, lens induction, segmentation.

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Drosophila -ll

First indication of segmentation seen with:

Morphological segmentation first becomes molecularly evident when pair-rule products accumulate in stripes at late syncytial blastoderm after gap domains refined. Unlike gap genes in broad domains, pair-rule proteins exhibit seven regularly spaced bands detectable by in situ hybridization, marking future parasegment boundaries before cellularization. Homeotic genes confer identity later but do not create periodic pattern. Gap genes precede pair-rule but lack periodicity. Observation of eve stripes represents earliest indication of segmentation converting continuous maternal gradients into discrete repeating units, linking nonperiodic information to repetitive body plan architecture characteristic of insect embryos and evolutionary patterning.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: First segmentation indication - pair-rule stripe patterning.

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Fertilization in mammals-lll

Gap genes regulate:

Drosophila embryonic segmentation proceeds hierarchically: maternal gradients activate gap genes then pair-rule then segment polarity. Gap genes like hunchback, Krüppel, knirps, giant are transcription factors expressed in broad overlapping domains along anterior-posterior axis, each controlling large contiguous blocks of segments. Mutations delete several adjacent segments creating gaps in larval cuticle pattern, hence name. They regulate pair-rule gene stripe expression. Segment polarity genes control subsegments and polarity within each segment via Wg and En, homeotic genes confer identity not segment number. Therefore gap genes establish coarse regionalization specifying large segmental domains upstream of finer patterning cascades in early embryogenesis.

Ref: St Johnston & Nüsslein-Volhard, Cell 1992: Gap genes establishing broad segmental domains in Drosophila embryonic patterning.

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