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#axis specification

3 public questions tagged with this topic.

Specification Determination-ll

Which of the following factors plays a major role in specifying embryonic axes in early development?

Embryonic axis formation relies fundamentally on secreted morphogens establishing graded concentrations across fields of cells. Molecules such as Bicoid in insects, Nodal, Wnt, BMP and FGF in vertebrates are released from localized sources, forming diffusion gradients that decline with distance. Cells measure local ligand concentration via receptor occupancy and downstream effectors like phosphorylated Smads or nuclear beta-catenin, activating distinct threshold-dependent transcriptional programs according to French flag model. High doses may specify organizer or dorsal mesoderm, intermediate lateral mesoderm, low ventral tissues, translating continuous gradient into discrete, spatially ordered fate domains along body axes.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Morphogens and Axis Formation.

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Drosophila -l

Dorsal-ventral axis in Drosophila is primarily determined by:

Dorsoventral polarity originates during oogenesis when Gurken, a TGF-alpha-like ligand, secreted from oocyte nucleus located dorsal-anterior position activates EGFR in adjacent follicle cells. Dorsal follicle fate represses Pipe sulfotransferase expression ventrally, restricting Pipe to ventral follicle cells that modify vitelline membrane. After fertilization, serine protease cascade downstream of Pipe cleaves Spätzle, activating Toll receptor ventrally. Toll signaling via Tube, Pelle degrades Cactus inhibitor, allowing Dorsal NF-kB transcription factor nuclear entry ventrally. Thus Gurken dorsalizes follicle cells and Dorsal patterns embryo ventrally, jointly establishing DV axis.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Dorsoventral patterning Gurken and Dorsal.

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Drosophila -l

Terminal regions (acron and telson) in Drosophila are specified by:

Terminal regions acron at anterior and telson at posterior are non-segmented ends of Drosophila body plan specified by terminal maternal effect group. Unlike Bicoid and Nanos localized determinants, Torso receptor tyrosine kinase is uniformly distributed in embryonic membrane with ligand Trunk secreted by follicle cells and activated only at poles by Torso-like. Ligand binding dimerizes Torso, triggering Ras-MAPK cascade via D-Raf, MEK, releasing transcriptional repression by Groucho and Capicua. This permits expression of tailless and huckebein specifically at poles, which pattern acron and telson irrespective of central segmentation programs.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Terminal patterning - Torso RTK pathway.

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