#embryonic patterning

amphibian -ll

The primary embryonic induction process involves:

Primary embryonic induction defined by Spemann and Mangold as organizer dorsal mesoderm inducing overlying ectoderm to become neural plate rather than epidermis, mediated by BMP antagonists Chordin, Noggin. This mesoderm-ectoderm interaction is archetypal induction: dorsal mesoderm secreting inhibitors instructs competent ectoderm whose fate changes upon contact requiring time for transcription. Secondary inductions like lens or limb involve similar principles. Although endoderm and blastocoel form earlier and neural crest arises later via Wnt and BMP border signals, fundamental conceptual breakthrough concerned mesoderm signaling to ectoderm generating central nervous system and body pattern.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 10: Primary embryonic induction - mesoderm-ectoderm interactions in neural induction.

View Details

amphibian -ll

Complete embryo dorsalization by lithium chloride (LiCl) occurs by:

Lithium chloride inhibits GSK-3 beta kinase by competing for magnesium binding, mimicking canonical Wnt signaling. GSK-3 inactivation prevents beta-catenin phosphorylation at Ser33/37/Thr41 and proteasomal degradation, leading ubiquitous beta-catenin stabilization throughout embryo. Nuclear beta-catenin activates Siamois and Twin everywhere, converting entire marginal zone into Nieuwkoop and organizer tissue expressing Goosecoid. Resulting embryo highly dorsalized forming expanded notochord, neural tissue and lacking ventral blood islands. Beta-catenin stabilization alone explains dorsalization, not direct Noggin activation, confirming Wnt-beta-catenin dorsal pathway and GSK-3 regulatory role.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Lithium chloride dorsalization via beta-catenin stabilization and GSK-3 inhibition.

View Details

amphibian -ll

Dorsal mesoderm during organizer formation specifically induces:

Dorsal mesoderm from Spemann organizer involutes as chordamesoderm: anterior prechordal plate induces forebrain, while posterior notochordal and paraxial dorsal mesoderm secretes Wnt antagonists Frzb plus FGF and retinoic acid gradients that posteriorize overlying neural ectoderm, converting default anterior neural fate to hindbrain rhombomeres expressing Krox20 and spinal cord trunk expressing HoxB9. Experiments grafting dorsal mesoderm beneath animal caps induced Krox20-positive hindbrain and HoxB9 trunk markers. Ventral mesoderm and epidermis lack this caudalizing ability, confirming dorsal mesoderm induces hindbrain and trunk, essential for AP neural patterning.

Ref: Wolpert, Principles of Development, 5th ed., Chapter 6: Dorsal mesoderm patterning hindbrain and trunk neural tissue.

View Details

chick development-l

Sonic hedgehog (Shh) signaling during chick development is crucial for:

Sonic hedgehog secreted from Hensen's node and left paraxial mesoderm plays pivotal role in left-right asymmetry, not just dorsoventral neural patterning. Shh induces Caronte, a Cerberus family BMP antagonist, on left side near node. Caronte inhibits BMP signaling locally, relieving BMP-mediated repression of Nodal in left lateral plate mesoderm via FoxH1 transcription. Right side FGF8 maintains BMP activity blocking Nodal. This Shh-Caronte-Nodal-Pitx2 cascade drives heart looping and gut coiling. Loss of asymmetric Shh randomizes situs, demonstrating crucial role in left-right asymmetry specification beyond neural tube closure.

Ref: Levin, Cell 1995; Gilbert, Developmental Biology, Chapter 12: Sonic hedgehog signaling in left-right asymmetry regulation.

View Details

Drosophila -l

The follicle cells receiving Gurken signaling in Drosophila become:

During mid-oogenesis, oocyte nucleus migrates to dorsal-anterior corner secreting Gurken ligand. Nearby follicle cells expressing EGFR Torpedo bind Gurken, triggering Ras-Raf-MEK-ERK cascade and expression of dorsal follicle markers such as Fos, Broad-Complex and Keel. These cells produce dorsal eggshell appendages and dorsalize embryo indirectly by repressing Pipe sulfotransferase. Ventral follicle cells not receiving Gurken retain Pipe expression necessary for ventralizing Spätzle cleavage later. Therefore follicle population receiving Gurken signal adopts dorsalized fate, secreting dorsal patterning cues and shaping eggshell asymmetry essential for embryonic dorsoventral polarity.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Gurken EGFR signaling dorsalizes follicle cells.

View Details

Drosophila -l

Anterior organizing center in Drosophila is specified by:

Anterior organizing center in Drosophila embryo is established by bicoid mRNA localization to anterior cortex mediated by Exuperantia, Swallow and Staufen RNA-binding proteins during oogenesis. Upon egg deposition, translation creates steep anterior-to-posterior Bicoid protein gradient acting as morphogen within syncytium. Bicoid homeodomain binds DNA to activate transcription of anterior gap genes like hunchback, orthodenticle and buttonhead, while also binding caudal mRNA to repress its translation anteriorly. High Bicoid concentration specifies acron and cephalic segments. Loss transforms anterior into posterior structures, demonstrating essential anterior patterning role.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Bicoid morphogen anterior patterning.

View Details

Drosophila -ll

Activation of Toll receptor results from binding of:

Activation of Toll depends on extracellular cleavage of Spatzle precursor in perivitelline space. Ventrally restricted Pipe sulfotransferase modifies vitelline membrane enabling protease cascade comprising Gastrulation defective, Snake, Easter that processes Spatzle into active fragment. Active Spatzle binds Toll leucine-rich repeats inducing dimerization and recruitment of MyD88, Tube, Pelle. Gurken acts earlier in follicle cells, Dorsal acts downstream transcriptionally, Pipe modifies eggshell indirectly. Spatzle represents only physiological ligand whose ventral processing converts eggshell polarity into embryonic Toll activity and Dorsal gradient essential for mesoderm induction.

Ref: Nature, Spatzle cleavage by Easter protease as Toll ligand - ventral activation of Toll receptor in Drosophila.

View Details

Drosophila -ll

Protein inhibiting dorsal nuclear localization:

Cactus protein shares homology with vertebrate I-kappa B inhibitor binding Rel domain factors. Cactus binds Dorsal covering nuclear localization signal, retaining complex cytoplasmically in future dorsal cells. Ventral Toll signaling activates scaffold Tube and kinase Pelle phosphorylating Cactus prompting ubiquitin-mediated degradation. Free Dorsal then enters nuclei establishing gradient. Spatzle activates Toll upstream, Tube and Pelle promote Cactus degradation indirectly. Cactus therefore serves as direct molecular brake on Dorsal nuclear entry, levels inversely correlating with Dorsal concentrations and defining ventralizing activity during DV patterning and mesoderm induction.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Cactus I-kB homolog inhibiting Dorsal nuclear localization.

View Details