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Organizers and Gastrulation

Normal Dorsal-Ventral Asymmetry in Xenopus

  • Organizer establishment

    • (A) Fertilization

    • Process begins with the sperm entering the egg.

    • Formation of the gray crescent essential for developing asymmetry.

    • Subsequent separation of blastomeres leads to further development.

    • (B) Cortical rotation

    • Initiates to lead to normal development.

    • Dsh (Dishevelled protein):

    • Enriched in the dorsal region of the embryo, facilitating proper structure formation.

    • (C) The Belly piece refers to structures formed from normal development.

Dorsal Enrichment Mechanisms

  • Dorsal enrichment of Dsh:

    • Contributes to inhibition of GSK-3 in the dorsal region.

  • Effects on B-catenin:

    • GSK-3 targets B-catenin for degradation in the absence of Dsh.

    • Dorsal processes result in B-catenin becoming stabilized, allowing entry into the nuclei of cells.

    • B-catenin plays a critical role in activating specific gene expression related to dorsal mesoderm and organizer formation.

Mesoderm Induction and Formation of the Organizer

  • (A) Fertilization

    • Sperm enters egg, initiating mesoderm induction processes through signaling proteins.

  • (B) Cortical rotation

    • Critical for setting up the dorsal-ventral axis.

  • (C) Organizer origin:

    • The Nieuwkoop center is crucial for generating dorsal mesoderm-inducing signals.

    • Signals from Dsh lead to the specification of dorsal structures.

Gradient of Xnrs Expression

  • Xnrs (Xenopus nodal-related proteins):

    • Established through RT-PCR and in situ hybridization to show highest expression in dorsal-vegetal regions.

  • Their function is crucial for mesoderm induction in a concentration-dependent manner:

    • Inhibition of Xnrs directly blocks mesoderm formation, demonstrating their vital role.

  • Xnrs can induce mesoderm without needing the vegetal hemisphere.

Nodal-Related Proteins and the TGF-β Family

  • Nodal proteins (Xnrs) are categorized as mesoderm inducers and are members of the Transforming Growth Factor-β (TGF-β) family.

  • There are four recognized Xnrs in Xenopus: Xnr1, Xnr2, Xnr3, and Xnr4.

Current Model for Mesoderm Induction

  • At stages 8, 9, and 10, gradients of various proteins are established:

    • VegT, Vg1, and B-catenin are key mesoderm inducing factors.

    • BMP4 and Xwnt8 establish gradients affecting ventral and lateral mesoderm development.

Functions of the Organizer

  • The dorsal blastopore lip (often referred to as “Spemann’s Organizer”) plays critical roles:

    1. Differentiation into dorsal mesoderm.

    2. Dorsalization of surrounding mesoderm areas likely to become ventral mesoderm.

    3. Inducing the formation of the neural tube from ectoderm.

    4. Initiation of gastrulation movements, essential for proper embryo organization.

    5. Induction of neural plate transformation into the neural tube.

Identification of Organizer Molecules

  • Research has focused on identifying specific organizer molecules using methods like:

    • cDNA library screening

    • mRNA injection techniques.

  • Key identified molecules:

    • Chordin

    • Noggin: expressed in the organizer immediately after formation.

    • Nodal-related proteins, Goosecoid, Cerberus, among others.

Rescue Experiments with Noggin mRNA

  • Excess noggin mRNA can lead to abnormal dorsal structures forming at the cost of ventral structures, demonstrating the pivotal role noggin plays in dorsalization.

    • Low vs. High Noggin concentrations impact organizer gene activity and structure orientation (back piece vs. belly piece).

Mechanisms of Organizer Activity

  • Cortical rotation facilitates the movement of important signaling molecules like Vg1, Dsh, and GBP, leading to:

    • Stabilization of B-catenin which enters the nucleus to trigger organizer gene expressions.

    • Induction of Xnrs which are essential for mesoderm formation.

Chordin's Role in Organizer Activity

  • Chordin binds to BMP ligands to inhibit BMP receptor binding, alongside noggin, effectively neutralizing ventralizing signals.

Secreted Proteins of the Organizer

  • The organizer produces secreted proteins that inhibit TGF-β and Wnt signaling pathways, including:

    • Frzb-1

    • Dickkopf-1

    • Crescent

    • Chordin

    • Noggin

    • Follistatin

    • Lefty/Antivin

BMP and Nodal Gradients

  • Gradients of BMP and Nodal morphogens coordinate dorsal-ventral patterning:

    • Interaction zones between BMPs and Nodal along the dorsal-ventral axis help define cell fate during gastrulation.

Role of BMPs in Development

  • BMPs are associated with ventralization and consequently inhibit neural formation in targeted ectodermal regions.

  • Such signaling is counteracted by BMP antagonists from the organizer, allowing formation of the neural plate.

Gastrulation Dynamics

  • Gastrulation specifically organizes germ layers (endoderm, mesoderm, ectoderm) into their correct configurations:

    • Endoderm: gut, liver, lungs.

    • Mesoderm: bone, muscle, cartilage, kidney, heart, blood.

    • Ectoderm: skin, nervous system.

  • The processes of gastrulation include:

    • Epiboly: thinning and spreading of cells.

    • Vegetal rotation: pressing against the blastocoel.

    • Bottle cell formation: localized constriction at the dorsal blastopore lip.

    • Involution and cell migration: tissue folding inward over internal surfaces, driving further development.

    • Convergence and extension: intercalation along the midline, facilitating anterior-posterior elongation.

Collective Cell Migration During Gastrulation

  • Illustrates the coordination between various cell layers (upper vs. following rows) and their movement patterns driven by lamellipodia and cellular dynamics.

Upcoming Topic

  • Left-Right Patterning will be discussed in the next class, focusing on how organisms establish asymmetry along their left-right axis during development.