Holoblastic Cleavage: Radial in Sea Urchins
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Holoblastic Cleavage: Radial in Sea Urchins

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Questions and Answers

What type of cleavage is characterized by having both meridional and equatorial axes of symmetry?

  • Meroblastic cleavage: Superficial
  • Meroblastic cleavage: Discoidal
  • Holoblastic cleavage: Spiral
  • Holoblastic cleavage: Radial (correct)

    • Which of the following is NOT a product of cleavage in the sea urchin?

  • Plasmomeres (correct)
  • Micromeres
  • Mesomeres
  • Macromeres

    • What is the role of the maternal determinant present in the vegetal pole of the sea urchin embryo?

  • To initiate the cleavage process
  • To ensure development of all three germ layers (correct)
  • To promote growth of the ciliated epidermal cells
  • To enhance the fertilization process

    • What is the primary function of Dishevelled in the context of cleavage?

    <p>To prevent degradation of β-catenin</p> Signup and view all the answers

    In holoblastic cleavage, which type of cells are formed by the cleavage process in a sea urchin?

    <p>Blastomeres</p> Signup and view all the answers

    Which feature is characteristic of a sea urchin's 1-cell zygote?

    <p>Biochemically asymmetric</p> Signup and view all the answers

    Which assertion correctly describes the ciliated epidermal cells formed by isolation of the animal pole cells?

    <p>They are permanently differentiated cells</p> Signup and view all the answers

    What defines a macro- versus a micro-mere in the context of sea urchin cleavage?

    <p>Size and cytoplasmic volume</p> Signup and view all the answers

    What is the proposed role of b-catenin in the fates of the micromeres?

    <p>It autonomously specifies the fate of skeleton-forming genes.</p> Signup and view all the answers

    What are the effects hypothesized from stabilizing b-catenin everywhere?

    <p>Altered fates of an1 and an2 cells.</p> Signup and view all the answers

    How does Pmar1 expression affect HesC?

    <p>Pmar1 inhibits HesC expression.</p> Signup and view all the answers

    What is the significance of the micromeres at the 64-cell stage?

    <p>They serve as the site for b-catenin expression.</p> Signup and view all the answers

    What would be the expected result of inhibiting b-catenin translocation to the nucleus?

    <p>It would alter the fates of veg1 and veg2 cells.</p> Signup and view all the answers

    Which of the following best describes a double-negative gate in gene regulatory networks?

    <p>A configuration where two inhibitors lead to activation.</p> Signup and view all the answers

    What type of signaling is described in the Notch signaling section?

    <p>Juxtacrine</p> Signup and view all the answers

    How do small micromeres differ from large micromeres in terms of cellular fate specification?

    <p>Small micromeres are conditionally specified, while large micromeres are autonomously specified.</p> Signup and view all the answers

    What does the presence of HesC indicate in relation to skeleton-forming genes?

    <p>HesC blocks skeletal gene expression.</p> Signup and view all the answers

    What substance is suggested to stabilize b-catenin in the hypotheses?

    <p>LiCl</p> Signup and view all the answers

    What role does the hyaline layer play in the gastrulation process?

    <p>It provides a structural scaffold for mesenchyme cell movement.</p> Signup and view all the answers

    In a non-neutral environment, what can be inferred about micromere cell fate?

    <p>It is affected by surrounding cells.</p> Signup and view all the answers

    What is the main function of a feed-forward circuit in gene expression?

    <p>To enhance the expression of downstream genes.</p> Signup and view all the answers

    What happens to lamin during the process described in figure 10.11?

    <p>It breaks down in the ingressing mesenchyme cells.</p> Signup and view all the answers

    How do mesenchymal cells interact with the extracellular matrix (ECM) during their ingression?

    <p>They secrete collagen parallel to their direction of travel.</p> Signup and view all the answers

    What does it mean for animal cell fate to be conditionally specified?

    <p>It can be influenced by external signals or the environment.</p> Signup and view all the answers

    What is the primary function of CSPG secretion during embryo development?

    <p>To cause water absorption and lamina expansion</p> Signup and view all the answers

    What occurs to micromeres upon ingression?

    <p>They lose b-catenin expression and become PMCs</p> Signup and view all the answers

    What role do primary mesenchymal cells (PMCs) play in relation to b-catenin?

    <p>PMCs aggregate at high b-catenin expression sites</p> Signup and view all the answers

    What marks the beginning of gastrulation in sea urchins?

    <p>Establishment of the blastopore</p> Signup and view all the answers

    Which characteristic is associated with the movement of the archenteron?

    <p>Convergent extension facilitating elongation</p> Signup and view all the answers

    What receptor activity is primarily involved in the signaling process of PMCs?

    <p>FGF receptor activity</p> Signup and view all the answers

    In the process of mesenchymal cell movement, what cytoskeletal structures are specifically involved?

    <p>Lamellipodia and filopodia</p> Signup and view all the answers

    What happens to micromeres as they ingress?

    <p>They transition into primary mesenchymal cells</p> Signup and view all the answers

    Study Notes

    Holoblastic Cleavage: Radial

    • "3" Axes of symmetry
    • 1 axis of symmetry
    • Example: Sea urchin

    Radial Cleavage - Sea Urchin

    • Early stage of embryonic development
    • Holoblastic type
    • Cleavage is a series of cell divisions that occur in the zygote, resulting in a multicellular embryo
    • Characterized by the orientation of the mitotic spindles, which are responsible for separating chromosomes during cell division
    • Radial cleavage occurs when the mitotic spindles are aligned perpendicular to the animal-vegetal axis, resulting in daughter cells that are arranged in a radial pattern
    • Early sea urchin embryos are biochemically isotropic (equal distribution in the cytoplasm)

    Maternal Determinants and Cell Fate in Sea Urchins

    • Maternal determinants are molecules that are deposited in the egg cytoplasm during oogenesis and provide positional information for the developing embryo.
    • Maternal determinants determine the early development of an embryo, especially in the absence of a functional zygote genome.
    • Example: Dishevelled in sea urchin embryos
    • Dishevelled is a maternal determinant that localizes to the vegetal pole of the sea urchin egg
    • Dishevelled prevents the degradation of beta-catenin
    • Beta-catenin plays a crucial role in regulating gene expression and cell fate determination
    • This results in the localization of beta-catenin in the vegetal cells of the embryo, which leads to the formation of the primary mesenchyme cells that will form the skeleton

    The Micromeres and Their Role in Sea Urchin Development

    • The large micromeres are located at the vegetal pole of the sea urchin embryo.
    • Autonomous specification - Cells have their fate determined internally
    • They are specifically fated to become the skeleton
    • The micromeres also have inductive potential, meaning they can influence the fate of other cells

    The Role of b-catenin in Sea Urchin Skeletal Development

    • Beta-catenin is a key regulator of skeletal development in sea urchins
    • It activates the expression of Pmar1
    • Pmar1 inhibits the expression of HesC
    • HesC repression allows the expression of genes that control skeletal formation

    Gastrulation in Sea Urchins

    • Gastrulation is a crucial stage of embryonic development, involving the formation of the three germ layers (endoderm, mesoderm, and ectoderm)
    • Invagination: Infolding process that begins in the archenteron formation
    • Archenteron: Primitive gut
    • A sheet of cells invaginates, forming a fluid-filled cavity
    • Cell shape changes, cell rearrangements, filopodial extensions, and convergent extension drive gastrulation
    • The archenteron extends towards the opposite pole of the embryo, eventually reaching the inner side of the blastocoel.

    Ingression and the Primary Mesenchyme Cells (PMCs)

    • Ingression: A process where a group of skeletogenic mesenchyme cells move from the outer surface of the embryo inward
    • PMCs move to the inner blastocoel, where they will form the skeleton
    • The ingression of PMCs is driven by chemotaxis: They follow chemical signals
    • As PMCs ingress, they lose expression of b-catenin, which is replaced by other molecules such as FGF and E- cadherin to promote the formation of mesenchymal cells

    FGF Signaling and its Role in Skeletogenic Mesenchyme Development

    • The PMCs migrate towards regions of high FGF expression in the blastocoel
    • FGF receptors on the PMC plasma membrane mediate this migration
    • FGF signaling activates downstream pathways, leading to rearrangements of the cytoskeleton and cell movement towards the FGF source

    Syncytial Cables and the Formation of the Skeleton

    • PMCs form syncytial cables, which are interconnected networks of cells that facilitate the efficient communication and movement of PMCs
    • The PMCs secrete calcium carbonate, which crystallizes and forms the mineralized skeleton of the sea urchin larva.

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    Description

    Explore the process of radial cleavage in sea urchins, a type of holoblastic cleavage during early embryonic development. This quiz will cover the axes of symmetry, the role of mitotic spindles, and the importance of maternal determinants in cell fate.

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