Developmental Biology Lecture 8: Cleavage
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Questions and Answers

What is a key characteristic of cleavage divisions in embryos?

  • They feature defined G1 and G2 phases.
  • They are very rapid compared to other cell divisions. (correct)
  • They are not transcriptionally silent.
  • They involve extensive cell growth between divisions.

    • What role does Cdk1-cyclinB play during cleavage divisions?

  • It promotes mitosis and is active during M-phase. (correct)
  • It maintains low levels of cyclin during M-phase.
  • It prevents mitosis from occurring.
  • It promotes cell growth during interphase.

    • How do cleavage divisions affect the size of cells?

  • Cell size varies greatly between divisions.
  • Cells grow larger with each division.
  • Cells get smaller with each division. (correct)
  • Cells maintain their size throughout division.

    • Why are cleavage divisions described as being transcriptionally silent?

    <p>The zygotic genome is inactive during cleavage.</p> Signup and view all the answers

    What is a typical pattern observed in cleavage divisions?

    <p>Cell divisions are influenced by yolk distribution.</p> Signup and view all the answers

    What type of cleavage occurs in mammals due to the low amount of yolk in the egg?

    <p>Holoblastic cleavage</p> Signup and view all the answers

    In frog cleavage, how do the divisions progress?

    <p>The first two divisions are vertical, and the third is horizontal</p> Signup and view all the answers

    What type of cleavage do fish and birds predominantly exhibit?

    <p>Meroblastic cleavage</p> Signup and view all the answers

    How does the plane of cleavage division in C.elegans differ from that in mammals?

    <p>C.elegans undergoes holoblastic and asymmetric cleavage</p> Signup and view all the answers

    What primarily influences the orientation of cleavage divisions?

    <p>Mitotic spindle orientation</p> Signup and view all the answers

    What is a characteristic of meroblastic cleavage reflected in division furrows?

    <p>Furrows stop upon reaching yolk</p> Signup and view all the answers

    Which type of cleavage is characterized by rotational cleavage patterns?

    <p>Holoblastic cleavage</p> Signup and view all the answers

    What is the primary consequence of cell ablation in C.elegans?

    <p>Specifically restricts fates of future cells</p> Signup and view all the answers

    What are the consequences of the lack of G1 and G2 phases during cleavage divisions?

    <p>Divisions happen rapidly with smaller cell sizes each time.</p> Signup and view all the answers

    Which factor primarily determines the stereotypical pattern of cleavage divisions across different organisms?

    <p>Distribution and thickness of yolk in the egg.</p> Signup and view all the answers

    Why do cleavage divisions result in transcriptional silence in the embryo?

    <p>The zygotic genome remains inactive, relying on oocyte stores.</p> Signup and view all the answers

    How does the activity of Cdk1-cyclinB (MPF) affect the timing of cleavage divisions?

    <p>It leads to continuous activation during the M-phase only.</p> Signup and view all the answers

    What is a significant characteristic of the cleavage embryo regarding cell divisions?

    <p>Divisions are typically synchronous across all embryos.</p> Signup and view all the answers

    Which cleavage type allows cell divisions to penetrate completely through the egg?

    <p>Holoblastic cleavage</p> Signup and view all the answers

    What defines meroblastic cleavage in fish and birds?

    <p>Division occurs only in the upper region of the egg</p> Signup and view all the answers

    During early cleavage stages, which characteristic is observed in C.elegans?

    <p>Asymmetric divisions that restrict cell fate</p> Signup and view all the answers

    What factor primarily determines the orientation of cleavage divisions?

    <p>Position of the mitotic spindle</p> Signup and view all the answers

    Which statement accurately describes the cleavage divisions in frogs?

    <p>Initial divisions are uneven due to yolk concentration</p> Signup and view all the answers

    What occurs during the sixth cleavage division in the fish and birds?

    <p>Cells separate completely from the yolk</p> Signup and view all the answers

    Which type of cleavage typically arises from a uniform distribution of yolk?

    <p>Holoblastic cleavage</p> Signup and view all the answers

    In what way do initial divisions in fruit fly cleavage differ from other types of cleavage?

    <p>Divisions happen without cell cleavage</p> Signup and view all the answers

    Study Notes

    Cleavage Overview

    • Cleavage refers to the initial mitotic divisions of the embryo, dividing the single-celled zygote into smaller cells called blastomeres.
    • This process occurs without growth, resulting in progressively smaller cells at each division.
    • Cleavage lacks defined G1 and G2 phases, driven by constitutive Cdk1-cyclinB activity, leading to rapid divisions.

    Cleavage Characteristics

    • Cleavage divisions occur rapidly relative to other cell divisions.
    • Cells remain transcriptionally silent; the zygotic genome is inactive, relying on maternal oocyte stores for resources.
    • Divisions are typically synchronous, creating a predictable cleavage pattern influenced by yolk distribution.
    • Parameters defining cleavage include yolk amount and distribution, the completeness of early cleavages, symmetry of daughter cells, and planes of division.

    Cleavage in Frogs

    • Frogs exhibit holoblastic cleavage with yolk concentrated in the egg's lower hemisphere.
    • The first two divisions are vertical; the third is horizontal, positioned higher in the egg.
    • Following initial divisions, upper cells divide more rapidly due to lower yolk content.

    Meroblastic Cleavage in Fish and Birds

    • Fish and bird embryos show meroblastic cleavage due to thick yolk that prevents complete division.
    • Initial cell divisions are vertical and incomplete, with cleavage furrows stopping upon reaching the yolk.
    • The sixth division is horizontal, separating the yolk from the upper blastomeres.

    Cleavage in Cephalopods

    • Cephalopods show variations in early embryonic stages, demonstrating both divergence and convergence in cleavage patterns.

    Cleavage in Mammals

    • Mammals typically exhibit holoblastic cleavage with evenly distributed yolk (isolecithal).
    • Cleavage divisions are complete through the egg, resulting in equally sized blastomeres.
    • Mammalian cleavage is slower compared to other species, with early zygotic transcription starting minimally within 24-48 hours.

    Rotational Cleavage

    • In holoblastic cleavage, the second cleavage occurs at right angles to the first, forming a rotational pattern.
    • The orientation of mitotic spindle determines the pattern of cleavage division.

    Cleavage in C. elegans

    • C. elegans demonstrates holoblastic and asymmetric cleavage.
    • Division results in lineage-restricted cell fates; specific cells inherit unique markers such as P granules.
    • Ablation of certain cells impacts developmental fate, exemplified by future vulva formation.

    Centrosome and Spindle Orientation

    • The plane of cell division is influenced by the orientation of mitotic spindles.
    • Centrosome positioning is critical and guided by proteins at the cell cortex that interact with centrosomes via astral microtubules.

    Meroblastic Cleavage in Fruitflies

    • Initial stages include DNA replication and nuclear division prior to actual cell cleavage.
    • Nuclei migrate to the cell periphery, setting the stage for subsequent cellular development.

    Cleavage Overview

    • Cleavage refers to the initial mitotic divisions of an embryo, dividing the zygote into smaller cells known as blastomeres.
    • These divisions occur without growth, resulting in increasingly smaller cells, termed altered mitosis.
    • Typical cleavage lacks defined G1 and G2 phases due to the presence of constitutive Cdk1-cyclinB (MPF), which is active during mitosis.

    Characteristics of Cleavage

    • Cleavage divisions are rapid compared to other cell divisions.
    • No cell growth occurs; instead, cells become smaller after each division.
    • Early cleavage divisions are synchronous and vary in pattern based on yolk distribution and thickness.
    • The zygotic genome remains inactive, so embryos rely on maternal oocyte stores for early development.

    Cleavage Types Based on Yolk Distribution

    • Amount and distribution of yolk significantly influence cleavage patterns across different organisms.
    • Cleavage can be categorized as either complete (holoblastic) or incomplete (meroblastic).
    • Symmetry of cleavage impacts whether daughter cells are identical or not.

    Cleavage in Specific Organisms

    • Frogs: Undergo holoblastic cleavage. Yolk is concentrated in the lower half, leading to distinct division patterns; first two divisions are vertical, followed by a horizontal division.
    • Fish & Birds: Features meroblastic cleavage due to thick yolk. Initial cell divisions are vertical and incomplete, leading to blastomeres still attached to yolk until later divisions.
    • Mammals: Exhibit holoblastic cleavage with minimal yolk distributed evenly. Cleavage is slower with early zygotic transcription starting within 24-48 hours.

    Cell Division Orientation

    • The orientation of cleavage divisions is determined by the alignment of mitotic spindles, which is influenced by centrosome positioning.
    • Centrosomes interact with the cell cortex through proteins, often aided by astral microtubules.

    Specific Patterns in Model Organisms

    • C.elegans: Display holoblastic and asymmetric cleavage, where specific cell lineages inherit unique fates demonstrated by the distribution of P granules.
    • Fruit Flies: Initial stages involve nuclear division and migration before the physical cleavage of cells occurs, signifying meroblastic patterns.

    Summary of Key Processes

    • Cleavage occurs quickly, resulting in smaller cells without growth, characterized by rapid division and specific egg yolk adaptations.
    • The roles of cytoplasmic factors, such as maternal contributions and zygotic transcription, play vital roles in early embryonic development.
    • Understanding cleavage patterns provides insight into developmental biology and organismal differences in reproduction.

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    Lecture 8 Cleavage PDF

    Description

    Explore the fascinating process of cleavage in embryonic development, where the single-celled zygote undergoes rapid divisions without growth. This quiz delves into altered mitosis and the role of Cdk1-cyclinB in promoting these crucial initial stages of cell division.

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