Embryonic Development: Cleavage and Mitosis
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Questions and Answers

What is the primary role of Hox genes in embryonic development?

  • Regulate muscle differentiation
  • Induce stem cell proliferation
  • Control placement of body segments (correct)
  • Facilitate nutrient absorption

    • Which transcription factor family is known for its role in the development of sense organs and the nervous system?

  • T box genes
  • Pax genes (correct)
  • Sox genes
  • Basic helix-loop-helix proteins

    • What common feature is associated with the Sox gene family?

  • Homeobox domain
  • Zinc finger motif
  • HMG domain (correct)
  • Helix-loop-helix structure

    • What is a key function of the Zinc Finger Protein (ZnF) family?

    <p>Bind DNA, RNA, and proteins</p> Signup and view all the answers

    What is the function of TGF-β family members in embryonic development?

    <p>Inducing mesodermal layers</p> Signup and view all the answers

    Which FGF family member is associated with kidney development?

    <p>FGF-2</p> Signup and view all the answers

    What developmental process do Limit proteins primarily influence?

    <p>Nervous system development</p> Signup and view all the answers

    What does the Dlx gene primarily regulate?

    <p>Limb and organ development</p> Signup and view all the answers

    What role do fibroblast growth factors (FGFs) play in embryonic development?

    <p>Cell migration and proliferation</p> Signup and view all the answers

    What characteristic distinguishes the Basic Helix-Loop-Helix (bHLH) proteins?

    <p>Two alpha helices separated by an amino acid loop</p> Signup and view all the answers

    What is the main outcome of the process known as cleavage during early embryogenesis?

    <p>Formation of blastomeres</p> Signup and view all the answers

    What does Mitosis Promoting Factor (MPF) primarily initiate in the cell cycle?

    <p>M phase</p> Signup and view all the answers

    Which type of cleavage is characterized as complete cleavage?

    <p>Holoblastic Equal Cleavage</p> Signup and view all the answers

    In which type of cleavage do the cells maintain the capacity to develop into a full organism if separated?

    <p>Indeterminate Cleavage</p> Signup and view all the answers

    What is a primary factor that influences cell specialization during embryonic cleavage?

    <p>Intrinsic and extrinsic factors</p> Signup and view all the answers

    What event marks the midblastula transition?

    <p>Activation of zygotic gene transcription</p> Signup and view all the answers

    Which of the following describes meroblastic cleavage?

    <p>Cleavage occurs only in certain regions of the egg</p> Signup and view all the answers

    Which structure forms the tissue of the chorion during mammalian cleavage?

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

    How do mammals differ from other vertebrates during cleavage?

    <p>They have the smallest and slowest cleavage process.</p> Signup and view all the answers

    What is the role of the internal cell mass (ICM) in early mammalian development?

    <p>It supports the trophoblast and develops into the embryo.</p> Signup and view all the answers

    What happens to cyclin B during the cell cycle?

    <p>It is synthesized and degraded in a cell cycle-specific manner.</p> Signup and view all the answers

    Which feature is characteristic of protostome development?

    <p>Cytoplasmic determinants influencing body axes</p> Signup and view all the answers

    What is an example of a transcription factor involved in embryonic development?

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

    At what stage does the human blastocyst form, and what key features does it introduce?

    <p>5 days, integration into the uterine wall begins</p> Signup and view all the answers

    Study Notes

    Cleavage

    • Rapid mitotic divisions where the egg's cytoplasm is divided into smaller cells with nuclei.
    • Creates blastomeres, which are the cells resulting from cleavage.
    • Also called blastulation.
    • Involves karyokinesis (nuclear division) and cytokinesis (cytoplasmic division).

    Mitosis Promoting Factor (MPF)

    • Induces the cell to enter the M phase (mitosis).
    • Activated MPF leads to:
      • Chromosome condensation
      • Nuclear envelope breakdown
      • RNA polymerase inhibition (transcription shutdown)
      • Myosin regulatory subunit phosphorylation (cytokinesis inhibition).
    • Plays a crucial role in the transition from fertilization to cleavage.
    • Controls meiotic cell division in the egg cell.
    • Consists of two subunits:
      • Cyclin B
      • Cyclin-dependent kinase (cdk2).

    Patterns of Embryonic Cleavage

    • Factors contributing to cell specialization:

      • Intrinsic factor (lineage): Information inherited from the mother cell during cell division.
      • Extrinsic factor (positional): Information received from the surrounding environment or neighboring cells.

    • Determining Body Axes:

      • Cytoplasmic Determinants (mom’s genome): mRNAs or proteins present in the egg before fertilization.
        • Example: Bicoid, a factor present in a concentration gradient across the unfertilized egg, determines anterior vs. posterior axis.
      • Yolk Polarity: Occurs in eggs with high yolk content.
        • Animal Pole defines the Anterior.
        • Vegetal Pole defines the Posterior.
      • Induction: Communication between cells leading to different cell fates among initially similar cells.
        • Relevant in mammalian embryos with low and evenly distributed yolk.

    Types of Cleavage Furrows

    • Holoblastic Cleavage: Complete cleavage.
      • Holoblastic Equal: In eggs with little yolk (microlecithal) or evenly distributed yolk (isolecithal). Blastomeres have equal size.
      • Holoblastic Unequal: In eggs with moderate yolk (mesolecithal). Blastomeres differ in size, with smaller micromeres and larger macromeres.
    • Meroblastic Cleavage: Incomplete cleavage.
      • Superficial Meroblastic: In eggs with centrally located yolk (centrolecithal). Cleavage is restricted to the outer cytoplasm.
      • Discoidal Meroblastic: In eggs with large yolk (macrolecithal). Cleavage furrows are only formed at the animal pole region.

    Types of Cleavage Plane/ Arrangement

    • Radial Cleavage: Division planes perpendicular to each other. Blastomeres are aligned directly above or to the side of each other.
    • Spiral Cleavage: Division planes are not perpendicular.
    • Bilateral Cleavage: Cleavage is symmetrical on both sides, creating left and right halves.
    • Rotational Cleavage: The first cleavage is meridional. In the second cleavage, one blastomere divides meridionally, and the other divides equatorially.

    Types of Cell Fate

    • Determinate Cleavage: The developmental fate of cells is determined early, leading to specialization.
    • Indeterminate Cleavage: Each cell can develop into a complete organism if separated.

    Midblastula Transition

    • Activation of Zygotic Gene Transcription: The zygote begins to produce its own mRNAs from its DNA.
    • Cell Cycle Changes: The cell cycle slows down, and G1 and G2 phases are added. Cell division becomes asynchronous.
    • Cell Migration: Central process in the development and maintenance of multicellular organisms.

    Cleavage in Fish Eggs

    • Discoidal and meroblastic cleavage.
    • Cleavage occurs in the blastodisc.
    • Calcium ions are essential for actin cytoskeleton formation.
    • Division time is approximately 15 minutes per division.
    • Midblastula Transition:
      • Gene transcription activation.
      • Slower cell division.

    Distinct Cell Populations in Fish Eggs

    • YSL (Yolk Syncytial Layer): Directs cell movements during gastrulation.
    • EVL (Enveloping Layer): The most superficial layer. Develops into periderm (extraembryonic covering).
    • Deep Cells: Located between EVL and YSL. Give rise to the embryo proper.

    Amphibian Cleavage

    • Radially symmetrical and holoblastic unequal cleavage.
    • Animal pole and vegetal pole establish polarity.
    • Formation of the morula (16-64 cell stage).
    • Blastocoel becomes evident at the 128-cell stage.

    Cleavage in Bird Eggs

    • Discoidal meroblastic cleavage.
    • Cleavage occurs in the blastodisc.
    • First cleavage furrow is centrally located.
    • Equatorial and vertical cleavages divide the blastoderm.

    Structures in Bird Eggs

    • Subgerminal Cavity: Space between the blastoderm and yolk.
    • Area Pellucida: Forms the majority of the embryo.
    • Area Opaca: Peripheral ring of the blastoderm that hasn't shed deep cells.
    • Marginal Zone: Transitional region between the area opaca and area pellucida.

    Cleavage in Mammalian Eggs

    • Meridional and equatorial (rotational cleavage).
    • Blastomeres do not divide simultaneously.
    • Smallest and slowest cleavage (human egg: 100µm)
    • Blastomeres do not divide at the same rate.

    Stages of Mammalian Cleavage

    • Compaction (8-cell stage): Blastomeres maximize contact, forming a compact ball of cells.
    • Morula (16-32 cell stage): Characterized by outer and inner cells.
    • Cavitation: Formation of the internal cavity (blastocoel) by fluid secretion.
    • Blastocyst: Embeds into the uterine wall and establishes the placenta.

    Mammalian Cleavage: Cell Types

    • Trophoblast:
      • Forms the tissue of the chorion.
      • Contains integrin, which binds to uterine collagen, fibronectin, and laminin.
      • Secretes proteases.
    • ICM (Inner Cell Mass):
      • Gives rise to the embryo and yolk sac, allantois, and amnion.
      • Supports the trophoblast.

    Human Cleavage

    • Marked by a slower pace compared to other vertebrates.
      • 2-cell stage: 1 day
      • 4-cell stage: 2 days
      • 16-cell stage: 3 days
      • Blastocyst: 4 days
      • With trophoblast and ICM: 5 days
    • Human eggs lack abundant ribosomes and RNA during oogenesis. Embryos rely on gene products.
    • The Oct4 gene plays a crucial role in early development.
    • Limited maternal mRNA in embryos.
    • Transcription products from maternal and paternal chromosomes guide early development.

    Blastocyst Attachment

    • The zona pellucida disintegrates, exposing the blastocyst to the uterine wall for attachment.
    • Integrins in the trophoblast bind to collagen, laminin, and fibronectin in the endometrium.
    • Proteases secreted by the trophoblast enable the blastocyst to embed into the uterine wall.

    Tissue Formation in Early Mammalian Embryo

    • Trophoblast: Provides nutrients and develops into a significant part of the placenta.
    • Decidua: Uterine lining that forms the maternal placenta, influenced by progesterone.

    Tissue Types within the Trophoblast

    • Cytotrophoblast (Langerhans Layer): Inner layer of the trophoblast.
    • Syncytiotrophoblast: Epithelial covering of embryonic placental villi that invades the uterine wall to establish nutrient circulation between the embryo and the mother.

    Tissue Formation in Early Mammalian Embryo

    • Mesodermal tissue extends outward from the embryo, derived from the yolk sac and primitive streak cells.
    • The connecting stalk of extraembryonic mesoderm connects the embryo to the trophoblast, forming the umbilical cord vessels.

    Molecular Basis for Embryonic Development

    • Transcription Factors: Proteins that bind to the DNA of promoter/enhancer regions of specific genes.
      • They interact with RNA polymerase 2, regulating the amount of mRNA and gene products.

    Transcription Factor Types

    • Homeobox: Nucleotides that code for the homeodomain. Homeobox genes encode transcription factors regulating gene expression, particularly in segmentation and axis formation.
    • Hox Genes: Homoebox gene complex (humans & other vertebrates).
      • 39 homologous homeobox genes.
      • Involved in cranial-caudal patterning.
        • Antennapedia: Controls leg placement
        • Bithorax: Governs abdominal and posterior thoracic segments.

    Transcription Factor Types (Continued)

    • Pax Genes: Play a critical role in developing sense organs and the nervous system.
    • Sox Genes: Family of transcription factors sharing a common HMG (high mobility group) domain.

      • Bind to various nucleotides, causing structural changes.

      • E.g.: SRY gene.

      • Basic helix-loop-helix proteins: Class of transcription factors containing two alpha helices separated by a loop. Regulate myogenesis.

    • Zinc Finger Protein (ZnF): Family of transcription factors with cysteine and histidine bound by zinc ions, creating a finger-like structure.
      • Bind to DNA, RNA, and proteins.

    Transcription Factor Types (Continued)

    • Lim Proteins: Bind to DNA in the nucleus. Absence leads to headless mammalian embryos.
    • T-box Genes: Induce formation of the mesoderm layer and specification of forelimbs and hindlimbs.

    Transcription Factor Types (Continued)

    • Dlx Gene: Patterning of outgrowth structures.
      • Appendage development.
      • Morphogenesis of jaws and the inner ear.
    • Msx Gene: Involved in embryonic development and epithelial-mesenchymal interactions in limbs and face.
      • General inhibitors of cell differentiation during prenatal development.
      • Postnatal, they contribute to tissue proliferative capacity.

    Signaling Molecules

    • Also known as cytokines.
    • Affect neighboring or distant cells.
    • Include growth factors.
    • Bind as ligands to receptor molecules.

    Growth Factor Families

    • TGF-β Family: Consisting of 30 molecules.
      • Roles in mesodermal induction, myoblast proliferation.
      • Activin: Granulosa cell proliferation.
      • Decapentaplegic: Signaling in limb development.
      • Left: Determination of body symmetry.
      • Sonic hedgehog (Shh): Impacts gene expression in target cells.

    Growth Factor Families (Continued)

    • FGF (Fibroblast Growth Factor) Family: Diverse functions, including:
      • Cell migration
      • Proliferation
      • Differentiation
      • Survival
      • Essential roles in development, metabolism, and tissue homeostasis.

    Specific FGF Family Members

    • FGF-1: Keratinocyte proliferation, liver induction.
    • FGF-2: Hair growth, induction of renal tubules.
    • FGF-3: Inner ear formation.
    • FGF-4: Trophoblast mitotic activity.
    • FGF-5: Ectodermal placode formation.
    • FGF-8: Midbrain patterning, limb outgrowth, teeth induction, filiform papillae induction.
    • FGF-10: Limb induction, prostate gland morphogenesis.

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    Early Embryogenesis and Developmental Biology PDF

    Description

    This quiz covers the processes of cleavage and mitosis promoting factor (MPF) during embryonic development. It explores the rapid mitotic divisions that lead to the formation of blastomeres and the role of MPF in cellular transitions. Test your understanding of these critical biological concepts in embryology.

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