Embryonic Development: Cleavage and Mitosis

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.