Mechanisms of Embryonic Development

Bio 162: Mechanisms of Embryonic Development

Overview of Embryonic Development Patterns

  • Similarities in both Animal and Land Plant embryonic development:
    • Polarity:
    • Establishes axes of symmetry.
    • In plants, this occurs at the two-cell stage.
    • In animals, this can occur either before fertilization or after, varying between groups.
    • Cleavage:
    • Growth occurs via cell division throughout embryo development.
    • Tissue specialization and organogenesis:
    • Involves the rearrangement of tissues to form distinct organs and organ systems.
Animal-Specific Patterns
  • Gastrulation:
    • Involves radical cell movements to reposition cells within the three germ layers, leading to the formation of a multi-layered organism.
    • Notably, plant cells cannot migrate due to the rigid structure of the cell wall.

Mechanisms Circulating Embryonic Development

  • Key molecular and cellular events in embryonic development include:
    • Patterning:
    • Differentiation in the expression of homeotic genes establishes specialized body regions and locates structures.
    • Cell fate and differentiation:
    • Involves differential gene expression and cell communication that provides positional information to cells across the embryo.
    • Results in cell determination and differentiation.
    • Cellular Signaling:
    • Involves signals sent between cells, where receiving cells respond through the activation or inhibition of different gene subsets.
    • Differential Gene Expression:
    • Expression of different subsets of genes leads to specialized cells that, while genetically identical, functionally and morphologically differ.

Basics of Gene Expression

  • External Signals:
    • External factors often trigger gene expression.
  • Transcription Factors:
    • Regulate the expression of target genes, possibly activating or inhibiting their expression.
  • Genotype vs Phenotype:
    • Genotype refers to the genetic makeup while phenotype describes observable traits.

Detailed Aspects of Differential Gene Expression

  • Differential Gene Expression:
    • Involves expressing different subsets of genes, resulting in specialized cells that are genetically identical yet functionally diverse.
    • Often triggered by external signals that provoke changes within the cell, such as releasing transcription factors.
    • Transcription and Translation Processes:
    • Transcription: The process whereby information is copied from DNA to RNA.
    • Translation: The conversion of RNA information into an amino acid chain (protein).
    • Production of specialized proteins contributing to polarity, cell determination, and signaling.

Induction Mechanisms in Development

  • Induction:
    • Secretion of chemical signals (morphogens) from one cell group to nearby target (responder) cells.
    • Inducer triggers a cascade of changes in responder cells, leading to further differential gene expression.
  • Factors Influencing Inductive Signal Response:
    • Inducer Concentration:
    • The graded response in target cells depends on the concentration of the inducer signal they receive.
    • Competence of Target Cells:
    • The ability of target cells to respond is influenced by the number and types of receptors for the inducer present on their surface.

Morphogen Concentration Gradients

  • Gradients:
    • Morphogen concentration gradients determine developmental patterns by providing positional information to embryonic plants and animals based on responder cell competency at varying concentration thresholds.

Consequences of Differential Gene Expression & Cellular Signaling

  • Polarity and Axis Determination:
    • Each cell receives molecular coordinates, helping to determine body axes:
    • In Animals:
      • Anterior-posterior, dorsal-ventral, and left-right axes.
    • In Land Plants:
      • Apical-basal axes.

Mechanisms Generating Polarity

  • Positional Information:
    • Strategies for generating polarity include:
    • Cytoplasmic Segregation:
      • Maternally-derived materials segregated before fertilization.
      • Regional localization of materials (e.g., mRNAs and proteins) during egg maturation.
    • Cell Division Orientation:
      • Specific orientation of planes to cause uneven division of materials.

Additional Positional Information Mechanisms

  • Communication and Signaling:
    • Beyond segregation, cell communication through signaling molecules (morphogens) establishes concentration gradients that yield varied responses from cells based on morphogen levels.

Determination and Cell Fate

  • Process of Determination:
    • Determination sets a cell’s fate and its descendants along a specific developmental pathway.
    • Initially, determined cells appear indistinguishable from others until Differentiation, where they achieve distinct identities.
  • Example of Germ Layer Differentiation in Animals:
    • Once polarity is established, differential gene expression leads to the formation of germ layers.

Determination and MyoD Gene Example

  • Determination:
    • The process that sets a cell's overall fate, as exemplified by the activation of the myoD gene, which produces the MyoD transcription factor.
  • Differentiation:
    • MyoD acts upon various genes initiating a signal cascade involving additional transcription factors.
    • Differentiation manifests as cells become specialized, transforming into desirable cell types.

Differentiation and Tissue Repair Mechanisms

  • Animal Tissue Repair:
    • Utilizes stem cells that can migrate to damaged sites and differentiate into appropriate cell types.
  • Plant Repair Mechanisms:
    • Involves cells that can de-differentiate into meristematic cells, then re-differentiate to replace structures.
  • Vegetative Propagation Mechanism:
    • Illustrates this repair strategy in plants.

Homeotic Genes and Patterning

  • Homeotic Genes:
    • Master control genes initiate cascades of gene expression affecting timing and protein synthesis across numerous genes.
    • Malfunction of these genes can lead to structural abnormalities, such as the formation of extra organs or total absence of organs.
  • Types of Homeotic Genes:
    • Include Hox genes (specific to animals) and Organ Identity Genes (OIGs) (specific to plants).

Special Characteristics of Hox Genes

  • Collinearity of Hox Genes:
    • The order of Hox genes on chromosomes corresponds to their activation positions along the body axes.
  • Conservation Across Species:
    • Hox proteins are highly conserved, which allows them to substitute across distantly related species, highlighting common ancestry approximately 670 million years ago.

Evolutionary Developmental Biology (EvoDevo)

  • Understanding how developmental mechanisms evolve to form different biological structures over time.
  • Involves creating novel body plans using similar materials, genes, and regulators.
    • Particularly relevant among multicellular eukaryotes.
  • Gene Expression Regulation Impact:
    • Variations in spatial expression, timing, and dosage of gene expression can lead to different phenotypes.

Developmental Toolbox

Heterotopy
  • Heterotopy:
    • Refers to differences in gene expression location ("where").
    • Example: Cervical versus Thoracic vertebrae in birds and mammals, including variance in expression of Hoxc6 affecting vertebrae count and morphology.
Heterochrony
  • Heterochrony:
    • Specifies differences in the timing of gene expression ("when").
    • Example: Giraffes’ delayed maturity regarding bone growth leads to proportionally longer cervical vertebrae compared to other mammals, managed by small regulatory changes.
Heterometry
  • Heterometry:
    • Addresses differences in the level of gene expression ("how much").
    • Example: Beak depth and length in certain bird species, driven by expression levels of BMP4 for depth and CaM for length.

Plant Patterning: Flower Structure

  • Flower Organization:
    • Composed of four whorls:
    • Carpels (uppermost whorl)
    • Stamens with anthers
    • Petals
    • Sepals

Organ Identity Genes (OIGs)

  • Types of OIGs: A, B, and C
    • Expression regulated by transcription factors (TFs): polypeptides categorized as A, B, or C.
    • Dimers of two polypeptides act as TFs initiating cascades of differential expression leading to distinct phenotypes.
    • Specific expressions:
    • Sepals express only A.
    • Petals express A & B.
    • Stamens express B & C.
    • Carpels express C.