Lectures 29-30: Developmental Biology

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  • Developmental Biology (Lec. 29/30):
    • How does a single cell become a complex multicellular organism?
      • Proliferation: incr. cell number
      • Apoptosis: selective cell death, ex. to form hands
      • Differentiation: change cell identity, i.e. which genes/ level of expression
        • Due to genetic lineage of cell
        • Due to environment of cell
          • Differentiate due to signalling molecules
            • Ex. Xenopus (frog embryo) and activin signalling molecule
              • Activin can bind receptors locally or distally, phos. activates SMAD dimer which binds specific DNA promoter sequences
              • Expression of genes is dose-dependent; for different activin amts, the xenopus blastomere formed skin, heart muscle, blood, etc.
              • Activin found in vegetal cells
          • Differentiate due to physical environment, ex. substrate, pH, temp
      • Morphogenesis: change cell shape/ physics
        • Ex. Mouse embryo development
          • 1. 8-cell embryo forms polarity and divides, forming a 16-cell outwardly-polar, inwardly-apolar embryo
          • 2. Blastocyst: cavitation of inside material
            • ICM and TE differentiate here by Cdx2 (homeobox gene) expression
              • Cdx2 is necessary to form TE, without which the ICM cannot hatch out of the zona to implant in uterus
          • 3. Inner cell mass (ICM) separates to one side of the cavity, surrounded by trophectoderm (TE)
            • ICM becomes embryo
            • TE becomes placenta
          • 4. Primitive endoderm (PrE) forms, hatches from zona, implants in uterus
          • 5. Post-implantation: proximal-distal polarity determined by distal visceral endoderm (VE) cells
            • VE cells secrete nodals (similar to activin signals) to migrate up the side of embryo
              • Polarity changes from prox-dist to anterior-posterior
          • 6. Anterior VE expresses inhibiting signals to induce head formation genes
      • Migration: change cell location


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