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Chapter 47 : Animal Development Terminology Review • Development is determined by: – zygote’s genome – cytoplasmic determinants: molecules in the egg • Cell differentiation: specialization of cells in structure and function • Morphogenesis: process by which an animal takes shape • Model organisms: species representative of a larger group and easily studied, ex. Drosophila, sea urchin, frog Principles of differentiation: 1. Genomic Equivalence: our cells have the same genes, but don’t have the same genes “on” 2. During early cleavage divisions, embryonic cells must become different from one another – If the egg’s cytoplasm is heterogenous, dividing cells vary in the cytoplasmic determinants they contain 3. Induction: process where interactions among embryonic cells influence their fate, via differential gene expression and cell-cell communication Fate Mapping • Fate maps = territorial diagrams of embryonic development • Frog studies indicate cell lineage in germ layers traces to blastula cells • In many species that have cytoplasmic determinants, only the zygote is totipotent Fig. 47-21 Epidermis Epidermis Central nervous system 64-cell embryos Notochord Blastomeres injected with dye Mesoderm Endoderm Blastula (a) Fate map of a frog embryo Neural tube stage (transverse section) Larvae (b) Cell lineage analysis in a tunicate Formation of the Vertebrate Limbs • Pattern formation: development of spatial organization • Positional information: molecular cues that control pattern formation – Tells a cell where it is with respect to the body axes Major limb-bud organizer regions • Apical ectodermal ridge (AER): thickened area at bud tip, allows outgrowth of the limb on the proper axes • Zone of polarizing activity (ZPA): block of tissue that organizes development of limb “sides” (i.e. littler finger vs. thumb) • Hox genes: “architect genes” – Polysyndactyly (“many fingers joined together”) is a limb disorder caused by a Hox mutation