Chapter 23: Development:- Development goes through an ordered phase in animal life cycle:o Fertilization, Cleavage, Gastrulation and organogenesiso Fertilization occurs when haploid sperm/egg cells fuse (to create diploid zygote) Sperm drawn to the egg Head of sperm binds to jelly layer of egg Triggers acrosome reaction (releases sperm contents) Flagella activity ramps up Sperm penetrates egg coat Membrane fusion of sperm and egg Sperm nucleus enters egg SEA URCHINS MODEL FOR FERTILIZATION - Undergoes external fertilization- Bindin: Protein on head of sea urchin sperm that binds to surface of sea urchin eggs (acts as key so that sperm only binds to eggs of same species) Interactions required for plasma membranes of sperm and egg to fuse - Animals have different mechanisms to avoid polyspermy (fertilization by more than one sperm)o Sea Urchins: fertilization stimulates creation of physical barrier (Ca+2 based signal is induced and propagated throughout the egg; fertilization envelope forms so no additional sperm is present) o Mammals: Cortical granules release enzymes that modify egg-cell receptors (prevents binding by additional sperm) Cleavage: Set of rapid cell divisions that happen in animal zygotes immediately after fertilization - First step in embryogenesis (makes single celled zygote into multicellular embryo)- Cleavage partitions egg cytoplasm without additional growth of zygoteo Blastomeres: Cells created by cleavage divisions When cleavage is complete, embryo has a mass of blastomere cells called a blastulao Cytoplasmic determinants are found in specific locations within egg cytoplasm. Ends up in specific populations of blastomeres, cleavage initiates step by step process that resultsin differentiation of cells Cleavage occurs in mammalian oviduct, connects ovary to uterus (where embryo develops) Cleave results in blastocyst, specialized blastula consisting of two populations of cells (external, thin walled hollow trophoblast surrounds inner cell mass (ICM) After blastocyst embeds in uterine wall, mixture of trophoblast and maternal cells form placenta (which provides nourishments and rids waste for embryo) ICM contains cells that undergo gastrulation and develops into the embryo - Gastrulation: Extensive/highly organized cell movements radically rearrangeembryonic cells into structure called the gastrula o Results information of embryonic tissue layerso Most early embryos have three primary tissue layers (ectoderm, mesoderm and endoderm AKA germ layers because they make adult tissues/organs) Arranged in layers o In Frog embryos: Frog blastula contains fluid filled space called blastocoel Gastrulation begins with formation of opening called blastopore cells from the periphery move inward through blastopore, forming tube-like structure that will eventually become the gut o Ectoderm: Forms outer covering of adult body/nervous systemo Mesoderm: Gives rise to muscle; most internal organs and connective tissues (i.e. bone/cartilage) o Endoderm: Produces lining of digestive tract/gut as well as some other organs o Major body axes also become visible and gut has formed through gastrulation - Organogenesis: Cells proliferate/become differentiated o Rod like notochord appears in dorsal mesoderm (unique to chordates) Notochord: Key organizing element but undergoes apoptosis Signals from notochord trigger reorganization of dorsal ectodermal cells (leading to neural tube formation Precursorto brain/spinal chord) THEN mesodermal cells become organized into blocks of tissues called somites (form on both sides of neural tube down the length of the body) - Somite cells form variety of structures but not determined. o As somite matures, they become irreversibly determined (differentiates in response to signals from nearby tissues; signals diffuse away from cells in the notochord, the neural tube and nearby ectoderm/mesoderm to act on specific populations of target cells in the somite)- Myoblast: Cell that’s determined to become muscle cell but hasn’t produced muscle specific proteins yeto myoD: Protein that causes muscle differentiation(Regulatory transcription factor, binds to enhancers upstream of muscle specific
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