Biol 118 1st Edition Lecture 21 Outline of Last Lecture I. Shared Developmental ProcessesII. Genetic Equivalence & Differential Gene Expression III. Regulatory GenesIV. Segmentation GenesV. Regulatory CascadeVI. Conservation of Hox Gene FunctionOutline of Current Lecture I. IntroductionII. FertilizationIII. Cleavage IV. GastrulationV. OrganogenesisVI. Cell DifferentiationCurrent LectureIntroduction- Gametes: Haploid reproductive cellso Sperm: Male gameteso Eggs: Female gametes- Ordered phases of developmento Fertilizationo Cleavage o GastrulationThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.o OrganogenesisFertilization- Fertilization: occurs when a haploid sperm & egg fuse, forming a diploid zygote (fertilized egg)- Steps:o Sperm are drawn to the eggo The head of the sperm binds to the jelly layer of eggo Triggers acrosome reaction, which releases sperm contentso Flagella activity ramps upo Sperm penetrates egg coat o Membrane fusion of sperm and eggo Sperm nucleus enters egg- Bindin: protein on the head of sea urchin sperm that binds to the surface of sea urchin eggso Acts as a key so a sperm binds to only eggs of the same species- Polyspermy: Fertilization by more than one spermo In sea urchins, fertilization stimulates creation of physical barrier Ca 2+ based signal is induced & rapidly propagated fertilization envelope forms, which keeps away additional spermo In mammals, cortical granules release enzymes that modify egg-cell receptors, preventing binding by additional spermCleavage- Cleavage: the set of rapid cell divisions that take place in zygotes immediately after fertilizationo First step in embryogenesis: process that makes a single-celled zygote into a multicellular embryo- Partitions the egg cytoplasm without any additional growth of the zygote- Blastomeres: the cells created by cleavage divisions o Blastula: mass of blastomere cells- Cytoplasmic determinants are found in specific locations within the egg cytoplasmo End up in specific populations of blastomeres- Cleavage initiates the step-by-step process that results in the differentiation of cells- In mammalso Occurs in oviduct: connects the ovary to the uteruso Results in a blastocyst: a specialized blastula consisting of two populations of cells The external, thin-walled hollow trophoblast surrounds the inner cell mass (ICM)o After blastocyst embeds in the uterine wall, a mixture of trophoblast & maternal cells form the placentao The ICM contains the cells that undergo gastrulation and develop into an embryoGastrulation- Extensive and highly organized cell movements radically rearrange the embryonic cells into a structure called the gastrula- Results in the formation of embryonic tissue layerso Tissue: integrated set of cells that function as unito 3 primary tissue layers: Ectoderm (outer), mesoderm (middle) & entoderm (inner) Called germ layers: give rise to adult tissues and organs- In frogs:o Blastocoel: fluid-filled space in blastulao Gastrulation begins with formation of blastopore opening- At end of gastrulation:o Three embryonic tissues are arranged inlayerso The gut has formedo Major body axes have become visibleOrganogenesis- Organogenesis: Process of tissue & organ formation- Begins once gastrulation is complete & the embryonic germ layers are in place- Cells proliferate and become differentiated: become a specialized cell typeo Have distinctive structure & function - Early in organogenesis, notochord appears in dorsal mesodermo Unique to chordates (vertebrates)o Functions as a key organizing element o Undergoes apoptosis eventually- Notochord triggers reorganization of dorsal ectodermal cells & forms neural tubeo Precursor to the brain & spinal cord- After neural tube, mesodermal cells organize into blocks of tissues called somiteso Form on both sides of neural tube down the length of bodyo Can form a variety of structures but aren’t initially determinedo Eventually become irreversibly determined and differentiate into specific cell type based on their location within the somiteDetermination- Determination: Somite cells differentiate in response to signals from nearby tissueso Signals diffuse away from cells in the notochord, the neural tube & nearby ectoderm and mesoderm to act on specific populations of target cells in the somite- In muscle cells:o Myoblast: cell that is determined to become a muscle cell but has not begun producing muscle-specific proteinso MyoD: protein that causes muscle cell differentiation Regulatory transcription factoro Steps: Fertilization triggers cleavage, resulting in a blastocyst Blastocyst cell signals trigger gene cascades, resulting in positional changes during gastrulation During organogenesis, the notochord signals specific somite cells to beginMyoD production, targeting these cells as belonging to specific muscles Muscle cells begin expressing muscle specific
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