Biol 118 1st Edition Lecture 21 Outline of Last Lecture I Shared Developmental Processes II Genetic Equivalence Differential Gene Expression III Regulatory Genes IV Segmentation Genes V Regulatory Cascade VI Conservation of Hox Gene Function Outline of Current Lecture I Introduction II Fertilization III Cleavage IV Gastrulation V Organogenesis VI Cell Differentiation Current Lecture Introduction Gametes Haploid reproductive cells o Sperm Male gametes o Eggs Female gametes Ordered phases of development o Fertilization o Cleavage o Gastrulation These 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 Organogenesis Fertilization Fertilization occurs when a haploid sperm egg fuse forming a diploid zygote fertilized egg Steps o Sperm are drawn to the egg o The head of the sperm binds to the jelly layer of egg o Triggers acrosome reaction which releases sperm contents o Flagella activity ramps up o Sperm penetrates egg coat o Membrane fusion of sperm and egg o Sperm nucleus enters egg Bindin protein on the head of sea urchin sperm that binds to the surface of sea urchin eggs o Acts as a key so a sperm binds to only eggs of the same species Polyspermy Fertilization by more than one sperm o In sea urchins fertilization stimulates creation of physical barrier Ca 2 based signal is induced rapidly propagated fertilization envelope forms which keeps away additional sperm o In mammals cortical granules release enzymes that modify egg cell receptors preventing binding by additional sperm Cleavage Cleavage the set of rapid cell divisions that take place in zygotes immediately after fertilization o 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 cytoplasm o End up in specific populations of blastomeres Cleavage initiates the step by step process that results in the differentiation of cells In mammals o Occurs in oviduct connects the ovary to the uterus o 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 placenta o The ICM contains the cells that undergo gastrulation and develop into an embryo Gastrulation Extensive and highly organized cell movements radically rearrange the embryonic cells into a structure called the gastrula Results in the formation of embryonic tissue layers o Tissue integrated set of cells that function as unit o 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 blastula o Gastrulation begins with formation of blastopore opening At end of gastrulation o Three embryonic tissues are arranged inlayers o The gut has formed o Major body axes have become visible Organogenesis 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 type o Have distinctive structure function Early in organogenesis notochord appears in dorsal mesoderm o Unique to chordates vertebrates o Functions as a key organizing element o Undergoes apoptosis eventually Notochord triggers reorganization of dorsal ectodermal cells forms neural tube o Precursor to the brain spinal cord After neural tube mesodermal cells organize into blocks of tissues called somites o Form on both sides of neural tube down the length of body o Can form a variety of structures but aren t initially determined o Eventually become irreversibly determined and differentiate into specific cell type based on their location within the somite Determination Determination Somite cells differentiate in response to signals from nearby tissues o 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 proteins o MyoD protein that causes muscle cell differentiation Regulatory transcription factor o 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 begin MyoD production targeting these cells as belonging to specific muscles Muscle cells begin expressing muscle specific proteins
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