Biology of Stem Cells
Biology of Stem Cells MCDB 4615
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This 3 page Class Notes was uploaded by Mrs. Willis Mante on Thursday October 29, 2015. The Class Notes belongs to MCDB 4615 at University of Colorado at Boulder taught by Staff in Fall. Since its upload, it has received 29 views. For similar materials see /class/231844/mcdb-4615-university-of-colorado-at-boulder in Molecular, Cellular And Developmental Biology at University of Colorado at Boulder.
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Date Created: 10/29/15
MCDB 4620 VERTEBRATE DEVELOPMENTAL BIOLOGY LECTURE NOTES 9302 Lecture 3 Fertilization and Cleavage Reading Gilbert pp 185192 198199 204206 223224 303304 345347 354358 The mature unfertilized animal egg is like a factory closed for the night it is stocked with energy reserves and is ready to spring into action any moment The recognition of and binding to the egg surface by the sperm is the signal that begins development When the sperm contacts the egg there is a rapid programmed response by both egg and sperm that results in 0 activation of the egg s metabolism transmembrane signals that block further sperm entry triggering of lipid and macromolecular synthesis in preparation for cleavage reinitiation of the cell cycle fusion of sperm and egg nuclei to produce a diploid zygote with a combination of maternal and paternal genetic information The egg There are two arrests in egg maturation Arrest 1 after replication at end of prophase of 1st meiotic division corresponding to the G2 checkpoint Arrest time is variable months in amphibia up to years in many mammals During this time the egg ovum grows and accumulates proteins and RNA39s Its large distinctive nucleus is the called the germinal vesicle GV First arrest is broken by progesterone from the follicle cells surrounding the egg Egg then continues through meiosis I Arrest 2 The timing of the second arrest varies depending on the organism 75 For most it is at the M phase of meiosis II interesting exceptions in dogs and foxes arrest is at the end of the first meiotic prophase so that both metaphases occur after fertilization Sperm egg interactions in mammals Summary figure 78B 1 Binding of sperm to egg zona pellucida Egg zona pellucida composed of3 glycoproteins ZPl ZP2 ZP3 ZP3 carbohydrates bind sperm 7 17 Many sperm proteins are candidates for egg binding proteins such as sp56 galactosyl transferase and zona receptor kinase However it is currently unclear which of these proteins is important for egg binding in vivo 2 Acrosome reaction in sperm induced by ZP3 7 l l Increases in calcium mediate the reaction Acrosome reaction necessary for sperm to reach and bind the egg plasma membrane Secondary binding of sperm to ZP2 3 Sperm binds and fuses with the egg plasma membrane Fertilins alpha and beta on the sperm membrane bind the egg membrane Fertilin alpha also may play a role in membrane fusion The cell surface molecule CD9 on the egg membrane is required for fusion with sperm 4 Egg activation Cortical granule reaction block to polyspermy ZP proteins modified such that acrosome reaction cannot take place 720 Mediated by rise in calcium causes release of meiotic arrest Ultimately results in fusion of egg and sperm pronuclei and initiation of development Cleavage The initial cell divisions quotcleavagesquot subdivide the egg and produce a multicellular embryo the blastula stage embryo In addition the first distinctions between different cell populations in the embryo are observed Spindle orientation determines cleavage plane orientation Control of this process must result from control of spindle orientation since cleavage plane orientation is dictated by the spindle Spindle orientation is determined by positioning of the centrosomes not understood Because of the way the centrosomes divide the spindle orientation at each cleavage is usually though not always orthogonal to the orientation of the preceding cleavage Non uniformity of egg cytoplasm gives the embryo a polarity Yolk content in the cytoplasm a source of nutrients is nonuniform in many eggs Eggs with yolk in one region of the egg have an established polarity prior to fertilization In contrast in mammalian eggs there appears to be little cytoplasmic nonuniformity and patterning is initiated at later stages Yolk distribution and extent of cleavages Whether cleavages go entirely through the embryo holoblastic or only partially meroblastic depends partly on the amount and distribution of yolk in the egg As you will see eggs differ markedly in this respect 85 by species Establisment of cell identities is dependent on cytoplasmic determinants or signaling Because organization of the egg cytoplasm occurs during oogenesis or just after fertilization the partitioning transmits maternally supplied information to the embryo thus differential partitioning of localized factors can give blastomeres different developmental potentials Although this is more generally true in invertebrate embryos there are some cytoplasmic determinants in vertebrates as well For example in the unfertilized Xenopus egg Dsh a member of the Wnt pathway is localized to the vegetal hemisphere of the embryo more on this later In mammals however there is apparently little or no prepatteming of the cytoplasm Patterning of the embryo depends entirely on cell signaling rather than segregated determinants Mammals While the embryo cleaves it moves along the oviduct to the uterus Interestingly unlike most other animals the embryonic genome begins to function as early as the 2cell stage Cleavage is rotational asynchronous and slow 12 to 24 hours per division 1121 While the egg is dividing and forming the blastocyst it is moving down the fallopian tube to the uterus where it will implant 1120 Blastulation At the 8 cell stage the loosely connected blastomeres undergo compaction 1122 The cells are now tightly packed together and the outside cells are connected via tight junctions preventing leakage of ions and small molecules At the 16cell stage there are only two inner cells all the rest are outer cells connected by tight junctions At this point the embryo is called a morula At about the 30 cell stage the blastocoele cavity forms and the embryo is organized into the inner cell mass which will form the embryo and some extraembryonic tissues and the trophoblast the outer ring of tightly joined epithelial cells which will form the chorion embryonic part of the placenta The inner cell mass then reorganizes into two layers the epiblast embryonic tissues only and hypoblast extraembryonic Chick quotCleavage is 39 39 quot 39 r39 th yolk does not cleave Fig ll8E and discoidal the cleaving cells lie on top of the yolk in the form of a at circular disk cleavage divisions are synchronous for the rst four only then become asynchronous as the cells divide and become several layers thick they secrete uid towards the yolk creating the subgerminal cavity that now separates the future embryo from the yolk After several cleavages the group of cells sitting atop the yolk is called the blastoderm These cells are linked by tight junctions characteristic of epithelial cells Blastulation The blastula is distinguishable from the cleavage stage embryo in that it has formed two layers an upper layer the epiblast and a lower layer the hypoblast The hypoblast forms by delamination of cells from the epiblast which join other migrating cells from the posterior marginal zone of the embryo only 119 The epiblast starts off as an epithelial sheet and will go on to form the three germ layers The hypoblast makes the extraembryonic membranes These membranes also exist in mammals but not in amphibia They are supporting membranes that accomplish a variety of important tasks such as waste storage the allantois respiration chorion and protection amnion The space between epiblast and hypoblast is the blastocoel not the subgerminal cavity Amphibia Amphibian eggs have yolk concentrated in the vegetal hemisphere so that cell divisions proceed more rapidly in the animal hemisphere 101 This leads to the production of large vegetal macromeres and small animal micromeres a blastocoele cavity becomes apparent at about the 32cell stage although it actually begins to form earlier There are no extraembryonic membranes all the cells become part of the embryo and unlike chicks even yolk lled cells divide Embryonic transcription does not begin until the midblastula transition many maternal RNAs and proteins are loaded into the egg so transcription is not initially required
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