U of M BIOLOGY 4361 - Fertilization (16 pages)

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Fertilization



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Fertilization

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16
School:
University of Minnesota- Twin Cities
Course:
Biology 4361 - Developmental Biology
Developmental Biology Documents
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Biology 4361 Developmental Biology Fertilization October 19 2006 Fertilization fusion of sperm and egg accomplishes A sex combining genes derived from two genomes B reproduction transmits genes from parents to offspring initiates reactions in the egg cytoplasm that allows development to proceed Generally four major events occur 1 Contact and recognition between sperm and egg also ensures that egg and sperm are of the same species 2 Regulation of sperm entry into the egg only one sperm can fertilize usually only one sperm is allowed to enter the egg others are excluded 3 Fusion of the genetic material of the sperm and egg 4 Activation of egg metabolism to start development Structure of the Gametes Sperm rather obscure took a bad rap for a long time e g von Leeuwenhoek co discoverer of sperm considered sperm to be parasites in the semen this parasite idea was popular through many generations of scientists note that sperm were discovered in 1676 but their role in fertilization was not fully elucidated until 1876 200 years a long time to wait Oscar Hertwig Herman Fol independently demonstrated sperm entry into the egg and union of cells nuclei Sperm structure head contains streamlined haploid nucleus DNA in pseudo crystalline form with protamines transcriptionally inactive acrosome acrosomal vesicle Golgi derivative contains enzymes that digest proteins and complex sugars used to lyse outer coverings of the egg modified secretory vesicle many species contain region of globular actin proteins between sperm nucleus and acrosomal vesicle 1 g actin used to extend acrosomal process filamentous actin during early stages of fertilization recognition between sperm and egg i e species specificity molecules located in on the acrosomal process Flagellum tail contains axoneme microtubules tubulin emanate form the basal centriole 9 2 construction 2 central microtubules 9 doublet microtubules one mt of doublet is complete 13 protofilaments second mt of doublet is incomplete C shaped 11 protofilaments NOTE 9 2 arrangement with dynein arms conserved in axonemes throughout the eukaryotic kingdoms dynein attached to microtubules motor protein ATPase dynein activity causes outer mt to slide past inner causing flagellar bending midpiece mitochondria mammals several in a spiral around axoneme sea urchins one large mitochondrion in a ring around axoneme Egg ovum all material necessary for the beginning of growth and development must be stored in the egg eggs actively accumulate material as they develop NOTE volume of eggs v sperm sea urchin egg 2 x 10 4 mm3 200 picoliters 200X volume of the sea urchin sperm proteins yolk made in other organs mostly liver ribosomes and tRNA burst of protein synthesis after fertilization mRNA encodes proteins for use in the early stages of development messages remain repressed until after fertilization morphogenic factors direct the differentiation of cells into certain types transcription factors paracrine factors localized regionally segregated into different cells during cleavage protective chemicals 2 DNA repair UV filters antibodies alkaloids defensive in many most species the final stages of egg meiosis take place while the sperm s nuclear material male pronucleus is traveling toward what will become the female pronucleus Recognition of egg and sperm The interaction of egg and sperm generally proceeds according to five basic steps 1 The chemoattraction of the sperm to the egg by soluble molecules secreted by the egg 2 The exocytosis of the acrosomal vesicle to release its enzymes 3 The binding of the sperm to the extracellular envelope vitelline layer or zona pellucida of the egg 4 The passage of the sperm through the extracellular envelope 5 Fusion of egg and sperm cell membranes NOTE sometimes steps 2 and 3 are reversed e g mammalian fertilization sperm binds to extracellular matrix of the egg before releasing acrosomal contends after these 5 steps sperm and egg pronuclei meet and development is initiated External Fertilization in Sea Urchins Fertilization challenges how to bring two very small cell together in a large place how to ensure that only sperm and eggs of the same species join Sperm attraction Action at a distance Species specific sperm attraction documented in numerous species including cnidarians molluscs echinoderms urochordates chemotaxis sperm are attracted towards eggs of their species sperm follow chemoattractant gradient Orthopyxis caliculata cnidarian regulates taxis movement and timing of sperm prior to 2nd meiotic division no chemotaxis after 2nd meiotic division chemotaxis chemotaxis mechanisms molecular agents can be different even in closely related species need to be sea urchins motility acquired shortly after exposure to seawater in testes pH is kept low pH 7 2 by high CO2 content in seawater pH elevated to 7 6 results in activation of dynein ATPase flagellar motor protein 3 chemotaxis guided by egg derived peptides e g resact Resact 14aa peptide isolated from egg jelly of Arbacia punctulata sperm in sea water swim in circles add resact sperm congregate resact specific for A punctulata will not activate sperm from Strongylocentrotus purpuratus S purpuratus chemotactic peptide speract A punctulata sperm have resact receptors in membrane resact binding activates guanylyl cyclase activity in cytoplasmic side of receptor cGMP activates a calcium channel Ca2 influx provides directional cue NOTE a single resact molecule can provide directional information for sperm swim up a concentration gradient until they reach the egg resact also acts as a sperm activating peptide resact respiration activating increased mitochondrial respiration and sperm motility increase in cGMP and Ca2 activates mitochondrial ATP generating apparatus and dynein ATPase that stimulates flagellar movement Acrosome reaction in marine invertebrates AR has two components 1 Fusion of the acrosomal vesicle with the sperm cell membrane exocytosis results in the release of acrosomal contents 2 Extension of the acrosomal process sea urchin AR initiated by contact of sperm with egg jelly specific complex sugar in the egg jelly polysaccharide bind to specific receptors located on the sperm cell membrane directly above the acrosomal vesicle egg jelly polysaccharides often highly specific to each species components from one species fail to activate AR in closely related species in S purpuratus AR initiated by repeating polymer of fucose sulfate sulfated carbohydrate binds to its sperm


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