Chapter 47- Animal Development I. Embryonic development A. three interdependent & simultaneous processes 1. cell division a. mitosis of zygote: 1. large number of cells 2. differentiation a. cells diversify 3. morphogenesis=creation of form a. cell & tissue movement b. programmed cell death II. Fertilization A. two important processes happen here: 1. combine two haploid sets of chromosomes into single cell 2. activation of egg a. contact with sperm initiates metabolic reactions B. sea urchin example (figure 47.2) 1. early development similar to vertebrates 2. external fertilization a. sea urchins are broadcast spawners a. sperm cell contacts egg jelly coat 3. acrosomal reaction a. release of hydrolytic enzymes from acrosome in sperm b. enzymes make hole in jelly coat i. acrosomal process a. lock & key molecular mechanism i. so that only same species can fertilize the egg b. extends thru jelly coat c. binds to receptor on egg's vitelline layer ii. plasma of egg & sperm fuse iii. sperm nucleus enters egg cytoplasm a. electrical change in plasma i. membrane depolarization ii. =fast block to polyspermy a. 1-3 seconds to 1 min. b. blocks entry of other sperm i. prevents abnormal chromosome # 4. cortical reaction (takes place in cortex=outer zone) a. fusion of sperm & egg causes signal transduction pathway i. calcium released from ER of egg ii. cortical granules (vesicles) in egg fuse with plasma membrane iii. enzymes raise & harden vitelline layer a. becomes sperm-proof fertilization layer b. =slow block to polyspermy i. after 1 minute 5. activation of the egg a. increased calcium also triggers metabolic changes in egg i. higher cellular respiration & protein synthesis b. binding of sperm a trigger, provides no materials i. calcium alone will cause activation a. can provide start to parthenogenesis c. sperm nucleus starts to swell i. merges with egg nucleus= ZYGOTE! C. mammalian fertilization 1. internal, unlike sea urchins 2. capacitation a. secretions by female alter sperm molecules i. increase sperm motility 3. mammalian egg (actually secondary oocyte) a. coated with follicle cells released during ovulation i. capacitated sperm must get thru follicles to zona pellucida (ZP) a. ZP is extracellular matrix of oocyte i. made of three glycoproteins ii. ZP3 a. glycoprotein on zona pellucida b. also sperm receptor i. meets complementary molecule on sperm iii. connection betw ZP3 & sperm a. causes acrosome on sperm to release contents by exocytosis i. protein-digesting enzymes ii. allow sperm to enter ZP iii. sperm protein binds to egg a. =depolarization i. fast block to polyspermy iv. cortical granules released a. harden zona pellucida i. slow block to polyspermy b. fingerlike extensions on egg (=microvilli) i. take whole sperm into egg a. including tail b. basal body of sperm flagellum i. divides and forms centrioles of zygote c. unfertizled mammalian eggs have no centrioles of their own c. haploid nuclei of sperm & egg do not fuse immediately i. nuclear envelopes disperse ii. chromosomes share common spindle in 1st division IV. After fertilization 3 processes take place & establish body plan: A. cleavage 1. special rapid cell divisions 2. creates multicellular embryo called blastula 3. embryo does not grow in size i. cytoplasm is divided into ever smaller cells ii. zygote partitioned into many blastomeres a. each with own nucleus iii. division of cytoplasm not always homogeneous 4. polarity of eggs i. most animals have egg polarity a. but not mammals! ii. defined by concentration gradients of cytoplasmic components: a. mRNA b. proteins c. yolk i. nutrients stored in egg ii. key factor in egg polarity a. most concentrated at vegetal pole b. least concentrated at animal pole i. animal pole is site where most polar bodies are budded off during meiosis 5. polarity of zygote (hemispheres, rather than poles) a. determined by egg polarity b. yolk impedes cell division i. results in unequal cell size in some animals a. e.g., amphibians ii. cleavage occurs more in animal hemisphere a. different sized cells result c. zygotes with less yolk can still have polarity i. but cleavage is more equal across zygote ii. e.g., sea urchins 6. continued cleavage a. morula: mulberry i. solid ball of cells 7. blastocoel forms a. fluid filled cavity i. centrally located in sea urchins ii. animal hemisphere in frogs a. due to unequal cell division B. gastrulation- a morphogenic process (figure 47.9 & 47.10) 1. spatial rearrangement of cells in blastula 2. pattern differs by animal group 3. common set of cellular changes: a. in cell motility b. in cell shape c. in cellular adhesion to other cells 4. result is 3-layered embryo=gastrula a. embryonic germ layers: i. ectoderm a. outer layer=skin ii. endoderm a. lines digestive tract iii. mesoderm a. in between other 2 layers 5. example: sea urchin gastrulation (figure 47.9) a. blastula=single layer of cells around blastocoel b. vegetal plate forms at vegetal pole c. mesenchyme cells detach from vegetal plate i. migrate into blastocoel d. vegetal plate invaginates (buckles inward) e. mesenchyme cells form filopodia f. endoderm cells form archenteron i. becomes blastopore ii. will become anus g. filopodial connections form between archenteron & other side of blastocoel h. contraction of filopodia drags archenteron to blastocoel wall i. second opening ii. will become mouth j. gastrulation is complete k. mesenchyme will become skeleton 6. compare with frog gastrulation (figure 47.10) a. blastocoel is off-center b. cells on side burrow inward i. becomes blastopore c. cells roll inward i. become mesoderm & endoderm d. cells from animal pole cover surface i. will become ectoderm e. blastopore becomes circular f. archenteron & advancing cells become archenteron i. displacing blastocoel g. yolk plug left in blastopore h. archenteron not yet through to other side when organogenesis takes place C. organogenesis=process by which 3 germ layers become organs 1. 3 kinds of changes occur w/in layered tissues a. folds b. splits c. condensation