Lecture'16'March'25,'2015'From'single'cell'to'organism'''Reading:''Chapter'22'Molecular'Biology'of'the'Cell'(MBOC),'pages'1305E1320.''Synopsis:''Developmental'Biology'seeks'to'understand'how'a'fertilized'egg'becomes'a'functioning'organism.'We'discussed'some'of'the'goals'of'the'second'half'of'the'course'and'listed'some'of'the'major'processes'that'developmental'biology'studies.'We'asked'why'we'study'developmental'biology.''In'addition'to'the'strong'motivation'provided'by'intellectual'curiosity,'we'briefly'mentioned'medical'and'agricultural'benefits.''We'study'developmental'biology'through'description,'perturbation'(both'experimental'and'genetic),'and'use'tools'of'cell'biology,'biochemistry'and'molecular'biology.''Most'studies'in'developmental'biology'rely'on'a'handful'of'model'organisms.''The'reason'we'can'learn'general'truths'about'developmental'biology'by'studying'diverse'animals'is'that'the'fundamental'mechanisms'and'molecules'that'drive'development'in'all'of'these'animals'are'highly'conserved.''As'many'as'half'of'the'genes'in'each'of'the'animal'model'organisms'are'present'in'humans'and'carry'out'similar'functions'in'both'species.''The'extent'of'this'conservation'is'demonstrated'by'the'ability'of'the'engrailed'gene'from'fruit'flies'to'replace'the'homologous'gene'from'mouse.''(This'gene'plays'an'important'role'in'pattern'formation'in'both'organisms;'pattern'formation'is'discussed'below.)''This'course'will'focus'on'the'development'of'animal'embryos,'a'process'called'embryogenesis.''The'major'events'of'embryogenesis'are:'''Fertilization,'in'which'the'egg'and'sperm'unite'to:'1)'create'a'genetically'unique'diploid'individual'through'the'fusion'of'their'haploid'nuclei'to'form'the'diploid'zygote.''2)'initiate'embryogenesis'by'activating'the'egg.''Cleavage,'in'which'the'newly'formed'embryo'undergoes'many'rounds'of'cell'division'without'growth.''During'cleavage'M'follows'S'without'intervening'G1'and'G2'phases.''Cleaving'cells'are'called'blastomeres.''At'the'end'of'the'cleavage'stage'the'embryo'is'referred'to'as'a'blastula.'''The'blastula'consists'of'large'numbers'of'cells'that'are'organized'as'a'hollow'ball'of'cells.''(The'organization'of'embryos'at'this'stage'can'vary'–'not'all'are'hollow'balls).'''''Gastrulation,'in'which'the'cells'of'the'blastula'undergo'a'dramatic'reorganization'which'results'in'the'formation'of'three'distinct'tissues:'ectoderm'on'the'outside,'endoderm'on'the'inside,'and'mesoderm'between'the'two.''Each'of'these'tissues'will'give'rise'to'unique'subsets'of'cell'types'in'the'embryo.''We'briefly'examined'gastrulation'in'sea'urchins'as'an'example'of'this'process.''''Embryogenesis: development of animal embryos. Steps: fertilization cleavage gastrulation neurulation organogenesisNeurulation,'is'a'process'that'forms'the'nervous'system'in'all'vertebrates,'but'not'invertebrates.'In'neurulation,'a'specific'region'of'the'ectoderm,'called'the'neural'plate,'folds'along'the'dorsal'midline'and'separates'from'the'rest'of'the'ectoderm'to'form'the'neural'tube,'the'precursor'to'the'central'nervous'system,'and'the'neural'crest,'which'will'give'rise'to'a'number'of'other'tissues'including'the'peripheral'nervous'system.''(You'can'learn'about'neural'crest'in'more'advanced'courses.)'''Organogenesis,'the'process'that'results'in'the'formation'of'specific'organs'such'as'lung,'stomach,'eye'or'limbs.'''Changes'in'cellular'behavior'provide'the'basis'for'development.''The'major'cellular'behaviors'include'cell'proliferation,'specialization'(differentiation),'cell'movement,'cell'interaction,'cell'communication,'cell'growth'and'cell'shape'changes.''We'considered'five'major'questions'of'developmental'biology'including:''1.'What'makes'cells'different?''A'partial'answer'to'this'question'is'differential;gene;expression,'the'understanding'that'all'cells'have'the'same'DNA'sequence'but'that'different'cell'types'express'different'genes,'and'therefore'have'different ' protein'compositions,'cellular'organization'and'behaviors.''We'considered'that'there'are'two'different'aspects'to'this'question:''First,'what'is'the'nature'of'the'switches'that'turn'genes'on'and'keep'them'on'and'what'are'the'events'that'cause'some'switches'to'be'on'while'other'switches'are'off.'!'2.'How'do'cells'come'to'be'different'in'an'organized'and'reproducible'way?''One'answer'to'this'question'comes'from'studies'of'regional;specification'or'axis;formation,'the'process'by'which'embryos'determine'which'cells'will'give'rise'to'head'and'tail'(anterior/posterior'axis)'back'and'bel ly'(dorsal/ventral'axis)'and'left'and'right'sides'(left/right'axis).''''3.'How'do'cells'organize'into'tissues'and'org ans?''This'question'deals'with'the'process'of'morphogenesis,'the'many'different'events'that'lead'to'the'precise'shapes'and'sizes'of'the'parts'of'the'body.''Gast ru