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Chapter  47  

Animal  Development  

BIOL  223  

•  Embryonic  development    

•  determined  by  the  zygote’s  genome    

•  and  molecules  in  the  egg  called  

cytoplasmic  determinants  

•  Cell  differen2a2on    

•  specializaBon  of  cells  in  structure  and  

funcBon  

•  Morphogenesis    

•  process  by  which  an  animal  takes  

shape  

Embryonic  Development  

Stages  of  embryonic  development  •  Three  stages  of  development  

•  Cleavage  

•  cell  division  creates  a  hollow  ball  of  cells    

•  called  a  blastula  

•  Gastrula2on  

•  cells  are  rearranged  into  a  three-­‐layered  gastrula  

•  Organogenesis  

•  the  three  layers  interact  and  move  to  give  rise  to  organs  

•  Due  to  posiBonal  informaBon  

PLAY

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FerBlizaBon  

•  FerBlizaBon    

•  brings  the  haploid  nuclei  of  sperm  and  

egg  together  

•  forming  a  diploid  zygote  

•  The  sperm’s  contact  with  the  egg’s  surface    

•  iniBates  metabolic  reacBons  in  the  egg    

•  that  trigger  the  onset  of  

embryonic  development  

The  Acrosomal  ReacBon  •  Acrosome    

•  Structure  at  the  Bp  of  the  sperm    

•  releases  hydrolyBc  enzymes    

•  that  digest  material  surrounding  the  egg  

•  acrosomal  reac2on    

•  Gamete  contact  and/or  fusion    

•  depolarizes  the  egg  cell  membrane    

•  sets  up  a  fast  block  to  polyspermy  

•  Not  seen  in  mammals  

•  Charge  inside  of  egg  changes  from  negaBve  to  posiBve  

The  CorBcal  ReacBon  

•  cor2cal  reac2on  

•  induces  a  rise  in  Ca2+    

•  that  sBmulates  cor2cal  granules    

•  to  release  their  contents  

outside  the  egg  

•  causes  formaBon  of  a  fer2liza2on  

envelope    

•  that  funcBons  as  a  slow  block  to  

polyspermy  

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Fig. 47-3-5

Basal body (centriole)

Sperm head

Sperm-binding receptors

Acrosome Jelly coat Vitelline layer

Egg plasma membrane

Hydrolytic enzymes

Acrosomal process

Actin filament

Sperm nucleus

Sperm plasma membrane

Fused plasma membranes

Fertilization envelope

Cortical granule

Perivitelline space

EGG CYTOPLASM

AcBvaBon  of  the  Egg  

•  sharp  rise  in  Ca2+  in  the  egg’s  cytosol    

•  increases  the  rates  of  cellular  

respiraBon  and  protein  

synthesis  by  the  egg  cell  

•  egg  is  now  acBvated  

•  sperm  nucleus  merges  with  the  egg  

nucleus    

•  and  cell  division  begins  

Cleavage  •  cleavage  

•  a  period  of  rapid  cell  division  without  growth  

•  parBBons  the  cytoplasm  of  one  large  cell    

•  into  many  smaller  cells  called  blastomeres  

•  Morula  

•  Solid  ball  of  cells  (~4-­‐32  cells)  

•  Blastula    

•  ball  of  cells  with  a  fluid-­‐filled  cavity  called  a  blastocoel  

(a) Fertilized egg (b) Four-cell stage (c) Early blastula (d) Later blastula

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•  The  eggs  and  zygotes  of  many  animals  

•  have  a  definite  polarity  

•  except  mammals  

•  Polarity  defined  by  distribuBon  of  yolk  (stored  nutrients)  

•  vegetal  pole  has  more  yolk  

•  animal  pole  has  less  yolk  

•  three  body  axes    

•  established  by  the  egg’s  polarity    

•  and  by  a  corBcal  rotaBon  following  binding  of  the  sperm  

•  CorBcal  rotaBon    

•  exposes  a  gray  crescent  opposite  to  the  point  of  sperm  entry  

Cleavage  

Fig. 47-7

(a) The three axes of the fully developed embryo

(b) Establishing the axes

Pigmented cortex

Right

First cleavage

Dorsal

Left

Posterior

Ventral

Anterior

Gray crescent

Future dorsal side

Vegetal hemisphere

Vegetal pole

Animal pole Animal hemisphere

Point of sperm nucleus entry

Fig. 47-8-6

Blastula (cross section)

Blastocoel Animal pole

4-cell stage forming

2-cell stage forming

Zygote 8-cell stage

Vegetal pole

0.25 mm 0.25 mm

•  Cleavage  planes    

•  usually  follow  a  paVern  that  is  relaBve    

•  to  the  zygote’s  animal  and  vegetal  poles  

Cleavage  

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•  Cell  division  is  slowed  by  yolk  

•  Holoblas2c  cleavage  

•  complete  division  of  the  egg  

•  occurs  in  species  whose  eggs  have  liVle  or  moderate  amounts  of  yolk  

•  sea  urchins  and  frogs  

•  As  well  as  mammals  

•  Meroblas2c  cleavage  

•  incomplete  division  of  the  egg    

•  occurs  in  species  with  yolk-­‐rich  eggs  

•  repBles  and  birds  

Cleavage  

GastrulaBon  •  Gastrula  

•  Three  layered  embryo  with  a  primiBve  gut  

•  Called  germ  layers  

•  Gastrula2on  

•  Process  of  making  a  gastrula  

•  rearranges  the  cells  of  a  blastula  

•  Into  the  three  layers  

•  embryonic  germ  layers  

•  ectoderm  forms  the  outer  layer    

•  endoderm  lines  the  digesBve  tract  

•  mesoderm  partly  fills  the  space  between  the  endoderm  and  ectoderm  

•  GastrulaBon  in  the  frog  (holoblasBc  cleavage)  

•  frog  blastula  is  many  cell  layers  thick  

•  Cells  of  the  dorsal  lip    

•  originate  in  the  gray  crescent    

•  and  invaginate  to  create  the  archenteron  

•  Cells  conBnue  to  move  from  the  embryo  surface    

•  into  the  embryo  by  involu2on  

•  These  cells  become  the  endoderm  and  mesoderm  

•  The  blastopore    

•  encircles  a  yolk  plug  when  gastrulaBon  is  completed  

•  The  surface  of  the  embryo  is  now  ectoderm  

•  innermost  layer  is  endoderm  

•  middle  layer  is  mesoderm  

Frog  GastrulaBon  

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Future ectoderm

Key

Future endoderm Future mesoderm

SURFACE VIEW

Animal pole

Vegetal pole Early gastrula

Blastopore

Blastocoel

Dorsal lip of blasto- pore

CROSS SECTION

Dorsal lip of blastopore

Late gastrula

Blastocoel shrinking Archenteron

Blastocoel remnant

Archenteron

Blastopore

Blastopore Yolk plug

Ectoderm Mesoderm

Endoderm

•  GastrulaBon  in  the  chick  (meroblasBc  cleavage)  

•  The  embryo  forms  from  a  blastoderm    

•  And  sits  on  top  of  a  large  yolk  mass  

•  upper  layer  of  the  blastoderm  (epiblast)    

•  moves  toward  the  midline  of  the  blastoderm    

•  and  then  into  the  embryo  toward  the  yolk  

•  primi2ve  streak  

•  Thickening  midline  

•  movement  of  different  epiblast  cells    

•  gives  rise  to  the  endoderm,  mesoderm,  and  ectoderm  

Chick  GastrulaBon  

Fig. 47-11

Endoderm

Future ectoderm

Migrating cells (mesoderm)

Hypoblast

Dorsal Fertilized egg

Blastocoel

YOLK

Anterior

Right

Ventral

Posterior

Left

Epiblast

Primitive streak

Embryo

Yolk

Primitive streak

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Organogenesis  (NeurulaBon)  •  organogenesis  

•  various  regions  of  the  germ  layers    

•  develop  into  rudimentary  organs  

•  Early  in  vertebrate  organogenesis  

•  the  notochord  forms  from  mesoderm  

•  and  the  neural  plate  forms  from  ectoderm  

•  neural  plate  soon  curves  inward  

•  forming  the  neural  tube  

•  will  become  the  central  nervous  system  

•  Neural  crest  cells    

•  develop  along  the  neural  tube  of  vertebrates    

•  and  form  various  parts  of  the  embryo  (nerves,  parts  of  teeth,  skull  bones,  and  so  on)  

•  Mesoderm  lateral  to  the  notochord  forms  blocks  called  somites  

•  Lateral  to  the  somites,  the  mesoderm  splits  to  form  the  coelom  

Fig. 47-12

Neural folds Tail bud

Neural tube

(b) Neural tube formation

Neural fold

Neural plate

Neural fold

Neural plate

Neural crest cells

Neural crest cells

Outer layer of ectoderm Mesoderm

Notochord

Archenteron

Ectoderm

Endoderm

(a) Neural plate formation

(c) Somites

Neural tube

Coelom

Notochord

1 mm 1 mm SEM

Somite

Neural crest cells

Archenteron (digestive cavity)

Somites Eye

ECTODERM MESODERM ENDODERM

Epidermis of skin and its derivatives (including sweat glands, hair follicles) Epithelial lining of mouth and anus Cornea and lens of eye Nervous system Sensory receptors in epidermis Adrenal medulla Tooth enamel Epithelium of pineal and pituitary glands

Notochord Skeletal system Muscular system Muscular layer of stomach and intestine Excretory system Circulatory and lymphatic systems Reproductive system (except germ cells) Dermis of skin Lining of body cavity Adrenal cortex

Epithelial lining of digestive tract Epithelial lining of respiratory system Lining of urethra, urinary bladder, and reproductive system Liver Pancreas Thymus Thyroid and parathyroid glands

DerivaBves  of  Embryonic  Germ  Layers  

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Developmental  AdaptaBons  of  Amniotes  

Embryo

Amnion

Amniotic cavity with amniotic fluid

Shell

Chorion

Yolk sac

Yolk (nutrients)

Allantois

Albumen

•  Amniotes  

•  Embryos  of  birds,  other  repBles,  and  mammals    

•  develop  in  a  fluid-­‐filled  sac  in  a  shell  or  the  uterus  

•  four  extraembryonic  membranes  form  around  the  embryo:  

•  chorion    

•  funcBons  in  gas  exchange  

•  Contributes  to  the  placenta  in  mammals  

•  amnion    

•  encloses  the  amnioBc  fluid  

•  yolk  sac    

•  encloses  the  yolk  

•  allantois    

•  disposes  of  waste  products  and  contributes  to  gas  exchange  

Mammalian  Development  •  The  eggs  of  placental  mammals  

•  Are  small  and  store  few  nutrients  

•  Exhibit  holoblasBc  cleavage  

•  Show  no  obvious  polarity  

•  GastrulaBon  and  organogenesis    

•  resemble  the  processes  in  birds    

•  and  other  repBles  

•  Early  cleavage    

•  is  relaBvely  slow  in  humans  and  other  mammals  

•  Blastocyst  

•  Formed  at  compleBon  of  cleavage  

•  inner  cell  mass    

•  Inside  blasotcoel  

•  A  group  of  cells  develops  into  embryo    

•  and  forms  the  extra-­‐embryonic  membranes  

•  trophoblast  

•  outer  epithelium  of  the  blastocyst  

•  iniBates  implantaBon  in  the  uterus  

•  and  the  inner  cell  mass  of  the  blastocyst  forms  a  flat  disk  of  cells  

•  As  implantaBon  is  completed  

•  gastrulaBon  begins  

Mammalian  Development  

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Fig. 47-16-1

Blastocoel

Trophoblast

Uterus

Endometrial epithelium (uterine lining)

Inner cell mass

Fig. 47-16-2

Trophoblast

Hypoblast

Maternal blood vessel

Expanding region of trophoblast

Epiblast

•  epiblast  cells    

•  invaginate  through  a  primiBve  streak    

•  to  form  mesoderm  and  endoderm  

•  Placenta    

•  allows  for  the  exchange  of  materials    

•  between  the  mother  and  embryo  

•  is  formed  from  the  trophoblast    

•  mesodermal  cells  from  the  epiblast  

•  and  adjacent  endometrial  Bssue  

•  By  the  end  of  gastrulaBon  

•  the  embryonic  germ  layers  have  formed  

Mammalian  Development  

Yolk sac (from hypoblast)

Hypoblast

Expanding region of trophoblast Amniotic cavity Epiblast

Extraembryonic mesoderm cells (from epiblast) Chorion (from trophoblast)

Yolk sac

Mesoderm

Amnion Chorion Ectoderm

Extraembryonic mesoderm Atlantois

Endoderm

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You  should  now  be  able  to:  

1.  Describe  the  acrosomal  reacBon  

2.  Describe  the  corBcal  reacBon  

3.  DisBnguish  among  meroblasBc  cleavage  and  holoblasBc  cleavage  

4.  Compare  the  formaBon  of  a  blastula  and  gastrulaBon  in  a  sea  urchin,  a  frog,  and  a  chick  

5.  List  and  explain  the  funcBons  of  the  extraembryonic  membranes