Development

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Development

• Animals Embryonic Development
• Plants Seed Germination

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Embryonic Development

1. Fertilization fusion of 2nd oocyte sperm
2. Cleavage rapid division w/o growth
3. Morula solid ball of cells
4. Blastula hollow sphere of cells
5. Gastrula germ layers form
6. Extraembryonic membranes form
7. Organogenesis cell differentiation

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Fertilization

• Recognition receptor binding between head of
  sperm zona pellucida (oocyte)
• Penetration plasma membranes fuse, sperm nucleus
  enters oocyte
• Fertilization membrane forms to block entry of
  any other sperm
• Meiosis II of oocyte triggered by sperm nucleus,
  polar body discharged
• Fusion of nuclei sperm ovum nuclei fuse to
  form zygote nucleus, DNA replication (creates
  sister chromatids)

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Cleavage

• Rapid division without growth to create
  blastomeres
• Each blastomere contains less cytoplasm than
  original zygote
• Embryo polarity animal pole (top) vegetal pole
  (bottom), more yolk at bottom (yolk heavier)

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Morula Blastula

• Morula solid ball of cells
• Liquid fills morula, pushes cells outward
• Blastula hollow sphere of cells (single layer)
• Blastocoel fluid-filled cavity inside of
  blastula

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Blastocyst

• Blastocyst blastula in mammals
• Trophoblast outer ring of cells
• Embeds in endometrium (implantation)
• Produces HCG (to maintain corpus luteum)
• Forms chorion (and eventually placenta)
• Embryonic disc inner mass of cells
• Inner cell mass (ICM) flattens into disc
• Gastrulation development forms embryo and
  membranes (besides chorion)

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PRIMATIVE STREAK Gastrulation (folding inward)
occurs along a line (fold) instead of a circular
depression
Embryonic disc will become the embryo
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Gastrulation

• Group of cells fold into blastocoel
  (invagination) to create 2-layered embryo
• 3rd layer forms between inner outer layers
• 3 primary germ layers ectoderm (outer), mesoderm
  (middle), endoderm (inner)
• Archenteron center cavity formed by
  gastrulation (surrounded by endoderm, will become
  digestive tract)
• Blastopore opening into archenteron (becomes
  either mouth or anus)

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Extraembryonic Membranes

• Form outside of the embryo
• Chorion outer membrane
  for gas exchange (birds,
  reptiles) or
  implantation into
  endometrium (mammals,
  later becomes placenta)
• Allantois buds off archenteron, below chorion,
  storage of waste (birds, reptiles) or transport
  of waste (mammals, later becomes umbilical cord)
• Amnion forms amniotic cavity, filled with fluid
  to cushion embryo
• Yolk sac contains yolk (birds, reptiles), empty
  in placental mammals (nutrition from placenta)

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Organogenesis

• Cell differentiation (become specific
  tissues organs)
• Notochord
• Along dorsal surface of mesoderm
• Stiff rod to provide support
• Vertebrae form from nearby cells
• Neural tube
• Ectoderm directly above notochord
• Neural plate first, then indents
  (neural groove) before
  rolling up
  into neural tube
• Develops into central nervous sys.
• Cells roll off top, form neural crest
  (will form teeth, bones,
  muscles of
  skull, pigment cells in skin, etc.)

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Germ Layer Derivatives
ENDODERM Lungs, thyroid, pancreas, stomach,
colon, bladder, intestines, liver MESODERM Muscle,
kidneys, RBC ECTODERM Epidermis, neurons,
melanocytes
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Factors Influencing Development

• Embryonic induction
• Influence of cells on neighboring cells
• Organizers cells that secrete chemicals to
  influence development of neighboring cells
• Homeotic genes
• Master genes that control developmental genes
• Turn genes on/off that code for substances that
  directly affect development (control placement
  and spatial organization of body parts)
• Homeobox (180 nucleotides) sequence in homeotic
  genes
• HIGHLY CONSERVED shows common ancestry
• Found in homeotic genes of fungi, plants, and
  animals

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Seed Anatomy

• Parts of seed seed coat, endosperm/cotyledons,
  embryo
• Parts of embryo
• Epicotyl top part, becomes shoot tip
• Plumule young leaves on epicotyl
• Hypocotyl below epicotyl, becomes shoot
• Radicle below hypocotyl, becomes root
• Coleoptile surrounds/protects embryo

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Seed Germination

1. Dormancy until encountering specific
   environmental cues (water, temperature, light,
   seed coat damage)
2. Absorption of water seed swells (cracks coat),
   activates enzymes (initiates respiration)
3. Growth of radicle into roots anchors seedling
4. Elongation of hypocotyl produces shoot
5. Primary growth at tips of roots shoots (apical
   meristems)

Gibberellins!
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Root Zones

• Root tip (cap) protects apical meristem
• Zone of cell division (meristematic cells)
• Zone of elongation new cells absorb water
  lengthen
• Zone of maturation/differentiation cells mature
  into xylem, phloem, parenchyma, or epidermal
  cells

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Similar zones at shoot tip, without a cap