Download Development - Westford Academy Ap Bio

Development
Animals – Embryonic Development
Plants – Seed Germination
Embryonic Development
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Fertilization: fusion of 2nd oocyte & sperm
Cleavage: rapid division w/o growth
Morula: solid ball of cells
Blastula: hollow sphere of cells
Gastrula: germ layers form
Extraembryonic membranes form
Organogenesis: cell differentiation
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)
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)
Morula & Blastula
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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
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)
PRIMATIVE STREAK
Gastrulation (folding
inward) occurs along a
line (fold) instead of a
circular depression
Embryonic disc will
become the embryo
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)
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)
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.)
Germ Layer
Derivatives
ENDODERM
Lungs, thyroid, pancreas,
stomach, colon, bladder,
intestines, liver
MESODERM
Muscle, kidneys, RBC
ECTODERM
Epidermis, neurons,
melanocytes
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
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
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!
Root Zones
1. Root tip (cap):
protects apical
meristem
2. Zone of cell division
(meristematic cells)
3. Zone of elongation:
new cells absorb
water & lengthen
4. Zone of
maturation/differentiat
ion: cells mature into
xylem, phloem,
parenchyma, or
epidermal cells
Similar zones at shoot tip, without a cap