Title: Bio 120 2005 Lecture 9
1Bio 120 2005 Lecture 9
- Cloning implications
- Insect development
2human reproductive cloning
- basic biology is likely to be similar --we are
mammals - but low success rates pose a problem
- USA no federally funded research
- UK, Canada restricted, but allowed
- Germany total ban
3Proposed outcomes (if it ever worked)
- Vanity cloning
- eternal life? but your clone would be even less
similar than an identical twin - Eugenics, Dysgenics
- clone Einstein, Hitler, armies
- Sentimental cloning
- clone dead child, parent, pet
- all the above rest on wrongheaded genetic
determinism, i.e., that DNA determines identity. - cloning is not photocopying
- Infertility treatment?
- there are easier ways...
4therapeutic cloning
- Goal is to make genetically matched ES cells
- Proof-of-concept in mice
- allows somatic (not germline) gene/cell therapy
Rideout WM et al. 2002. Correction of a genetic
defect by nuclear transplantation and combined
cell and gene therapy. Cell 109 17-27.
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6therapeutic cloning
- goal is to make ES cells
- ethical worries are with use of blastocysts
- isnt there any other way to convert regular
cells into ES cells? - not yet
- adult stem cells very controversial, and not as
pluripotent as ES
7therapeutic cloning questions
- what makes blastocysts special
- biologically?
- ethically? why are other cells not so
privileged? - they are potential individuals
- but any cell is
8The ethical debate
- Kant humans are an end in themselves, not a
means - Bentham the greatest good for the greatest
number - is a zygote an individual? totipotency means any
cell is a potential individual!
Immanuel Kant
Jeremy Bentham
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10The questions
- How are body axes set up?
- How are germ layers specified
- How are germ layers subdivided and patterned?
11Arthropods
- Hardened exoskeleton, articulated body segments,
Jointed appendages - probably monophyletic
- gt 1 million described species, estimated 10-30
million - Outnumber humans by ratio of 108 1
- Subphylum Uniramia
- Class Myriapoda centipedes, millipedes
- Class Insecta insects
- Other subphyla Crustacea, Chelicerates
12The insect body plan
- Three body regions
- head (5-6 segments)
- thorax (3)
- abdomen (8-11)
- body is segmented unsegmented terminal regions
- 3 pairs of legs (on the three thoracic
segments)-Hexapoda
m ? f
13Insects
Wingless insects (e.g. silverfish)
Hemimetabolous insects Incomplete
metamorphosis (dragonflies, bugs, roaches,
earwigs, lice)
Holometabolous insects Complete
metamorphosis (beetles, butterflies, wasps, flies)
14Drosophila life cycle
Fig 2.29
15Drosophila development
http//flymove.uni-muenster.de/ Movies of
development and anatomy Interactive animations
of genetic mutants e.g. Processes/segmentation/
Highly recommended
16The Drosophila egg
- About 0.5 mm long
- Yolk in center
- Visible AP and DV asymmetry before fertilization
- eggshell (chorion) is also polarized--made by
follicle cells - Sperm entry via micropyle (little gate) in
eggshell
dorsal
A
P
ventral
17Cleavage and cellularization
Fig 2.30
Tubulin actin
Tubulin myosin
- nuclei divide every 9 min without cytokinesis
- Cellularization all at once
- Movies from Bill Sullivans lab
http//bio.research.ucsc.edu/people/sullivan/
18Gastrulation
- Ventral blastoderm invaginates to form mesoderm
(muscles etc) - Anterior, posterior invaginations form gut
Cross sections of embryos immunostained for
Twist, a bHLH protein expressed in mesoderm
Fig 2.31
19The germ band
- Ventral blastoderm, after gastrulation, will give
rise to most of embryo - Undergoes extension then retraction
- segmentation first visible during extended germ
band stage (top) - embryonic units are parasegments, different from
larval segments
20long-germ versus short-germ insects
- Drosophila (and other advanced insects)
Long-germ development - germ band develops from most of blastoderm
- Segments form simultaneously
- Beetles ( other primitive insects) Short or
intermediate germ development - Part of blastoderm first forms anterior segments
- Posterior segments develop progressively from
growth zone (cf. somitogenesis) - different routes to similar extended germ band
stages
Fig 5.34
21The Drosophila larva
- Feeding machine
- Segmented, obvious AP and DV pattern in cuticle
- Pupates, larval tissues self-destruct (autolysis)
- Adult (imago) rises from the ashes via imaginal
discs
Figs 2.33, 2.34
22Today axis formation
- 1. Experimental embryology suggests that simple
mosaic models are not enough - 2. Morphogen gradient models of pattern
formation - 3. Using genetics in Drosophila to identify the
morphogens
The red-banded leaf-hopper (not Euscelis)
23Experimental embryology of insects
- Leaf hoppers short-horned bugs (Hemimetabola)
- Euscelis incisus (formerly E. plebejus)
- Intermediate-germ development
- Large eggs, soft egg shell, amenable to
manipulations - Endosymbiotic bacteria in posterior
- See section 5.18
241. evidence for a posterior organizer
- Suck out tiny bit of cytoplasm from posterior
- Lose thorax and abdominal segments
- Long-range effect the activation center
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Friedrich Seidel (1897-1992)
252. Ligature experiments
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- Klaus Sander (1950s) ligate egg with thread
- lose segments in middle of pattern
- remaining segments in correct order, spread out
- Earlier ligature gives bigger gap
26Cytoplasmic transfer experiments
(1)
(2)
2 hours
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(same experiment as Fig 5.36)
- Move posterior cytoplasm by poking with needle
- ligate immediately--posterior bicaudal,
anterior makes nothing
- same experiment except wait 2 hours before
ligating - now anterior forms complete pattern
27Conclusions
- Posterior cytoplasm is special
- Probably nothing to do with the bacteria, these
just a convenient marker - Rest of egg highly regulative
- Long-range effects, not explained by simple
mosaic model - Sanders model diffusible morphogen made in
posterior - Also independence of A-P and D-V axes
28Two questions of pattern formation
- How can cell fate be determined by relative
position? (what is the positional information) - How can a cells response vary depending on its
history?
29Morphogenetic fields
shoulder
limb
X
- Newt limb development (Spemann)
- Remove limb disc, limb flank regulates
- Disk flank constitute a developmental field
region in which cell fate determined by relative
position
30Response to signals depends on history (I.e.
genome)
- Spemann Schotte 1932
- Transplant newt ventral cells into frog gastrula,
newt teeth where frog mouth (no teeth) should be - cells fates were appropriate for their position
and for their ancestry
31Fields
- Embryonic territories that communicate to form a
structure - E.g. the limb field etc
- Cells in a field are equivalent in developmental
potential (at first) - Cells become different in response to signals
- Signals produced from signaling centers, and have
concentration-dependent effects
32The French Flag analogy
- Flag area field
- Cells read out position relative to boundary
(flagpole) - Response depends on
- Local concentration of morphogen relative to
threshold values - Cells own history
Fig 1.22
33Response depends on history (genotype)
- Cells are newt (UK) or frog (French) in genotype
- Both respond to same signals
- Response (UK or French) depends on history
(newt mouth)
(frog gastrula)
Fig 10.36
34How do you get gradients?
- Localized source of morphogen that can diffuse
over gt1 cell diameter - Morphogen must be unstable if degraded
everywhere, a dispersed sink - Exponential decay gradient from source
- Localized source/Dispersed sink (LSDS) model
- Gradient could form over small (gt 1 mm)
territories in 1 hour given known diffusion
constants
morphogen
Distance from source
35Simple LSDS models
- Can explain
- Why organizers can pattern large groups of
cells--because they are morphogen sources - How ordered patterns form--because morphogen
gradient has polarity - defect regulation--why pattern reforms if small
bits of field removed or added (because source
still intact) - Have difficulty explaining
- Size invariance (e.g. Dictyostelium)
- Ability of sources to re-form (e.g. limb field)
- And do not address
- how cells can read out local morphogen
concentrations - Box 10A reaction-diffusion models that
self-organize
36Gradient explanation of gaps
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37Gradient explanation of cytoplasmic transfers
Wait a couple of hours
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