Developmental biology

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• Developmental biology
• Definition
• Field of biology that studies how a single cell
  (the fertilized egg) gives rise to a fully formed
  organism.
• Draw out embryonic development from 1-cell stage,
  cleavage, blastula, gastrula, germ layer
  formation, body plan development, neurulation,
  organogenesis
• Why study it (why do people get excited about the
  topic? Complexity, beauty, imaging, relevance
  (human congenital defects, cancer)
• Why have a course in it? (most of what you have
  studied so far has been intracellular biology,
  focusing on molecules and what happens inside
  individual cells one important aspect of biology
  that has not yet been covered is how cells
  communicate with each other, how they form
  multicellular organisms, how cells form different
  tissues, and how these different tissues form
  organs that in turn interact with each other).

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• Questions
• axis determination (AP, DV, LR) Q. how do you
  break the symmetry of the egg? Sperm entry,
  localized determinants
• cell differentiation, cell proliferation, cell
  growth
• cell migration, polarization (symmetric vs
  asymmetric cell division), cell shape (giving
  cells different morphologies, e.g. neurons, EMT),
  cell death
• morphogenesis (how cells come together to form
  tissues and how these tissues migrate)
• organogenesis
• timing (biological clock)
• aging
• germ cell development, fertilization
• stem cells (what are they (multipotency, ability
  to self-renew), why did they become so trendy
  (Dolly the sheep, 1997, showed that cloning was
  possible development of human ES cells, 1998
  nobel for mouse ES cells induced pluripotent
  cells ))
• regeneration
• evo-devo (a field of biology that compares the
  developmental processes of different animals in
  an attempt to determine the ancestral
  relationship between organisms and how
  developmental processes evolved.)
• Q. How efficient is human development? (talk
  about implantation)

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Human reproduction is an inefficient process
50 of concepti do not implant (implantation 8-10
dpf, Heart beat at 21 dpf). a further 30 die
and abort after implantation.
3-4 of all live births possess a
macroscopically visible congenital defect
(120,000 babies/year in the USA).
1 of all babies are born with a heart defect.
20 of neonatal deaths are caused by congenital
defects (the leading cause of neonatal death in
the USA)
congenital disorders are the cause of 50 of
pediatric admissions in the USA
Developmental defects seen at birth are caused by
defects in the cellular processes of development.
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• Developmental biology
• Model systems
• Worm (C. elegans)
• Fly (Drosophila melanogaster)
• Fish (zebrafish, medaka)
• Mouse
• Xenopus (laevis, tropicalis)
• Chick (Quail)
• Sea urchin (echinoderm), ascidians (ciona
  intestinalis, savigni) (urochordate (tunicate)
  (notochord in larvae), hydra (cnidaria,
  invertebrate), planarians (flatworm), arabidopsis
  (weed) ( other plants), axolotls (and other
  salamanders), chlamydomonas (unicellular green
  alga, protozoan), yeast (?),
• Many systems available which one to use most
  powerful one where you can study your questions
  of interest. (also, many of these model systems
  are used to study processes other than
  developmental biology).

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Friday, May 30th 17th Annual Developmental
Biology Symposium Developmental insights
from atypical model systems Speakers
Eduardo R. Macagno, Ph.D., University of
California, San Diego (leech) Gary Wessel,
Ph.D., Brown University (starfish) William
Smith, Ph.D., University of California, Santa
Barbara (ascidians) Richard Behringer, Ph.D.,
University of Texas (Bats) Kathleen K. Smith,
Ph.D., Duke University (marsupials) Host
The Developmental Biology Journal Club Rock
Hall Auditorium
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• Developmental biology
• Tools
• - Forward genetics
• - Reverse genetics (is gene x necessary?)
  (mutation, RNAi, antisense oligos)
• - Gain-of-function expts (is gene x sufficient)
  (mRNA injections, transgenics various levels of
  sophistication, gal4-uas, temporal and spatial
  control)
• - Where is function of gene x required? Creating
  genetic mosaics (genetic, transplants)
• Imaging (live, gene and protein expression)
• - Experimental embryology (cell or tissue
  ablations, bead implantation, chick-quail
  chimeras)

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C. elegans (adult 1 mm in length)
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C. elegans development (embryo about 50 ?m in
length)
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Xenopus laevis
AP
V
D
VP
D
A
P
V
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Early developmental stages of Xenopus laevis
2.5 hpf
5 hpf
10 hpf
3.5 hpf
hpf hours post-fertilization
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Early developmental stages of Xenopus laevis
Cleavage movie
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Developmental stages of Xenopus laevis
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Gastrulation the ultimate cell migration problem
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Gastrulation and Neurulation highly
coordinated tissue movements
Keller neurulation movie
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Zebrafish embryonic development
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http//anatomy.ucsf.edu/devbio/course.htm
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http//www.nature.com/milestones/development/miles
tones/index.html
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The Question
1 cell type
200 different cell types
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Answer by the blastula stage (2000 cells)

• Evidence
• morphological differences
• fate map data
• e.g. ventral cells give rise to blood
• dorsal cells give rise to notochord
• - gene expression

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Answer at the blastula stage (2000 cells)

• Evidence
• morphological differences
• fate map data
• e.g. ventral cells give rise to blood
• dorsal cells give rise to notochord
• - gene expression

AP animal pole VP vegetal pole D dorsal V
ventral
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The turning point of Hans Spemanns career
partial constriction
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Hilde Mangold and Hans Spemann, 1924
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Hilde Mangold and Hans Spemann, 1924
a
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Tissue from the dorsal side of the embryo
(dorsal lip of the blastopore) is transplanted
to the ventral side of the embryo.
This induces cells on the ventral side of the
embryo to form a second neuraxis ventrally
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Modern version of the Mangold and Spemann
experiment
Gastromaster movie http//www.xenbase.org/methods/
methods.html
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What is wrong with this representation of the
Mangold and Spemann experiment?
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A small region of the embryo, the organizer or
dorsal lip, has the ability when grafted to the
opposite (ventral) side to cause pronounced
cell non-autonomous effects. Neighboring cells
in the ectoderm are induced to form central
nervous system and those in the mesoderm to form
dorsal structures such as somites. These
inductive effects should be mediated by
diffusible molecules and the isolation of the
molecular signals involved has been a challenge
for generation of embryologists (Viktor
Hamburger, 1988).
Seeking the molecules responsible for the
activity of the Organizer (using the tools
available in late 1980s). a
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Seeking the molecules responsible for the
activity of the Organizer I. Inducing the
formation of an ectopic axis
Inject RNA (from Organizer) into
ventrovegetal blastomere
V
D
Lateral view
Formation of an ectopic axis
Dorsal view
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• Richard Harland (UC Berkeley)
• used an expression screen for activities that
  induce
• dorsal/head structures in Xenopus embryos
• - identified Noggin (Smith and Harland, Cell,
  1992)

• Eddy De Robertis (UCLA)
• used an expression screen for activities that
  rescue
• UV ventralized Xenopus embryos
• - identified Chordin (Sasai et al., Cell, 1994)

Two novel secreted proteins. What next?
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Data suggesting that Noggin and Chordin could
function by antagonizing BMP signaling.
BMPs act as ventralizers and are expressed in
the ventral side of the embryo (1992)
DN-BMP receptors have dorsalizing activity (1994)
The Drosophila homologue of Chordin
(Short-gastrulation) functions genetically as an
antagonist of Dpp (BMP homologue)
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The model
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Additional members of the pathway were initially
identified genetically in Drosophila
Tolloid (clearly functions positively in BMP
signaling)
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Xolloid (Xenopus tolloid) cleaves Chordin,
reducing its affinity for BMP, thereby allowing
BMP binding to its receptor
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Model of BMP4 gradient leading to different
mesodermal cell types
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What is responsible for the formation of
the dorsal organizer?
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Embryologically By 4 cell stage, the embryo is
asymmetric in its potential (Spemann, 1938)
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Embryologically By 4 cell stage, the embryo is
asymmetric in its potential (Spemann, 1938)
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• Molecularly
• Wnt pathway implicated
• Wnt injection leads to secondary axis
• (Andy McMahon and Randy Moon, Cell, 1989)
• depletion of beta-catenin inhibits dorsal tissue
  formation
• (Janet Heasman, Chris Wylie, et al. Cell, 1994)
• Wnt11 appears to be the initiating event
• (Janet Heasman, Chris Wylie, et al. Cell, 2005)

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Wnt signaling, 2006 Xenopus assays, fly
genetics, yeast 2-hybrid
http//www.stanford.edu/rnusse/wntwindow.html
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http//faculty.washington.edu/rtmoon/cell2.html
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Proteins secreted by the dorsal and ventral
signaling centers in the Xenopus gastrula
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• Summary
• Key concepts
• Induction (organizing centers)

• Conservation of signaling pathways (vertebrates,
  fly)

• Activators and Inhibitors (negative feedback
  loops)

• Complexity