Transcript No Slide Title

Bio 108 - 3/6/2000 Morphogenetic Movements (contd)
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Contact information
– Bruce Blumberg
– Office 4216 BioSci II
• office hours
W 3-4
F 12-1 (or by appointment in exceptional
cases)
– phone 824-8573
– [email protected] (preferred contact mode)
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You are responsible for what I lecture about and what is
covered in the assigned reading in Alberts, et al.
– Generally speaking, I will stick close to the book as
did the other lecturers
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Today
– morphogenetic movements
• neurulation
• neural crest migration
– cell determination
BioSci 108 lecture 23 (Blumberg) page 1
©copyright
Bruce Blumberg 2000. All rights reserved
Neurulation
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Last time we finished with gastrulation
The next developmental step is when the gastrula
ectoderm is induced to form neural tissue by the
underlying mesoderm and notochord (Figure 21-10)
– neural plate thickens and rolls up into a tube
– neural tube pinches off from the rest of the cell sheet
– the inducing mesoderm can lead to the formation of
an ectopic neural tube if transplanted to a new
embryo, much like the organizer transplant
– process is driven by changes in cell shape (21-11)
• cells become columnar
• apical actin bundles contract and cause curling
of neural plate
subsequently, dorsal cells of the neural tube pinch off and
migrate out through the mesoderm
– these neural crest cells will contribute to numerous
structures and tissues
• peripheral nervous system
• sympathetic ganglia
• Schwann cells
• pigment cells
– head neural crest will differentiate into cartilage,
bone and connective tissue
• these typically derive from mesoderm in other
parts of the body
BioSci 108 lecture 23 (Blumberg) page 2
©copyright
Bruce Blumberg 2000. All rights reserved
Cell adhesion
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Changes in cell adhesion molecules are important for
morphogenetic movements
– classic experiments showed that cells can recognize
others of the same type (figure 21-14)
• experiment:
– dissociate gastrula cells
– allow to reassociate
• observation:
– cells sort out according to tissue type
– outer epidermis
– central neural tube
– mesoderm in between
• inference:
– cells can identify each other
– basis of identification is cell surface molecules called
cadherins (21-15)
• Ca-dependent glycoproteins that are
differentially expressed in the early embryo
• cadherins control the formation and dissolution
of cell sheets AND
• provide an anchor for cytoskeleton within the
cells
• recognition of cadherin types and combinations
mediates interactions between cells and their
neighbors.
BioSci 108 lecture 23 (Blumberg) page 3
©copyright
Bruce Blumberg 2000. All rights reserved
Cells and the extracellular matrix
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In addition to adhering to each other, cells need to
interact with the extracellular matrix
– mediated through another class of cell surface
proteins called integrins
– integrins serve as transmembrane linkers between the
ECM and cytoskeleton
– How to test the role of this interaction?
• experiment:
– block cells from binding to fibronectin with
a synthetic peptide that saturates the
binding site
• observation:
– cells do not migrate and appear only loosely
adherent
• conclusion
– cell interaction with integrins is very
important for migration
– Mutations in integrins lead to embryonic patterning
defects
• Drosophila mutation lethal (1) myospheroid
leads to failure of muscle attachment
– embryos burst at about 24 hours after
fertilization when the first muscular
movements take place
– mutation is in an integrin subunit found in
muscles
BioSci 108 lecture 23 (Blumberg) page 4
©copyright
Bruce Blumberg 2000. All rights reserved
Invasion and migration
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Many types of cells migrate in the early embryo from
their point of origin
– muscle cells
– neural crest
– blood cells
– primordial germ cells
– neurons
how to follow migrations?
– Mark cells at the beginning of their journey and
follow them
• nontoxic dye
• heritable genetic marker
– quail/chicken chimeras
Neural crest follows distinct migratory pathways (21-17)
– much of this work was done by Marianne BronnerFraser who was previously at UCI
– migrating cells extend cellular processes that “test”
for interactions with neighboring cells
• depending on the types of interactions found, the
cells go one direction or another
• cell surface receptors again mediate this process
– some types attract or promote adhesion
– others inhibit movement and repel
connections
– Corey Goodman and neural cell migration
BioSci 108 lecture 23 (Blumberg) page 5
©copyright
Bruce Blumberg 2000. All rights reserved
Cell Diversification in the early embryo
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All of the information needed to form any type of
organism is contained in a single cell
– the central goal of developmental biology is to
understand how
• what are the molecules?
• how do they interact?
• what are the physical processes?
Question - does the genome remain intact and totipotent
during development or do cells become specialized by
genomic modifications?
– Answer seems obvious but an important early
developmental model was the worm Ascaris.
• Significant parts of the genome are lost during
development and differentiation
– experiment: (Figure 21-20)
• transplant the nucleus from a differentiated cell
from adult frog to egg lacking nucleus
• what is the developmental potential of the
resulting embryo?
– Observation:
• normal tadpoles developed that had
characteristics of the transplanted nucleus
(albino)
– conclusion:
• genome remains intact during development
• cells differ because they express different genes
BioSci 108 lecture 23 (Blumberg) page 6
©copyright
Bruce Blumberg 2000. All rights reserved
Cell diversification (contd)
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– Resulting animals are clones, exact copies of the
donor animal
– Such experiments were previously difficult and not
so reproducible
– today, the technology has advanced sufficiently that
mammals can be readily cloned
• for $250,000 you can have your pet cloned!
• it is only a matter of time before someone does
this with humans
Cytoplasmic determinants
– in most plants and animals, the egg is chemically
asymmetrical
• certain components are concentrated in specific
regions
• these localized cytoplasmic determinants vary in
importance between species
– development was previously characterized in two
broad ways
• in mosaic development, if a blastomere is
removed, the structures derived from it fail to
form e.g. sea urchin
– Inference is that determinants are important
• in regulative development the embryo develops
normally despite blastomere removal
– determinants are not important
– no animals are completely either way!
BioSci 108 lecture 23 (Blumberg) page 7
©copyright
Bruce Blumberg 2000. All rights reserved
Embryonic induction
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Induction - the switching of cells from one pathway into
another by the influence of an adjacent group of cells
– one of the most important processes during early
development of virtually all organisms
– experiments that demonstrated induction are some of
the most famous experiments in biology
Organizer transplant (Hans Spemann)
– until recently, the only Nobel Prize awarded for
Developmental Biology
– experiment:
• transplant the dorsal lip of the blastopore to the
ventral side of a host embryo
– observation:
• a complete secondary axis was formed from both
host and graft tissue
– inference:
• the transplant was able to organize the host
tissue into a secondary axis
• factor(s) from the graft were able to do this
– over the years, many such experiments were done
until it became clear that many, many types of
molecules could induce secondary axes in
salamanders
• put field into disrepute for many years
• turns out that salamander is promiscuous for
neural induction
BioSci 108 lecture 23 (Blumberg) page 8
©copyright
Bruce Blumberg 2000. All rights reserved
Embryonic induction (contd)
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Xenopus saved the day
– unlike salamander mesoderm and neural tissue could
only be induced in Xenopus by a few factors
– Pieter Niewukoop performed a classic induction
experiment (figure 21-21)
• isolated animal pole and vegetal pole cells
• if cultured in isolation, the animal pole cells
formed epidermis and the vegetal pole cells
stayed as they were.
• But when put together, mesoderm was formed!
• This simple mesoderm induction assay was the
basis for nearly all of the advances in molecular
embryology in recent years
– Tiedemanns and Grunz spent many years purifying
factors from embryos that could induce mesoderm
• these turned out to be growth factors in the FGF
and TGF-beta families
Later, a number of laboratories began to study the
molecular nature of the organizer
– Viktor Hamburger - Hans Spemann and the
Organizer, The Heritage of Experimental
Embryology
– Prompted by this book, professor Cho and I
constructed the first cDNA library from the dorsal lip
and identified many factors that were important for
its properties (Science 253, 194-196.
BioSci 108 lecture 23 (Blumberg) page 9
©copyright
Bruce Blumberg 2000. All rights reserved
Embryonic induction (contd)
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Sequential induction is responsible for much embryonic
patterning (Fig 21-22)
– a series of inductive interactions can generate many
kinds of cells, starting from only a few
Inductive abilities of cells differs
– although all vegetal cells can induce mesoderm, only
cells from the dorsal side can induce dorsal
mesoderm (organizer) (Fig 21-23)
– There are at least three different signals ( and
probably many more) responsible for mesoderm
induction in the early Xenopus embryo
– fertilization -> cortical rotation
– cortical rotation -> asymmetrical distribution of
dorsal determining factors (several possibilities)
• dorsal and ventral endoderm
– mesoderm and organizer induced by signal from
vegetal cells (signals 1 and 2)
– organizer dorsalizes adjacent mesoderm (signal 3)
mesoderm induction involves a variety of signaling
molecules
– TGF-beta family (activin, Vg-1, BMP2,4)
– Wnt family
– FGF family (FGF4)
– inhibitors of above
• chordin, noggin, cerberus
BioSci 108 lecture 23 (Blumberg) page 10
©copyright
Bruce Blumberg 2000. All rights reserved
Actions of inducing molecules
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One model for how the organizer is induced
– cortical rotation leads to activation of Vg-1 on the
dorsal side of the embryo
– Vg-1 protein in vegetal cells induces the organizer
– attractive model but not completely correct
• many so-called dorsalizing signals are, in fact,
inhibitors of ventral mesoderm and ectoderm
induction
Location (Fig 21-26)
– can be intracellular or extracellular
– molecules can act at both short and long range
nature
– secreted molecules
– bound to cell surface
secreted signals are called morphogens
– morphogens are diffusible substances that pattern the
embryo
– in its strictest definition, a morphogen patterns
tissues depending on its concentration
• model originated with Alan Turing a British
mathemetician who almost single-handedly
finished off the Germans in WWII
– he solved the famous “enigma” code that
allowed the Allies to decipher German
military transmissions
BioSci 108 lecture 23 (Blumberg) page 11
©copyright
Bruce Blumberg 2000. All rights reserved
Morphogens and gradients
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– Gradient model was popularized by Child, Huxley
and De Beer
• essential feature is that activity gradients led to
the formation of specialized regions of the
embryo
– Louis Wolpert provided many important theoretical
treatments, observed that positional signaling occurs
over small distances, -.1-2 mm
– Francis Crick provided legitimacy to the model with
a quantitative treatment of the activity of diffusible
substances
how do morphogens work (Fig 21-26)
– essential feature is that morphogens diffuse from a
source and are degraded or captured by a sink
• if there was no loss or destruction of the
morphogen there would ultimately be no
difference in the concentration
– cells sense their position in a morphogen gradient
and respond according to the concentration of
morphogen they encounter
• ectodermal cells exposed to little or no activin
become epidermis
• low levels of activin induce muscle
• higher levels induce notochord
BioSci 108 lecture 23 (Blumberg) page 12
©copyright
Bruce Blumberg 2000. All rights reserved