Development
Differential Gene Expression
Development
• development consists of a series of changes
zygote => embryo => immature => mature
• developmental changes continue until death
development
in
multicellular
eukaryotes
Figure 19.1
The Processes of Development
• development consists of
– growth
• cell division + cell expansion
– differentiation
• cellular specialization
– morphogenesis
• pattern formation
• due to the regulated expression of appropriate
sets of genes
The Processes of Development
• determination
– for “early” embryonic cells
• developmental potential is greater than
developmental fate
– “later” embryonic cells become determined
• developmental fate is fixed
– determined cells eventually differentiate
into mature forms
– determination precedes differentiation
Cell Differentiation:
Differential Gene Expression
• the zygote is totipotent
– contains the entire genetic constitution
(information)
– capable of forming all adult tissues
(expression)
• differentiated cells retain genetic constitution
– some cells remain totipotent
plant
parenchyma
cells
remain
totipotent
in
mature, fully
developed
tissues
Figure 19.3
Cell Differentiation:
Differential Gene Expression
• early embryonic animal cells are naturally
totipotent
– nuclei transferred to unfertilized eggs can
direct normal development
• genomic equivalence
• nuclear “fate swapping”
– separation of early embryo cells - naturally
or artificially - produces identical twins
Cell Differentiation:
Differential Gene Expression
• adult animals do not possess naturally
totipotent cells
– some differentiated adult cell nuclei can be
“made” totipotent again
some differentiated
animal cells can be
“reprogrammed”…
Figure 19.4
Figure 19.5
culture of
undifferentiated
embryonic
stem cells
Figure 19.6
Cell Differentiation:
Differential Gene Expression
• embryonic stem cells are naturally nearly
totipotent
– can be cultured in an undifferentiated state
– appropriate treatment causes differentiation
• therapeutic cloning would produce
replacement tissues from stem cell cultures
– nuclear transfer would produce compatible
stem cell cultures
Cell Differentiation:
Differential Gene Expression
• adult stem cells can be isolated from
differentiated tissues
– exhibit varying degrees of determination
– pleuripotent (or pluripotent)
• able to form all cell types belonging to
their source tissue
Cell Differentiation:
Differential Gene Expression
• as cells differentiate, different genes are
expressed
– molecular tools can demonstrate genomic
equivalence and differential gene expression
• Southern blotting can locate any gene in
any cell type
• Northern blotting locates only certain
mRNAs in each cell type
Cell Differentiation:
Differential Gene Expression
• as cells differentiate, different genes are
expressed
– MyoD1 encodes a transcription factor that
directs myoblasts to develop into skeletal
muscle fibers
• experimentally, MyoD1 can cause other
precursor cells to become muscle fibers
Cell Differentiation:
Differential Gene Expression
• as cells differentiate, different genes are
expressed
– each cell type has a master gene that begins
its specific expression program
– expression of a cell’s master gene is
regulated by signals peculiar to its location
Polarity Contributes to Cell Determination
• eggs, zygotes, and embryos have unequal
distributions of cellular components
– one end differs from the other
– different developmental events occur at
different locations
sea
urchin:
apical/basal
polarity…
Figure 19.7
unequal
distribution
of
cytoplasmic
determinants
Figure 19.8
initial
divisions
segregate
materials
of the
egg cytoplasm
into
different
embryo cells
Embryonic Induction Contributes to
Cell Determination
• inducers secreted by some embryonic tissues
direct the development of neighboring cells
• induction may be reciprocal between
neighboring tissues
reciprocal induction in the embryonic
development of the frog’s eye
Figure 19.9
Embryonic Induction Contributes to
Cell Determination
• Caenorhabditis elegans
– developmental model system
– fertilized egg => 959 cell adult
• pattern of division & development is
mapped
a full millimeter of developmental
excitement!!
Figure 19.10
Embryonic Induction Contributes to
Cell Determination
• Caenorhabditis elegans
– developmental model system
– fertilized egg => 959 cell adult
• pattern of division & development is
mapped
– induction directs vulval development
• anchor cell secretes inducer
• the nearest neighbor receives the signal;
activates gene 1
• adjacent neighbors receive two signals;
activate genes 1 and 2
Pattern Formation in Organ Development
• apoptosis contributes to pattern formation by
removing groups of cells
– programmed cell death
– mediated by specific gene products
apoptosis in development of the human hand
Figure 19.11
flower
organ
identity
genes
Figure
19.12
Pattern Formation in Organ Development
• organ identity genes interact to form flower
parts
– mutations produce inappropriate organs in a
flower
leafy activates transcription of A, B & C
Figure 19.13
Pattern Formation in Organ Development
• cells of a developing tissue receive positional
information in the form of morphogens
– signals are secreted by specific groups of
cells
– diffusion of signal molecules produces a
morphogen gradient
– the concentration of morphogen at a
particular cell determines its developmental
response
Establishing Body Segmentation:
Differential Gene Expression
• the Drosophila adult consists of dissimilar
body segments that develop from similar
larval segments
– maternal effect genes establish polarity
• express morphogens in nurse cells
–deposited locally in an egg
–gradients of morphogens develop
• some determine dorsal-ventral axis
• others determine anterior-posterior
axis
Bicoid
maternal
effect genes
Figure 19.14
Establishing Body Segmentation:
Differential Gene Expression
• segmentation gene products organize larval
segmentation
– gap genes, activated by maternal effect
genes, organize anterior-posterior regions
– pair rule genes define pairs of segments
– segment polarity genes define boundaries &
anterior-posterior organization of segments
– homeotic genes determine roles of segments
maternal effect
genes
induce
gap genes
induce
pair rule genes
induce
segment polarity
genes
&
homeotic genes
Figure 19.15
antennapedia is a homeotic mutation
Figure 19.16
Establishing Body Segmentation:
Differential Gene Expression
• homeotic gene mutations can produce bizarre
developmental effects
– bithorax mutants produce two winged
segments
– antennapedia produces legs in place of
antennae
• homeotic genes are homologous
– share the 180-bp homeobox
– encode the 60-a.a. homeodomain in their
polypeptide products