Transcript Developmental Biology

Developmental Biology
AP Bio
18:4
21:6
47:2 (part), 3
Wild-type mouse embryo 9.5
days post coitum
Our focus:
• Timing and Coordination
• Gene Expression
• Interactions, Cell Signaling
Insights into development have
been obtained froms studying
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Slime Molds
Nematode worm C. elegans
Fruit Flies
Zebrafish
Frog Embryos
Chick Embryos
Mice
Fig. 18-14
(a) Fertilized eggs of a frog
(b) Newly hatched tadpole
Zebrafish are often used for
embryological research.
Their embryos are transparent.
A program of differential gene
expression leads to the different cell
types in a multicellular organism
• During embryonic development, a fertilized egg
gives rise to many different cell types
• Cell types are organized successively into
tissues, organs, organ systems, and the whole
organism
• Gene expression orchestrates the
developmental programs of animals
A Genetic Program for Embryonic
Development
• The transformation from zygote to adult
results from cell division, cell
differentiation, and morphogenesis
• Cell differentiation is the process by which
cells become specialized in structure and
function
• The physical processes that give an
organism its shape constitute
morphogenesis
• Differential gene expression results from
genes being regulated differently in each
cell type
• Materials in the egg can set up gene
regulation that is carried out as cells divide
Cytoplasmic Determinants and
Inductive Signals
• An egg’s cytoplasm contains RNA, proteins, and
other substances that are distributed unevenly
in the unfertilized egg
• Cytoplasmic determinants are maternal
substances in the egg that influence early
development
• As the zygote divides by mitosis, cells contain
different cytoplasmic determinants, which lead
to different gene expression
Fig. 18-15a
Unfertilized egg cell
Sperm
Fertilization
Nucleus
Two different
cytoplasmic
determinants
Zygote
Mitotic
cell division
Two-celled
embryo
(a) Cytoplasmic determinants in the egg
• The other important source of
developmental information is the
environment around the cell, especially
signals from nearby embryonic cells
• In the process called induction, signal
molecules from embryonic cells cause
transcriptional changes in nearby target
cells
• Thus, interactions between cells induce
differentiation of specialized cell types
• Work with the nematode C. elegans has shown
that induction requires the transcriptional
regulation of genes in a particular sequence.
Fig. 18-15b
Early embryo
(32 cells)
Signal
transduction
pathway
Signal
receptor
Signal
molecule
(inducer)
(b) Induction by nearby cells
NUCLEUS
Sequential Regulation of Gene
Expression During Cellular
Differentiation
• Determination commits a cell to its final
fate: it is the progressive restriction of
developmental potential as the embryo
develops
• Determination precedes differentiation
• Cell differentiation is marked by the
expression of tissue-specific proteins
For example
• Myoblasts produce muscle-specific proteins and
form skeletal muscle cells
• MyoD is one of several “master regulatory
genes” that produce proteins that commit the
cell to becoming skeletal muscle
• The MyoD protein is a transcription factor that
binds to enhancers of various target genes
Fig. 18-16-1
Nucleus
Master regulatory gene myoD
Embryonic
precursor cell
Other muscle-specific genes
DNA
OFF
OFF
Fig. 18-16-2
Nucleus
Master regulatory gene myoD
Embryonic
precursor cell
Myoblast
(determined)
Other muscle-specific genes
DNA
OFF
OFF
mRNA
OFF
MyoD protein
(transcription
factor)
Fig. 18-16-3
Nucleus
Master regulatory gene myoD
Embryonic
precursor cell
Other muscle-specific genes
DNA
Myoblast
(determined)
OFF
OFF
mRNA
OFF
MyoD protein
(transcription
factor)
mRNA
MyoD
Part of a muscle fiber
(fully differentiated cell)
mRNA
Another
transcription
factor
mRNA
mRNA
Myosin, other
muscle proteins,
and cell cycle–
blocking proteins
Why doesn’t myoD change any
type of embryonic cell?
• Probably a combination of regulatory
genes are necessary for differentiation is
required.
Pattern Formation: Setting Up the
Body Plan
• Pattern formation is the development of a
spatial organization of tissues and organs
• In animals, pattern formation begins with the
establishment of the major axes
• Positional information, the molecular cues
(cytoplasmic determinants and inductive
signals) control pattern formation, and tell a
cell its location relative to the body axes and to
neighboring cells
• Pattern formation has been extensively
studied in the fruit fly Drosophila
melanogaster
• Combining anatomical, genetic, and
biochemical approaches, researchers have
discovered developmental principles
common to many other species, including
humans
The Life Cycle of Drosophila
• In Drosophila, cytoplasmic determinants
in the unfertilized egg determine the axes
before fertilization
• After fertilization, the embryo develops
into a segmented larva with three larval
stages
Fig. 18-17a
Head
Thorax
Abdomen
0.5 mm
Dorsal
BODY
AXES
(a) Adult
Anterior
Left
Ventral
Right
Posterior
Fig. 18-17b
Follicle cell
1 Egg cell
developing within
ovarian follicle
Nucleus
Egg
cell
Nurse cell
Egg
shell
2 Unfertilized egg
Depleted
nurse cells
Fertilization
Laying of egg
3 Fertilized egg
Embryonic
development
4 Segmented
embryo
0.1 mm
Body
segments
5 Larval stage
(b) Development from egg to larva
Hatching
Genetic Analysis of Early Development:
Scientific Inquiry
• Edward B. Lewis, Christiane NüssleinVolhard, and Eric Wieschaus won a Nobel
1995 Prize for decoding pattern formation in
Drosophila
• Homeotic genes control pattern formation in
the late embryo, larva, and adult.
Fig. 18-18
A mutation in regulatory genes, called homeotic genes, caused this.
Eye
Leg
Antenna
Wild type
Mutant
Homeotic Genes
• One example are the Hox and ParaHox genes
which are important for segmentation,
another example is the MADS-box-containing
genes in the ABC model of flower
development.
Chap 21:6
The Homeobox
• Homeotic genes contain a 180 nucleotide
sequence called a homeobox found in
regulatory genes .
• This homeobox has been found in inverts and
verts as well as plants.
• The homeobox DNA sequence
evolved very early in the history of
life and has been conserved virtually
unchanged for millions of years.
• Differences arise due to different
gene expressions.
ABC model of flower development
Molecular basis of differentiation:
• The A, B, and C genes are transcription
factors. Different transcription factors are
needed together to turn on a developmental
gene program--such as A and B needed to
initiate the program for petals. What turns on
the different transcription factors in different
cells?
• Induction and inhibition by one cell signaling
to a neighboring cell.
Fate Mapping
• Fate maps are general territorial diagrams
of embryonic development
• Classic studies using frogs indicated that
cell lineage in germ layers is traceable to
blastula cells
*Chap 47 (3)
http://education-portal.com/academy/lesson/how-fatemapping-is-used-to-track-cell-development.html
Fig. 47-21
Epidermis
Epidermis
Central
nervous
system
64-cell embryos
Notochord
Blastomeres
injected with dye
Mesoderm
Endoderm
Blastula
(a) Fate map of a frog embryo
Neural tube stage
(transverse section)
Larvae
(b) Cell lineage analysis in a tunicate
Fig. 47-21a
Epidermis
Epidermis
Central
nervous
system
Notochord
Mesoderm
Endoderm
Blastula
(a) Fate map of a frog embryo
Neural tube stage
(transverse section)
Axis Establishment
• Maternal effect genes encode for
cytoplasmic determinants that initially
establish the axes of the body of
Drosophila
• These maternal effect genes are also
called egg-polarity genes because they
control orientation of the egg and
consequently the fly
Bicoid: A Morphogen Determining Head
Structures
• One maternal effect gene, the bicoid gene,
affects the front half of the body
• An embryo whose mother has a mutant bicoid
gene lacks the front half of its body and has
duplicate posterior structures at both ends
Fig. 18-19a
EXPERIMENT
Tail
Head
T1
T2
T3
A1
A2
A3
A4
A5 A6
A7
A8
Wild-type larva
Tail
Tail
A8
A8
A7
Mutant larva (bicoid)
A6
A7
Fig. 18-19b
RESULTS
100 µm
Bicoid mRNA in mature
unfertilized egg
Fertilization,
translation
Anterior end
of bicoid
Bicoid protein in early
mRNA
embryo
Fig. 18-19c
CONCLUSION
Nurse cells
Egg
bicoid mRNA
Developing egg
Bicoid mRNA in mature
unfertilized egg
Bicoid protein
in early embryo
Bicoid mRNA, Bicoid Protein (red)
• This phenotype suggests that the product of
the mother’s bicoid gene is concentrated at the
future anterior end
• This hypothesis is an example of the gradient
hypothesis, in which gradients (amounts) of
substances called morphogens establish an
embryo’s axes and other features
• The bicoid research is important for three
reasons:
– It identified a specific protein required for
some early steps in pattern formation
– It increased understanding of the mother’s
role in embryo development
– It demonstrated a key developmental
principle that a gradient of molecules can
determine polarity and position in the
embryo
Cancer results from genetic changes
that affect cell cycle control
• The gene regulation systems that go wrong
during cancer are the very same systems
involved in embryonic development
Types of Genes Associated with Cancer
• Cancer can be caused by mutations to genes that
regulate cell growth and division
• Tumor viruses can cause cancer in animals
including humans
• Oncogenes are cancer-causing genes
• Proto-oncogenes are the corresponding normal
cellular genes that are responsible for normal cell
growth and division
• Conversion of a proto-oncogene to an oncogene
can lead to abnormal stimulation of the cell cycle
Fig. 18-20
Proto-oncogene
DNA
Translocation or
transposition:
Gene amplification:
within a control element
New
promoter
Normal growthstimulating
protein in excess
Point mutation:
Oncogene
Normal growth-stimulating
protein in excess
Normal growthstimulating
protein in excess
within the gene
Oncogene
Hyperactive or
degradationresistant protein
• Proto-oncogenes can be converted to oncogenes
by
– Movement of DNA within the genome: if it
ends up near an active promoter,
transcription may increase
– Amplification of a proto-oncogene: increases
the number of copies of the gene
– Point mutations in the proto-oncogene or its
control elements: causes an increase in gene
expression
Tumor-Suppressor Genes
• Tumor-suppressor genes help prevent uncontrolled
cell growth
• Mutations that decrease protein products of
tumor-suppressor genes may contribute to cancer
onset
• Tumor-suppressor proteins
– Repair damaged DNA
– Control cell adhesion
– Inhibit the cell cycle in the cell-signaling pathway
Interference with Normal Cell-Signaling
Pathways
• Mutations in the ras proto-oncogene and p53 tumorsuppressor gene are common in human cancers
• Mutations in the ras gene can lead to production of
a hyperactive Ras protein and increased cell division
Fig. 18-21a
1 Growth
factor
1
MUTATION
Ras
3 G protein
GTP
Ras
GTP
2 Receptor
Hyperactive
Ras protein
(product of
oncogene)
issues
signals
on its own
4 Protein kinases
(phosphorylation
cascade)
NUCLEUS
5 Transcription
factor (activator)
DNA
Gene expression
Protein that
stimulates
the cell cycle
(a) Cell cycle–stimulating pathway
Fig. 18-21b
2 Protein kinases
MUTATION
3 Active
form
of p53
UV
light
1 DNA damage
in genome
DNA
Protein that
inhibits
the cell cycle
(b) Cell cycle–inhibiting pathway
Defective or
missing
transcription
factor, such
as p53, cannot
activate
transcription
Fig. 18-21c
EFFECTS OF MUTATIONS
Protein
overexpressed
Cell cycle
overstimulated
(c) Effects of mutations
Protein absent
Increased cell
division
Cell cycle not
inhibited
• Suppression of the cell cycle can be important
in the case of damage to a cell’s DNA; p53
prevents a cell from passing on mutations due
to DNA damage
• Mutations in the p53 gene prevent suppression
of the cell cycle
The Multistep Model of Cancer
Development
• Multiple mutations are generally needed for fullfledged cancer; thus the incidence increases with
age
• At the DNA level, a cancerous cell is usually
characterized by at least one active oncogene and
the mutation of several tumor-suppressor genes
Hedgehog Signaling Pathway
http://www.youtube.com/watch?v=FCNJp6Y901M
• The Hedgehog Pathway is very important in
development but after adult state is
reached it is used in maintenance of stem
cells.
Development in Animals
Chap 47 (part of 2, 3)
• Timing and coordination to produce stages
• After fertilization, embryonic development
proceeds through cleavage, gastrulation, and
organogenesis
• The sperm’s contact with the egg’s surface
initiates metabolic reactions in the egg that
trigger the onset of embryonic development
• Important events regulating development occur
during fertilization and the three stages that build
the animal’s body:
– Cleavage: cell division creates a hollow ball of
cells called a blastula
– Gastrulation: cells are rearranged into a threelayered gastrula
– Organogenesis: the three layers interact and
move to give rise to organs
Fig. 47-6
(a) Fertilized egg
(b) Four-cell stage
(c) Early blastula
(d) Later blastula
Gastrulation
• Gastrulation rearranges the cells of a blastula into a
three-layered embryo, called a gastrula, which has a
primitive gut
• The three layers produced by gastrulation are called
embryonic germ layers
– The ectoderm forms the outer layer
– The endoderm lines the digestive tract
– The mesoderm partly fills the space between the
endoderm and ectoderm
These germ layers become:
• Ectoderm – skin and nervous system
• Mesoderm – skeleton, muscles, circulatory,
lining of body cavity
• Endoderm – lining of digestive and
respiratory tract, liver, many glands
(pancreas, thymus, thyroid, parathyroid)
Fig. 47-14
ECTODERM
Epidermis of skin and its
derivatives (including sweat
glands, hair follicles)
Epithelial lining of mouth
and anus
Cornea and lens of eye
Nervous system
Sensory receptors in
epidermis
Adrenal medulla
Tooth enamel
Epithelium of pineal and
pituitary glands
MESODERM
ENDODERM
Notochord
Skeletal system
Muscular system
Muscular layer of
stomach and intestine
Excretory system
Circulatory and lymphatic
systems
Reproductive system
(except germ cells)
Dermis of skin
Lining of body cavity
Adrenal cortex
Epithelial lining of
digestive tract
Epithelial lining of
respiratory system
Lining of urethra, urinary
bladder, and reproductive
system
Liver
Pancreas
Thymus
Thyroid and parathyroid
glands
Fig. 47-9-6
Key
Future ectoderm
Future mesoderm
Future endoderm
Archenteron
Animal
pole
Blastocoel
Blastocoel
Filopodia
pulling
archenteron
tip
Blastocoel
Archenteron
Blastopore
Mesenchyme
cells
Vegetal
plate
Ectoderm
Vegetal
pole
Mouth
Mesenchyme
cells
Blastopore
50 µm
Mesenchyme
(mesoderm
forms future
skeleton)
Digestive tube
(endoderm)
Anus (from
blastopore)
http://www.gastrulation.org/Movie9_3.mov
Gastrulation in the frog
• Early in vertebrate organogenesis, the
notochord forms from mesoderm, and
the neural plate (which will becomes the
nervous system) forms from ectoderm
Fig. 47-12
Eye
Neural folds
Neural
fold
Tail bud
Neural plate
SEM
1 mm
Neural
fold
Somites
Notochord
Neural
crest
cells
Coelom
Somite
Neural tube
Neural
plate
Neural crest
cells
1 mm
Notochord
Ectoderm
Endoderm
Archenteron
Archenteron
(digestive
cavity)
Outer layer
of ectoderm
Mesoderm
Neural crest
cells
(a) Neural plate formation
Neural tube
(b) Neural tube formation
(c) Somites
Fig. 47-21a
Epidermis
Epidermis
Central
nervous
system
Notochord
Mesoderm
Endoderm
Blastula
(a) Fate map of a frog embryo
Neural tube stage
(transverse section)
In humans,
• At completion of cleavage, the blastocyst
forms
• A group of cells called the inner cell mass
develops into the embryo
• The trophoblast, the outer epithelium of the
blastocyst, initiates implantation in the
uterus.
• As implantation is completed, gastrulation
begins
Fig. 47-16-1
Endometrial
epithelium
(uterine lining)
Uterus
Inner cell mass
Trophoblast
Blastocoel
will become embryo
Restriction of the Developmental
Potential of Cells
• In many species that have cytoplasmic
determinants, only the very early stages of
the embryo are totipotent.
• That is, only the zygote can develop into
all the cell types in the adult
• As embryonic development proceeds,
potency of cells becomes more limited
Stem Cells of Animals
• A stem cell is a relatively unspecialized cell that
can reproduce itself indefinitely and
differentiate into specialized cells of one or
more types
• Stem cells isolated from early embryos at the
blastocyst stage are called embryonic stem
cells; these are able to differentiate into all cell
types
• The adult body also has stem cells, which
replace nonreproducing specialized cells
Spemann Experiment
Gray crescent not
bisected equally
• The gray crescent acts as an “organizer”
by inducing cells to become certain parts
of the embryo.
• A signaling protein perhaps?
Importance of Apoptosis in
development
• Elimination of transitory organs and
tissues. Examples include tadpole tails
and gills.
• Tissue remodeling.
Vertebrate limb bud development,
removal of interdigital skin.
• Nutrients are reused!
When the grim and reaper
genes work together, they help
guide cells in flies through
their death process,
apoptosis—much like that
spectre of 15th century
folklore, the Grim Reaper.
Comparing plant and animal
development
• Since plants have rigid cell walls, there is no
morphogenetic movement of cells.
• plant development depends upon differential
rates of cell division then directed
enlargement of cells.
All postembryonic growth occur at meristems
which give rise to all adult structures (shoots,
roots, stems, leaves and flowers) and have the
capacity to divide repeatedly and give rise to a
number of tissues (like stem cells).
Two meristems are established in the embryo, one
at the root tip and one at the tip of the shoot.
The developmental patterning of organs therefore
continues throughout the life of the plant.
• Their fate is determined largely by their position
but they do have signaling.
• Homeotic genes control organ identity (ABC
model) but genes are called Mad-box genes
instead of Hox genes
Similarities in development of
plants and animals
• Both involve a cascade of transcription
factors
• But differences in regulatory genes as stated
in previous slide
Evo-Devo
Comparing developmental processes
of different multicellular organisms
• Many groups of animals and plants, even distantly
related ones, share similar molecular mechanisms
for morphogenesis and pattern formation.
• These mechanisms can be thought of as “genetic
toolkits”.
• Development produces morphology
and much of morphological evolution
occurs by modifications of existing
development genes and pathways
rather than the introduction of
radically new developmental
mechanisms.
Our
common
ancestor