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LECTURE PRESENTATIONS
For CAMPBELL BIOLOGY, NINTH EDITION
Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson
Chapter 47
Animal Development
Lectures by
Erin Barley
Kathleen Fitzpatrick
© 2011 Pearson Education, Inc.
Overview: A Body-Building Plan
• A human embryo at about 7 weeks after
conception shows development of distinctive
features
Eye
Heart
Vertebrae
• Development occurs at many points in the life
cycle of an animal
• This includes metamorphosis and gamete
production, as well as embryonic development
EMBRYONIC DEVELOPMENT
Sperm
Zygote
Adult
frog
Egg
Metamorphosis
Blastula
Larval
stages
Gastrula
Tail-bud
embryo
• Although animals display different body plans,
they share many basic mechanisms of
development and use a common set of
regulatory genes
• Biologists use model organisms to study
development, chosen for the ease with which
they can be studied in the laboratory
Fertilization
• Fertilization is the start of embryonic development
– the formation of a diploid zygote from a haploid egg and sperm
– takes place in the first third of the human fallopian tube
• two types in animals
• 1. Internal – mammals, reptiles, amphibians, worms
• 2. External – echinoderms, cnidarians, fish
• molecules and events at the egg surface play a crucial role in each
step of fertilization
– sperm penetrate the protective layer around the egg
– receptors on the egg surface bind to molecules on the sperm
surface
– changes at the egg surface prevent polyspermy = the entry of
multiple sperm nuclei into the egg
The Acrosomal Reaction
•
•
•
•
first studied with sea urchin eggs
any similarities with mammals
the acrosomal reaction is triggered when the sperm meets the egg
binding of the sperm to a receptor on the egg triggers this reaction
– docking onto the vitelline layer activates the acrosome at the tip of the sperm
– acrosome releases hydrolytic enzymes that digest the coating surrounding the egg
– the sperm extends an acrosomal process through the VL
– process docks with a receptor on the egg  depolarization of the egg and fast block to
polyspermy
Sperm
plasma
membrane
Sperm
nucleus
Basal body
(centriole)
Sperm
head
Acrosome
Jelly coat
Sperm-binding
Acrosomal
process
Fertilization
envelope
Actin
filament
Cortical
Fused
plasma granule
membranes
Hydrolytic enzymes
Perivitelline
Vitelline layer
space
EGG CYTOPLASM
Egg plasma membrane
The Cortical Reaction
• depolarization/fast block does not last very long
• IN ADDITION: the sperm binding to receptors on the egg’s plasma membrane
results in release of intracellular calcium
• calcium release stimulates exocytosis from cortical granules
– Not sure about the contents of these granules
• granule contents build up in between the egg’s membrane and the VL - do two
things:
– 1. removes the sperm receptors from the plasma membrane
– 2. hardens the vitelline layer and forms a fertilization envelope
• VL is now impervious to more sperm = slow block to polyspermy
Sperm
plasma
membrane
Sperm
nucleus
Basal body
(centriole)
Sperm
head
Acrosome
Jelly coat
Sperm-binding
receptors
Acrosomal
process
Fertilization
envelope
Actin
filament
Cortical
Fused
granule
plasma
membranes
Hydrolytic enzymes
Perivitelline
space
Vitelline layer
EGG CYTOPLASM
Egg plasma membrane
• the continued build up of these granules draws water into the space
between the plasma membrane of the egg and the fertilization envelope
– lifts is away and “rips” all other sperm off the egg – except for the one that is
attached by its acrosomal process
• followed by the entry of the sperm’s nucleus
• eventual fusion of the sperm and egg nuclei
• BUT – the egg must be activated first
Sperm
plasma
membrane
Sperm
nucleus
Basal body
(centriole)
Sperm
head
Acrosome
Jelly coat
Sperm-binding
receptors
Fertilization
envelope
Acrosomal
process
Actin
filament
Cortical
Fused
granule
plasma
membranes
Hydrolytic enzymes
Perivitelline
space
Vitelline layer
Egg plasma membrane
EGG CYTOPLASM
Fertilization in Mammals
• fertilization in mammals and other
terrestrial animals is internal
• secretions in the mammalian female
reproductive tract alter sperm motility
and structure
– this is called capacitation
– must occur before sperm are able to
fertilize an egg
• plasma membrane of the egg is
surrounded by an extracellular matrix =
zona pellucida
– similar to the vitelline layer
• and a ring of follicular cells = corona
radiata (nourishment)
1. several sperm enter zona pellucida
-one of the glycoproteins within the ZP (ZP3) acts as a receptor for the
sperm
-initiates an acrosomal reaction:
-production of hyaluornidase to digest away the corona radiata
-production of acrosin to digest away the ZP and oocyte membrane
-exposes a protein on the sperm (GalT) that allows it to bind to the egg’s
plasma membrane
2. the first sperm to contact the plasma membrane of the egg triggers the slow block
to polyspermy
3. oocyte releases the hardened zona pellucida away from the egg surface
4. entry of the entire sperm into the egg’s cytoplasm
5. fusion of the sperm with nucleus of the egg
-before entry and fusion by the sperm - the secondary oocyte must complete
meiosis II and form the ovum
Zona pellucida
• No fast block to
polyspermy has been
identified in mammals
Follicle cell
Sperm Cortical
Sperm nucleus granules
basal body
Egg Activation
• fusion of the two nuclei requires egg
activation
• the rise in Ca2+ in the cytosol of the egg
increases the rates of cellular respiration
and protein synthesis by the egg cell
– when this happens - egg is said to be
activated
• the proteins and mRNAs needed for
activation are already present in the egg
• one key event in oocyte activation =
completion of meiosis II to form the
ovum
-triggered by calcium release inside cell
• following activation the sperm nucleus
can merge with the egg nucleus and cell
division begins
Gamete fusion in Mammals
-sperm and egg membranes fuse  entire sperm taken into the egg
-about 4 hours later – nuclei fusion
-the nuclear envelopes of the egg and sperm disappear
-sperm and egg chromosomes organized onto the same mitotic spindle
-after this = diploid nucleus and a zygote
– in mammals - the first cell division occurs 1236 hours after sperm binding
Zona pellucida
-the sperm contributes its genome and one
centriole
-other organelles of the sperm are rapidly
degraded – including its mitochondria which
are thought to be mutated because of the
stress of “swimming” to the oocyte
(higher metabolism causes stress damage!)
Follicle cell
Sperm Cortical
Sperm nucleus granules
basal body
The One-celled zygote
• fertilized egg = zygote
• Greek for “to join” or “to yolk”
• comprised of:
– diploid nucleus
– cytoplasm – contains yolk proteins that are taken up
during oogenesis via endocytosis
• yolk proteins (i.e. yolk) is the source of nutrients for the
developing embryo
• a zygote with very little yolk will need to be implanted as an
embryo for survival
• mitosis and cytokinesis of a one-celled zygote will
duplicate the nucleus and partition the cytoplasm
into smaller and smaller cells
– known as Cleavage
– cleavage partitions the cytoplasm of the zygote (one large
cell) into many smaller cells called blastomeres
– division of the one celled-zygote creates the embryo
The One-celled zygote
• the difference in yolk distribution as cleavage
proceeds results in animal and vegetal hemispheres
that differ in appearance
• two poles in the zygote – animal and vegetal with a
marginal zone in between called a gray crescent
• animal pole – region of the zygote where
blastomere division is rapid
– little yolk within these cells
– also the location of sperm entry
– cells will become the embryo’s ectoderm and
endoderm
• vegetal pole - has more yolk in its blastomeres
–
–
–
–
cells are larger
divide slower
cells will become the embryo’s endoderm
in some animals – also forms the extra-embryonic
membranes
Yolk proteins
Cleavage
• zygote undergoes cleavage - a period of
rapid cell division without growth
– cell cycle consists mainly of the S phase and M
phase (DNA synthesis and mitosis)
– very little protein synthesis done – skips the
G1 and G2 phases
• first 5 to 7 divisions produces the blastula
– a hollow ball of cells with a fluid-filled cavity
called a blastocoel
50 m
(a) Fertilized egg (b) Four-cell stage (c) Early blastula (d) Later blastula
Cleavage Patterns
• cleavage patterns studied in the frog Xenopus
• in frogs and many other animals- the
distribution of yolk is a key factor
influencing the pattern of cleavage
• types of cleavage:
– 1. Determinate: results in the
developmental fate of the cells within an
embryo being set very early on
• e.g. 2 cell stage
– 2. Indeterminate: fate is not established
until later on in development
• cleavage patterns:
– A. Holoblastic
– B. Meroblastic
• Holoblastic cleavage = complete
division of the egg
– occurs in species whose eggs have
little or moderate amounts of yolk
– e.g. sea urchins and frogs and
mammals
– types:
•
•
•
•
1. bilateral
2. radial
3. rotational
4. spiral
Worms
Humans
Frog
• Meroblastic cleavage = incomplete
division of the egg
– occurs in species with yolk-rich eggs
– the cleavage furrow cannot pass
completely through the cell
– e.g. reptiles and birds
– types:
• 1. discoidal
• 2. superficial – mitosis with no cytokinesis
Radial
Spiral
Cleavage Pattern in the Frog
• 2 cell stage: first cleavage furrow
extends from the animal to the vegetal
pole
– cuts through the gray crescent
– establishes the anterior-posterior axis of the
animal
• 4 cell stage: second cleavage furrows
also extends from the animal to the
vegetal pole
– forms four equally sized blastomeres
Zygote
2-cell
stage
forming
Gray crescent
0.25 mm
4-cell
stage
forming
Animal
pole
8-cell
stage
8-cell stage (viewed
from the animal pole)
0.25 mm
Vegetal pole
Blastocoel
Blastula
(cross
section)
Blastula (at least 128 cells)
Cleavage Pattern in the Frog
• 8 cell stage: the third cleavage is
asymmetric and is along the “equator”
between the two poles
– BUT: the amount of yolk in the cells of the
vegetal pole displaces the cleavage furrow
toward the animal pole
– forms unequally sized blastomeres
– animal pole blastomeres are smaller in size
(micromeres)
• this displacing effect continues from this
point
-more cells found at the animal pole
• 16-cell stage: known as the morula
• rearrangement of the cells of the morula
results in the blastula
• cleavage is considered done as the embryo
transitions from morula to blastula
Zygote
2-cell
stage
forming
Gray crescent
0.25 mm
4-cell
stage
forming
Animal
pole
8-cell
stage
8-cell stage (viewed
from the animal pole)
0.25 mm
Vegetal pole
Blastocoel
Blastula
(cross
section)
Blastula (at least 128 cells)
Cleavage Patterns in Amniotes
• in amniotes - rearrangement of the cells of the morula results in
a mass of cells at one end of the blastula
– mass of cells = inner cell mass
– fluid-filled cavity = blastocoel
– outer covering = trophoblast
Blastula
The Human
Blastocyst
-human embryonic development:
-displacement of cleavage furrow doesn’t happen in mammals like humans – very little yolk in these cells
-so cleavage pattern produces equal sized blastomeres
-day 4– formation of 16 celled morula
-day 5 -fluid begins to collect in the morula and reorganizes the cells around a blastocoel
-embryo is now called a blastocyst (7 cleavages or ~130 cells)
-outer layer = trophoblast
-epithelial layer that forms extra-embryonic tissues (e.g. placenta, yolk
sac)
-also plays a role in implantation by breaking down the endometrium
through the secretion of enzymes
-inner cell mass at one end - totipotent embryonic stem cells
Blastula vs. Blastocyst
• Blastula – hollow ball of cells (blastoderm) surrounding a fluidfilled cavity called a blastocoel
– e.g. sea urchins
• Blastula – hollow ball of cells (trophoblast) surrounding an
inner cell mass at one end and a blastocoel at the other
– found in amniotes – animals that make an egg with an amniotic cavity
• e.g. frogs, chickens
• Blastocyst – blastula found in mammals
– e.g. humans
Morphogenesis in animals involves specific changes in
cell shape, position, and survival
• after cleavage, the rate of cell division slows and the normal cell
cycle is restored
• Morphogenesis = the process by which cells occupy their
appropriate locations and the embryo takes on its shape
– occurs through the production of soluble factors that control the
differentiation of cells = morphogens
– form gradients within the embryo
– many act as transcription factors and bind gene promoters
– others control cell migration and cell-cell adhesion
• morphogenesis involves:
– 1. Gastrulation = the movement of cells from the blastula
surface to the interior of the embryo
– 2. Organogenesis = the formation of organs
Gastrulation
• Gastrulation rearranges the cells of a blastula into a
three-layered embryo = called a gastrula
• 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
• Each germ layer contributes to specific structures in
the adult animal
ECTODERM (outer layer of embryo)
• Epidermis of skin and its derivatives (including sweat glands,
hair follicles)
• Nervous and sensory systems
• Pituitary gland, adrenal medulla
• Jaws and teeth
• Germ cells
MESODERM (middle layer of embryo)
• Skeletal and muscular systems
• Circulatory and lymphatic systems
• Excretory and reproductive systems (except germ cells)
• Dermis of skin
• Adrenal cortex
ENDODERM (inner layer of embryo)
• Epithelial lining of digestive tract and associated organs
(liver, pancreas)
• Epithelial lining of respiratory, excretory, and reproductive tracts
and ducts
• Thymus, thyroid, and parathyroid glands
Gastrulation Definitions
• invagination – infolding of a cell sheet into an
embryo
– forms the mouth, anus and archenteron
• involution – turning in of a cell sheet
• blastopore – opening of the archenteron
– forms at the point where cells enter the embryo
• deuterostome – blastopore becomes the anus
• protostome – blastopore becomes the mouth
Gastrulation in Sea Urchins
•
•
•
•
gastrulation begins at the vegetal pole of the
blastula
cells known as mesenchyme cells (red cells)
detach and migrate throughout the blastocoel
the remaining cells (yellow cells) near the
vegetal plate flatten and buckle inward
through a process called invagination
the depression becomes bigger and deeper
and becomes a cavity called the archenteron
–
–
–
•
–
–
Blastocoel
Mesenchyme
cells
Vegetal plate
Vegetal
pole
Blastocoel
Filopodia
lined with endodermal cells
will become the digestive tract
the opening of the archenteron is the blastopore which will become the anus
mesenchymal cells at the tip of the
archenteron form projections called filopodia
–
Animal
pole
Mesenchyme
cells
filopodia “drag” the archenteron through the
blastocoel
fuses with the wall of the blastocoel
that opening becomes the mouth
• since the mouth forms second
the sea urchin is known as a
Deuterostome
Key
Future ectoderm
Future mesoderm
Future endoderm
Blastopore
Archenteron
50 m
Blastocoel
Ectoderm
Mouth
Mesenchyme
(mesoderm forms
future skeleton)
Archenteron
Blastopore
Digestive tube (endoderm)
Anus (from blastopore)
Video: Sea Urchin Embryonic Development
© 2011 Pearson Education, Inc.
Gastrulation in Frogs
•
•
begins when a group of cells on the dorsal side of
the blastula begins to change shape and invaginate
invagination forms a crease
–
–
•
•
•
•
these cells will form the mesoderm and endoderm
mesoderm stays at the periphery of the embryo
the endoderm fills the embryo
as more cells invaginate - the crease/blastopore
expands and extends around the embryo (red
arrows)
the ectoderm expands and reduces the blastopore
to a small area at the vegetal pole
cells continue to enter into the embryo and the
mesoderm and endoderm expands and forms the
archenteron
–
•
will form the blastopore
the part above the crease is called the dorsal lip of
the blastopore
cells move into the interior (involution - dashed
arrow)
–
–
–
1
CROSS SECTION
SURFACE VIEW
Animal pole
Blastocoel
Dorsal
lip of
blastopore
Early
Vegetal pole
gastrula
Blastopore
Blastocoel
shrinking
2
3
Blastocoel
remnant
the blastocoel shrinks and eventually disappears
Dorsal
lip of
blastopore
Archenteron
Ectoderm
Mesoderm
Endoderm
late in gastrulation – blastopore forms the anus
Key
Future ectoderm
Future mesoderm
Future endoderm
Blastopore
Late
gastrula
Blastopore
Yolk plug
Archenteron
Gastrulation in Chicks
• prior to gastrulation – the chick
embryo is composed of an upper
and lower layer that form an
embryonic disk
Fertilized egg
Primitive
streak
Embryo
– the epiblast and hypoblast
– epiblast = embryo
– hypoblast (‘roof’ of the yolk sac)
= supports embryology & forms
part of the yolk sac
Yolk
Primitive streak
Epiblast
Future
ectoderm
Blastocoel
Migrating
cells
(mesoderm)
Endoderm
Hypoblast
YOLK
Gastrulation in Chicks
• during gastrulation - epiblast cells
move toward the midline of the
blastoderm
• pile-up of cells at the midline forms
a thickening called the primitive
streak
• epiblast cells migrate through the
primitive streak toward the yolk
Fertilized egg
Primitive
streak
Embryo
Yolk
Primitive streak
– some cells push the hypoblast to
the side to become the endoderm
– those remaining cells 
Future
mesoderm
ectoderm
– leftover epiblast cells  ectoderm
• animals with three germ layers are
known as Triploblastic
• Gastrulation is marked by
increased transcription and
translation
Epiblast
Blastocoel
Migrating
cells
(mesoderm)
Endoderm
Hypoblast
YOLK
Gastrulation in Humans
Endometrial epithelium
(uterine lining)
1 Blastocyst reaches uterus.
•
human eggs have very little yolk
Uterus
Trophoblast
– but we do have a yolk sac (site of
hematopoeisis)
•
•
•
the blastocyst is the human equivalent
of the blastula
early stages are very similar to chick
gastrulation
end of day 5 – blastocyst squeezes out
of the zona pellucida and is ready for
implantation (day 7)
–
•
•
•
Blastocoel
2 Blastocyst implants
(7 days after fertilization).
Expanding region of
trophoblast
Maternal
blood
vessel
Epiblast
Hypoblast
Trophoblast
3 Extraembryonic membranes
start to form (10–11 days),
and gastrulation begins
(13 days).
trophoblast is critical for implantation
following implantation - the trophoblast
continues to expand and forms the
extraembryonic membranes
Day 13 - gastrulation follows – similar to
the chick embryo
the front of the primitive streak forms
the blastopore
Inner cell mass
Expanding region of
trophoblast
Amniotic cavity
Epiblast
Hypoblast
Yolk sac (from hypoblast)
Extraembryonic mesoderm cells
(from epiblast)
Chorion (from trophoblast)
4 Gastrulation has produced a
three-layered embryo with
four extraembryonic
membranes.
Amnion
Chorion
Ectoderm
Mesoderm
Endoderm
Yolk sac
Extraembryonic mesoderm
Allantois
Developmental Adaptations of Amniotes
• colonization of land by vertebrates was made possible only after the
evolution of two things:
– 1. shelled egg of birds and other reptiles as well as monotremes (egglaying mammals)
OR
– 2. uterus of marsupial and eutherian mammals
• in both - embryos are surrounded by fluid in a sac called the
amnion
– protects the embryo from desiccation and allows reproduction on dry
land
• mammals and reptiles including birds are called amniotes for
this reason
•
four extraembryonic membranes that form around the embryo
– 1. chorion - functions in gas exchange
•
•
•
two layers – forms from the extraembryonic mesoderm and trophoblast
projections enter into the endometrium = chorionic villi
inner layer contacts the amnion
– 2. amnion - encloses the amniotic fluid; protection of
embryo
•
•
•
•
from EE mesoderm and ectoderm
early development not seen in humans
sac forms just after formation of the blastocyst
floor is the epiblast and then the ectoderm
– 3. yolk sac - encloses the yolk in bony fishes, sharks,
reptiles and birds
•
•
•
also exists in mammals
humans – primitive circulatory system and liver
roof is the hypoblast and then the endoderm
– 4. allantois - disposes of waste products and contributes to
gas exchange
•
•
•
•
•
not seen in fish and amphibians
filled with blood vessels – O2 transport
storage site for nitrogenous wastes
placental mammals – forms part of the umbilical cord
humans – also a connection point to the fetal bladder
Organogenesis
• during organogenesis - various regions of the germ layers develop into
rudimentary organs
• two important ones to a vertebrate: notochord and the neural plate
– the notochord forms from mesoderm
– the neural plate forms from ectoderm
-four weeks of development - embryo forms a tubular structure and undergoes
Neurulation
-embryo begins to form definitive structures – undergoes morphogenesis PLUS
organogenesis:
** neurulation occurs by induction (one tissue influences the development of
another)
-e.g. formation of the nervous system requires the mesodermal cells
of the notochord
-in front of the primitive streak - primitive node/Hensen’s knot  secretion of
numerous growth factors for neural development
-in front of the primitive node - the ectoderm folds to form the neural folds  head
and associated structures
-the groove between the folds (neural groove) deepens and the folds “fold over” to
form a tube = neural tube
-the neural groove develops into three ‘vesicles’  forebrain, midbrain and hindbrain
-mesodermal cells of the primitive node form a tube that runs the length of the
embryo
- becomes the notochord (day 22-24)  vertebral column
• notochord:
Neural folds
– develops from the mesoderm of
the embryo
– supportive rod that extends most
of the animal’s length – extends
into the tail
– dorsal to the body cavity
• located between the nerve cord and
the digestive tract
– flexible to allow for bending but
resists compression
1 mm
Neural Neural
fold
plate
• also a point of swimming muscle
attachment in some species – e.g.
amphioxus
– composed of large, fluid-filled cells
encased in a fairly stiff fibrous
tissue
– will become the vertebral column
in many chordates
• in humans, remnants of the
notochord can be found in the
intervertebral discs
Notochord
Ectoderm
Mesoderm
Endoderm
Archenteron
(a) Neural plate formation
Video: Frog Embryo Development
•
the ectoderm on top of the notochord is the
neural plate
•
•
Neural
fold
Neural plate
requires the formation of the notochord first –
induction by the mesoderm
the neural plate curves inward bringing the
neural folds together and forming the neural
tube (Neurulation)
– tissue on top of the neural tube is ectoderm
and will form the outer covering of the animal
•
the neural tube will become the central
nervous system (brain and spinal cord)
– will form into a dorsal nerve cord (spinal cord)
– anterior expansion are called neural folds and
will form the brain
•
•
cells that do not form the neural plate or stay
in the ectoderm are called Neural crest cells
neural crest cells migrate throughout the
embryo to form many tissues
–
nerves, parts of teeth, skull bones, part of the heart
(b) Neural tube formation
Neural
crest cells
Neural
crest cells
Neural
tube
Outer layer
of ectoderm
• portions of the mesoderm that do not
form the notochord form blocks called
somites
– located lateral to the notochord
– arranged serially along the length of
the notochord
– differentiate into sclerotomes
(cartilage and tendons, vertebrae),
myotomes (skeletal muscle) and
dermatomes (dermis)
– other parts differentiate into
mesenchymal cells - migratory
• in vertebrates - one of the major
functions of the somites is to form the
vertebrae, muscles of the vertebrae
and the ribs
• lateral to the somites - the mesoderm
splits into two layers
Eye
SEM
Neural tube
Notochord
Coelom
Somites
Tail bud
1 mm
Neural
crest
cells
Somite
– defines the coelom (body cavity)
(c) Somites
Archenteron
(digestive
cavity)
• Organogenesis in the chick is quite similar to that
in the frog
Neural tube
Notochord
Eye
Forebrain
Somite
Coelom
Endoderm
Mesoderm
Ectoderm
Archenteron
Lateral
fold
Heart
Blood
vessels
Somites
Yolk stalk
These layers
form extraembryonic
membranes.
(a) Early organogenesis
Yolk sac
Neural
tube
YOLK
(b) Late organogenesis
• The mechanisms of organogenesis in
invertebrates are similar
– but the body plan is very different
– e.g. the neural tube develops along the ventral side of
the embryo in invertebrates, rather than dorsally as
occurs in vertebrates
Mechanisms of Morphogenesis
• morphogenesis = creation of form or shape
• morphogenesis in animals but not plants involves
movement of cells
– cell wall of plants prevents complex processes like
gastrulation and organogenesis
• movement in parts of the cell
– changes in the cytoskeleton can result in cell shape
changes
• movement of the cell itself = migration
The Cytoskeleton in Morphogenesis
• reorganization of the cytoskeleton is a
major force in changing cell shape
during development
•
e.g. neurulation - microtubules oriented
from dorsal to ventral in a sheet of
ectodermal cells help lengthen the cells
along that axis and creates a long, rolled
tubular embryo
• 1. cuboidal ectodermal cells form a
continuous sheet
• 2. microtubules elongate the cells
• 3. actin filaments contract and deform
the sheet into a wedge – invagination
results
• 4. a wedge at the bottom and at the top
creates a tube
• 5. pinching off forms the tube
Ectoderm
Neural
plate
Microtubules
Actin
filaments
Neural tube
• the cytoskeleton also directs cell migration
– during organogenesis
• cells “crawl” during embryonic development
– cytoskeleton produces cellular extensions that adhere to
substrates and retracts
• the production of an extracellular matrix within the
embryo can direct where these cells will go
– production of cellular adhesion molecules (CAMs) on the
surface of the migrating cells interact with the ECM
– these CAMs interact with the cytoskeleton
• production of growth factor gradients can also direct
migration
Programmed Cell Death
• also known as apoptosis
• at various times during development - individual cells, sets of
cells, or whole tissues stop developing and undergo apoptosis
– are engulfed by neighboring cells
• e.g. loss of the tail by the tadpole  frog
• e.g. loss of the webbing between fingers and toes  humans
• e.g. many more neurons are produced in developing embryos
than will be needed
– extra neurons are removed by apoptosis
• numerous pathways regulate apoptosis
– production and activation of proteins called caspases
– also a role for the mitochondria in triggering the activation of these
caspases
Cytoplasmic determinants and inductive signals contribute to
cell fate specification
•
Cells in a multicellular organism share the same genome
–
•
how does one cell differ from another??
Differences in cell types are the result of the expression of different sets of
genes
–
cell is said to have acquired a specific fate
• there are two ways cell fate can be determined
– 1. Irreverisibly  Determination
– 2. Reversibly  Specification or Differentiation
• Determination is the term used to describe the process by which a cell or
group of cells becomes permanently committed to a particular fate
– e.g. dorsal vs. ventral structures
– occur VERY early on in embryonic development
– determination cannot be reversed!
• Specification or Differentiation refers to the resulting specialization in
structure and function
– this can be reversed
Cytoplasmic determinants and inductive signals contribute to
cell fate specification
• determination and specification can be done
one of two ways:
– autonomous is controlled by cytoplasmic determinants
• no role for its surrounding environment
• factors made within the cytoplasm
• usually are transcription factors that alter gene expression
– conditional is controlled by extracellular factors such as
surrounding cells or growth factors
Cytoplasmic determinants and inductive signals contribute to
cell fate specification
• fate determination and differentiation is a series of complex
events that involve changes in gene expression
– inductive signals for these changes are also complex
– can involve many things – such as:
• production of growth factors within the embryo – triggers specific
signaling pathways within the target cell  changes in gene
expression
• production of the extracellular matrix – can affect signaling
pathways within the cell also  changes in gene expression
• physical interaction between two cells
Determination and Specification
• the sea squirt (tunicate) and conditional specification:
– transplantation of the part of the cytoplasm (yellow crescent) produces muscle
cells
– endodermal cells produce FGF  notochord development by anteriorly positioned
cells; mesenchyme by posteriorly positioned cells
• the sea urchin:
– anterior-posterior axis lies along the animal-vegetal axis
– the blastomeres in the vegetal pole induce nearby tissue to become endoderm and
only endoderm
• cell fate determination
• role for a protein called beta-catenin
• present in the nuclei of cells in the vegetal pole – autonomous specification
– the blastomeres of the animal pole induce nearby cells to become either
ectoderm, mesoderm or endoderm
• cell fate specification because those cells that choose ectoderm can reverse and become
endoderm or mesoderm
• autonomous and conditional specification
Fate Mapping
• Fate maps are diagrams showing
organs and other structures that arise
from each region of an embryo
• classic studies using frogs indicated
that cell lineage in germ layers is
traceable to blastula cells
– 1920s – Walther Vogt
– marked specific cells in a frog blastula with
non-toxic dyes
– embryos sectioned and the dyes detected
• also done in tunicates (primitive
chordates)
Epidermis
Central
nervous
system
Epidermis
Notochord
Mesoderm
Endoderm
Blastula
Neural tube stage
(transverse section)
(a) Fate map of a frog embryo
64-cell embryos
Blastomeres
injected with dye
Larvae
(b) Cell lineage analysis in a tunicate
• later studies of C. elegans used the ablation (destruction) of single cells to
determine the structures that normally arise from each cell
– Caenorhabditis elegans is a roundworm
– 1mm long
– transparent body
• researchers were able to determine the lineage of each of the 959 somatic
cells in the worm
Time after fertilization (hours)
– determined that exactly 131 cells dies during normal development
– if one gene in these cells is mutated – they all live
– gene was found to be critical in apoptosis across a wide range of animals
Zygote
0
First cell division
Nervous
system,
outer skin,
musculature
10
Musculature, gonads
Outer skin,
nervous system
Germ line
(future
gametes)
Musculature
Hatching
Intestine
Intestine
Anus
Mouth
Eggs
Vulva
1.2 mm
ANTERIOR
POSTERIOR
• cell fate mapping in C. elegans done using germ cells
– are determined cells that give rise to sperm or eggs
• in all animals - complexes of RNA and protein are involved in the
specification of germ cell fate
• in C. elegans - such complexes are called P granules
– found in the zygote and persist throughout development
– can be detected in the gonads of the adult worm
– labelling of P granules in a fertilized C. elegans egg can allow researchers to track what
happens as a germ cell’s fate is determined
100 m
• P granules are distributed throughout the
newly fertilized C.elegans egg and move to
the posterior end before the first cleavage
division
– as a result only the most posterior of the two
cells formed by the first mitotic division contains
P granules
20 m
1 Newly fertilized egg
• with each subsequent cleavage, the P
granules are partitioned into the
posterior-most cells
– this cell is now a germ cell
2 Zygote prior to first division
• P granules act as cytoplasmic
determinants - fixing germ cell fate at the
earliest stage of development
– fate is not reversible
• tracking labelled P granules allows us to
see where germ cells develop and migrate
in an embryo
3 Two-cell embryo
4 Four-cell embryo
Axis Formation in the frog
• a body plan with bilateral symmetry is found across a range of animals
– nematodes, annelids, several invertebrates and also vertebrates
– this body plan exhibits asymmetry across the dorsal-ventral and anterior-posterior axes
– the right-left axis is largely symmetrical
• the anterior-posterior axis of the frog embryo is determined during oogenesis
– cleavage along the animal-vegetal pole indicates where the anterior-posterior axis forms
• the dorsal-ventral axis is not determined until fertilization
• once the A-P and D-V axes are established – the bilateral left-right axis is fixed
• upon fusion of the egg and sperm - the egg surface (plasma membrane and
inner cortex) rotates with respect to the inner cytoplasm
– called cortical rotation
– always toward the point of sperm entry
• rotation brings molecules from the cytoplasm of the animal hemisphere to
interact with molecules in the cortex of the vegetal pole
– inductive interactions activate regulatory factors in the vegetal cortex –
changes gene expression in that region
– leads to expression of dorsal- and ventral-specific gene expression
Dorsal
Right
Anterior
Posterior
Left
Ventral
(a) The three axes of the fully developed embryo
Animal
hemisphere
Animal pole
Point of
sperm
nucleus
entry
Vegetal
hemisphere
Vegetal pole
(b) Establishing the axes
Gray
crescent
Pigmented
cortex
Future
dorsal
side
First
cleavage
• in chicks - gravity is involved in establishing the anterior-posterior axis
– as the egg travels down the oviduct prior to being laid
• later, pH differences between the two sides of the blastoderm establish the
dorsal-ventral axis
– if the pH is reversed above and below the blastoderm – reverses cell’s fates
– the side facing the egg white becomes the ventral part of the embryo
– the side facing the yolk becomes the dorsal part
• in mammals - experiments suggest that orientation of the egg and sperm
nuclei before fusion may help establish embryonic axes
Restricting Developmental Potential
• Hans Spemann performed experiments to determine a cell’s developmental
potential (range of structures to which it can give rise)
• embryonic fates are affected by distribution of determinants and the pattern
of cleavage
• the first two blastomeres of the frog embryo are totipotent (can develop into
all the possible cell types)
– cells in the inner cell mass of a human embryo are also totipotent
Control egg
(dorsal view)
1a Control
group
1a. fertilized salamander eggs allowed to
divide normally  gray crescent in between Gray
crescent
the first two blastomeres
1b. fertilized eggs constricted by a thread
-shifted the gray crescent toward one blastomere
2. two blastomeres separated and allowed to
develop
RESULTS: blastomere that didn’t receive any material
from the gray crescent – loss of dorsal structures
Normal
Experimental egg
(side view)
1b Experimental
Gray
group
crescent
Thread
2
Belly piece
Normal
Cell Fate Determination and Pattern Formation
by Inductive Signals
• Question: when can cell fate can be modified?
• in mammals - embryonic cells remain totipotent until the 8-cell stage
– much longer than other organisms
• BUT - progressive restriction of developmental potential is a general feature of
development in all animals
• in general: tissue-specific fates of cells are fixed (i.e. determined) by the late
gastrula stage
• as embryonic cells acquire distinct fates - they influence each other’s fates by
induction
– conditional specification
– i.e. through the production of inductive signals
The “Organizer” of Spemann and Mangold
•
•
Spemann and Mangold transplanted tissues between early gastrulas and found that
the transplanted dorsal lip triggered a second gastrulation in the host
the dorsal lip functions as an organizer of the embryo body plan, inducing changes in
surrounding tissues to form notochord, neural tube, and so on
– production of a growth factor called BMP4 is critical for early embryogenesis
• causes the development of ventral structures
– involved in cell fate determination and specification
– Spemann’s Organizer works to inactive BMP4 signaling on the dorsal side of the embryo
•
determines what will be dorsal and what will be ventral
EXPERIMENT
Dorsal lip of
blastopore
RESULTS
Primary embryo
Secondary
(induced) embryo
Pigmented
gastrula
(donor embryo)
Nonpigmented
gastrula
(recipient embryo)
Primary structures:
Neural tube
Notochord
Secondary structures:
Notochord (pigmented cells)
Neural tube
(mostly nonpigmented cells)
Induction: Formation of the Vertebrate
Limb
• Inductive signals also play a major role in pattern formation
– the development of spatial organization
• the molecular cues that control pattern formation are called
positional information
– this information tells a cell where it is with respect to the body axes
– it also determines how the cell and its descendants respond to future
molecular signals
Drosophila Embryogenesis
early pattern formation work done in Drosophila – translates to
higher organisms
– three kinds of pattern formation genes
• 1. maternal effect genes – genes of the oocyte
–
–
–
–
responsible for the axes of the embryo
genes translated make proteins that form gradients within the embryo
protein gradients are created using the cytoskeleton
e.g. concentration of Bicoid protein at anterior end of the embryo results in head
structures
• 2. segmentation genes – establish the segmented body plan from A to P
• 3. homeotic genes – produce proteins with a highly conserved DNA binding region
(homeodomain)
– found on chromosome 3 in a series of clusters (multiple HOX genes per cluster)
– control A-P axis formation along with maternal effect and segmentation genes
– after the segments form along the AP axis – the hox genes determine what type of
segments they will become
» Antennepedia cluster of HOX genes  leg formation
-HOX genes are very active in human emrbyogenesis
•
•
•
the wings and legs of chicks, like all vertebrate limbs, begin as bumps of tissue called limb
buds
one limb bud–regulating region is the apical ectodermal ridge (AER)
the second region is the zone of polarizing activity (ZPA)
Anterior
Limb bud
AER
ZPA
Limb buds
2
Posterior
50 m
Digits
Apical
ectodermal
ridge (AER)
Anterior
3
4
Ventral
Proximal
Distal
Dorsal
Posterior
(a) Organizer regions
(b) Wing of chick embryo
• apical ectodermal ridge (AER)
– thickened ectoderm at the bud’s tip
– removal blocks the outgrowth of the limb along the proximal-distal axis
– AER secretes growth factors in the FGF family
• zone of polarizing activity (ZPA)
– mesodermal tissue under the ectoderm where the posterior side of the bud is attached
to the body
– necessary for pattern formation along the anterior-posterior axis
– e.g. in humans – determines where the thumb and 5th digit are positioned
– cells nearest the ZPA become the most posterior of the digits
Anterior
Limb bud
AER
ZPA
Limb buds
50 m
2
Posterior
Digits
Apical
ectodermal
ridge (AER)
Anterior
Ventral
Proximal
Dorsal
Distal
Posterior
(a) Organizer regions
3
4
(b) Wing of chick embryo
• tissue transplantation experiments support
the hypothesis that the ZPA produces an
inductive signal that conveys positional
information indicating “posterior”
EXPERIMENT
Anterior
New
ZPA
Donor
limb
bud
Host
limb
bud
ZPA
• inductive signal of the ZPA was found to
be Sonic hedgehog
– also involved in segmentation in the fruit fly
(called hedgehog)
– important roles in humans too
– SHH gradient results in the limb - decreasing
levels of SHH determines anterior
Posterior
RESULTS
4
3
2
• BUT before that: Hox genes also play
roles during limb pattern formation
– Hox gene expression determines whether a
limb will be a forelimb or a hindlimb
– so HOX and SHH determine arms vs. legs and
toes vs. fingers
2
4
3
Cilia and Cell Fate
• Ciliary function is essential for proper specification of cell fate in
the human embryo
• two kinds of cilia
– 1. Motile
– 2. Non-motile – monocilia
• found on nearly all cells
• motile cilia play roles in left-right specification
– creation of inductive signal gradients within the embryo? redistributes
cells within the embryo
– study of certain developmental problems called Kartagener’s syndrome
• non-motile sperm in males, sinus infections and bronchial infections – motile cilia no
longer work
• situs inversus - reversal of normal left-right symmetry
–
right to left flow by cilia creates an asymmetry between left and right halves of the body
• monocilia play roles in normal kidney development
– function as an “antenna” for multiple signaling molecules