Lecture

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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
– 1. sperm penetrates the protective layers around the egg
– 2. receptors on the egg surface bind to molecules on the
sperm surface
– 3. changes at the egg surface prevent polyspermy = the entry
of multiple sperm nuclei into the egg
Sperm
• three functions
– 1. reach the oocyte
– 2. penetrate the oocyte
– 3. donate its chromosomes to the
oocyte
Sperm
• major parts
– 1. head: contains the nucleus with 23
highly condensed chromosomes (one
chromatid)
– 2. acrosome: covers the anterior 2/3 of
the head
• specialized organelle that contains digestive
enzymes to dissolve the protective barriers
surrounding the oocyte
• e.g. hyaluronidase and proteases
– 3. midpiece
• contains mitochondria & pair of centrioles for the
production of the microtubules for the tail
• only centrioles are donated to the oocyte
– 4. tail or flagellum
• principal piece – longest portion of the tail
• end piece – terminal portion of the tail
• microtubules of the flagellum – arranged in a 9+2
pattern and are known as the axoneme
• made through oogenesis
– from an egg stem cell = oogonium
• bounded by a plasma membrane with several
proteins studded in it
• surrounded by 1 to 3 membranes
– mammalian oocyte – thin zona pellucida then a thicker
corona radiata
– other animals – innermost coat is called the vitelline layer
or the jelly coat
• depends on the animal
• e.g. amphibians = jelly coat
• innermost layer (e.g. ZP or VL) is made by the
oocyte
• the outermost layers are produced by the cells
of the oviduct
– e.g. layers of the shell
Oocyte
• components of oocyte:
– 1. nucleus – 23 chromatid
chromosomes
– 2. cytoplasm – numerous
components
• yolk granules
• also contains mRNAs and proteins for
fertilization, cleavage, cell fate
determination, embryo axis
orientation
– 3. maternal contributions –
nucleolus, mitochondria, centriole
pair, ribosomes
Oocyte
The Acrosomal Reaction
•
•
•
•
first studied with sea urchin eggs
many 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
– 1. docking onto the jelly layer activates the acrosome at the tip of the sperm
– 2. acrosome releases hydrolytic enzymes that digest the coating surrounding the egg
– 3. the sperm extends an acrosomal process through the jelly layer
– 4. 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
receptors
Fertilization
envelope
Acrosomal
process
Actin
filament
Cortical
Fused
granule
plasma
membranes
Hydrolytic enzymes
Perivitelline
space
Vitelline layer
Egg plasma membrane
EGG CYTOPLASM
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
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
The Cortical Reaction
• 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
Fertilization
envelope
Acrosomal
process
Actin
filament
Fused
plasma
membranes
Acrosome
Jelly coat
Sperm-binding
receptors
Hydrolytic enzymes
Vitelline layer
Egg plasma membrane
Cortical
granule
Perivitelline
space
EGG CYTOPLASM
• the continued build up of these cortical granules draws water into the space
between the plasma membrane of the egg and the fertilization envelope
– lifts the envelope away from the oocyte 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
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:
-e.g. production of hyaluronidase to digest away the corona radiata
-e.g. 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 – cortical
granule role
4. entry of the entire sperm into the egg’s cytoplasm
5. fusion of the sperm with nucleus of the egg
Zona pellucida
Follicle cell
• No fast block to
polyspermy has been
identified in mammals
Sperm Cortical
Sperm nucleus granules
basal body
-before entry and fusion by the sperm - the secondary
oocyte must complete meiosis II and form the ovum
Zona pellucida
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
• 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 from oocyte – contains yolk that is
taken up during oogenesis via endocytosis
• mitosis and cytokinesis of the one-celled
zygote will duplicate the nucleus and
partition the cytoplasm into the progeny
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
two nuclei fusing to form the zygote
What is Yolk?
•
•
•
•
food source stored in the egg’s cytoplasm
for growth of the embryo
made up of yolk proteins and lipids
amount of yolk differs from animal to animal
– fish, amphibians, birds and some insects have “yolky” eggs
– mammals – small oocytes with very little yolk
• most yolk is not made by the oocyte
– made by the liver of the mother and are transported by the
circulatory system of the animal to the forming oocyte
• the yolk is heavy and is found at the bottom of the
oocyte = site of the vegetal pole or vegetal
hemisphere
• in mammals – the yolk is distributed throughout the
oocyte
Yolk proteins
The one-celled Zygote
• the zygote is a single cell with ONE nucleus
(containing 46 single chromatid
chromosomes) plus a cytoplasm containing
yolk proteins and lipids (i.e. yolk)
• in zygotes with large amounts of yolk:
– because the yolk is heavy it settles to the
bottom of the zygote (i.e. vegetal pole)
– this “squishes” the nucleus up toward the
opposite pole = animal pole
– as the zygote splits and forms the embryo –
it forms on top of the yolk
The one-celled Zygote
• as the zygote splits and forms
the embryo – cytokinesis will
partition the yolk into the
blastomeres of the embryo
– those blastomeres containing the most
yolk stay at the vegetal pole of 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 and the
blastomeres are smaller
– little yolk within these cells
– also the location of sperm entry
– cells will become the embryo’s ectoderm and
endoderm
Yolk proteins
The One-celled zygote
• vegetal pole - has more yolk in its
blastomeres
– cells are larger & divide slower
– some of the cells will become the
embryo’s endoderm
– in some animals – also forms the extraembryonic 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 – one cell  ectoderm; 2nd cell 
endoderm
– 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
Radial
Spiral
• 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
Worms
Humans
Frog
Radial
Spiral
Radial Cleavage
• 2 cell stage: first cleavage furrow
extends from the animal to the vegetal
pole
– 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
• 8 cell stage: the third cleavage is along
the “equator” between the two poles
– at this point – cleavage can differ between
animals
– e.g. sea urchin – this cleavage is right at the
‘equator’ between the poles
– e.g. frog – cleavage plane is above it due to the
yolk
1st cleavage: from animal to
vegetal pole
2nd cleavage: A to V pole but at
right angles to the 1st
3rd cleavage: right angle to the
A-V axis
Cleavage Patterns in the Frog
• up to the 8 cell stage – radial
cleavage
• 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
Zygote
2-cell
stage
forming
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
-produces more cells at the animal pole
• 16-cell stage: known as the morula
Gray crescent
Blastocoel
Blastula
(cross
section)
Blastula (at least 128 cells)
Spiral & Rotational Cleavage
• in animals with spiral cleavage pattern – the
cleavage planes may not be parallel or at
right angles to the animal-vegetal axis
• in humans/mammals – rotational cleavage
pattern
•
•
•
•
1st cleavage – parallel to A-V axis
2nd cleavage is along the equator – and only splits
one of the two blastomeres
the second blastomere is split at another angle
complex series of divisions
Rotational
• rearrangement of the cells of the morula results in the blastula
• cleavage is considered done as the embryo transitions from morula to blastula
• what occurs now is cell division (mitosis) with Growth
Morula
Blastula
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 the first few cleavage patterns produces equal sized blastomeres
-then we undergo rotational cleavage
-day 3 – formation of 16-celled morula
-day 4 to 5 - fluid begins to collect as the morula continues to divide
-the blastomeres reorganize around a blastocoel
-embryo is now called a blastocyst (7 cleavages or ~130 cells)
The Human
Blastocyst
-day 6 - embryo is now called a blastocyst (7 cleavages or ~130 cells)
-outer layer = trophoblast
-epithelial layer that forms extra-embryonic tissues
-also plays a role in implantation by enzymatically breaking down
the endometrium
-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
NEXT CLASS:
MORPHOGENESIS &
ORGANOGENESIS
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 – folding in of a cell sheet into an embryo
– forms the mouth, anus and archenteron
• involution – turning in of a cell sheet at an opening in
the embryo
– requires migration
– results in invagination
• blastopore – opening of the archenteron
– forms at the point where cells enter the embryo
• deuterostome – blastopore becomes the anus
– i.e. mouth forms second
• 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 fold inward
through a process called invagination
the depression becomes bigger and
deeper and becomes a cavity called the
archenteron
–
–
–
lined with endodermal cells
will become the digestive tract
the opening of the archenteron is the
blastopore - which will become the anus
Animal
pole
Blastocoel
Mesenchyme
cells
Vegetal plate
Vegetal
pole
Blastocoel
Filopodia
Mesenchyme
cells
Blastopore
Archenteron
50 m
Blastocoel
Ectoderm
Key
Future ectoderm
Future mesoderm
Future endoderm
Mouth
Mesenchyme
(mesoderm forms
future skeleton)
Archenteron
Blastopore
Digestive tube (endoderm)
Anus (from blastopore)
Gastrulation in Sea Urchins
Animal
pole
Blastocoel
Mesenchyme
cells
• mesenchymal cells at the tip of the
archenteron form projections called
filopodia
Vegetal plate
– 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
Vegetal
pole
Blastocoel
Filopodia
Mesenchyme
cells
Blastopore
Archenteron
50 m
Blastocoel
Ectoderm
Key
Future ectoderm
Future mesoderm
Future endoderm
Mouth
Mesenchyme
(mesoderm forms
future skeleton)
Archenteron
Blastopore
Digestive tube (endoderm)
Anus (from blastopore)
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
–
–
•
crease 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
Dorsal
lip of
blastopore
Archenteron
these cells will form the mesoderm and
endoderm
mesoderm stays at the periphery of the
embryo
the endoderm fills the embryo
3
Blastocoel
remnant
Ectoderm
Mesoderm
Endoderm
Key
Future ectoderm
Future mesoderm
Future endoderm
Blastopore
Late
gastrula
Blastopore
Yolk plug
Archenteron
Gastrulation in Frogs
•
•
•
as more cells invaginate and involute 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
–
•
1
CROSS SECTION
SURFACE VIEW
Animal pole
Blastocoel
Dorsal
lip of
blastopore
Early
Vegetal pole
gastrula
Blastopore
Blastocoel
shrinking
2
Dorsal
lip of
blastopore
Archenteron
the blastocoel shrinks and eventually
disappears
late in gastrulation – blastopore forms
the anus
3
Blastocoel
remnant
Ectoderm
Mesoderm
Endoderm
Key
Future ectoderm
Future mesoderm
Future endoderm
Blastopore
Late
gastrula
Blastopore
Yolk plug
Archenteron
Gastrulation in Frogs
Zygote
•
•
•
•
•
in some amphibians – the region of
blastopore formation is less pigmented
than the animal hemisphere above it
known as the gray crescent
found in the amphibian zygote/embryo
pigments in these cells produce a gray
coloration
this region will become the blastopore
and the dorsal portion of the animal
2-cell
stage
forming
Gray crescent
Gastrulation in Chicks
Fertilized egg
• prior to gastrulation – the chick
embryo is composed of an upper
and lower layer that form an
embryonic disk
Primitive
streak
Embryo
Yolk
– the epiblast and hypoblast
– epiblast = embryo
– hypoblast (‘roof’ of the yolk sac)
= supports embryology & forms
part of the yolk sac
Primitive streak
Epiblast
Future
ectoderm
Blastocoel
http://www.youtube.com/watch?v=P
edajVADLGw
Migrating
cells
(mesoderm)
Endoderm
Hypoblast
YOLK
Gastrulation in Chicks
Fertilized egg
• 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
– some cells push the hypoblast to
the side to become the
endoderm
– those remaining cells 
mesoderm
– leftover non-migrating epiblast
cells  ectoderm
Primitive
streak
Embryo
Yolk
Primitive streak
Epiblast
Future
ectoderm
Blastocoel
Migrating
cells
(mesoderm)
Endoderm
Hypoblast
YOLK
Gastrulation in Chicks
Fertilized egg
Primitive
streak
• animals with three germ layers are
known as Triploblastic
• Gastrulation is marked by
increased transcription and
translation
Embryo
Yolk
Primitive streak
Epiblast
Future
ectoderm
Blastocoel
Migrating
cells
(mesoderm)
Endoderm
Hypoblast
YOLK
Gastrulation in the Human Blastocyst
Endometrial epithelium
(uterine lining)
1 Blastocyst reaches uterus.
•
human eggs have very little yolk
Uterus
Trophoblast
– yolk sac (site of hematopoeisis)
•
•
•
•
•
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)
following implantation - the trophoblast
will continue to expand and form the
extra-embryonic membranes
Day 13 - gastrulation follows – similar to
the chick embryo
the front of the primitive streak forms
the blastopore
Inner cell mass
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).
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
•
http://www.youtube.com/watch?v=x-p_ZkhqZ0M
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 extra-embryonic membranes that form
around the embryo
• all derived from the trophoblast
• cells form extra-embryonic mesoderm and
extra-embryonic endoderm
• develop into the:
– 1. chorion - functions in gas exchange
• two layers – forms from the extra-embryonic
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
• in front of the primitive streak primitive node/Hensen’s knot 
secretion of numerous growth factors
for neural development
Neural folds
• notochord:
– develops from the mesoderm of
the embryo – humans day 22 to 24
– supportive rod that extends most of
the animal’s length – extends into
the tail
1 mm
Neural Neural
fold
plate
– dorsal - located between the nerve
cord and the digestive tract
– flexible to allow for bending but
resists compression
• also a point of swimming muscle
attachment in some species – e.g.
amphioxus
Notochord
– composed of large, fluid-filled cells Ectoderm
encased in a fairly stiff fibrous tissue Mesoderm
– will become the vertebral column in Endoderm
many chordates
• in humans, remnants of the
notochord can be found in the
intervertebral discs
Archenteron
(a) Neural plate formation
• 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)
Neural
crest cells
– tissue on top of the neural tube is
ectoderm and will form the outer
covering of the animal
Neural
crest cells
(b) Neural tube formation
Neural
tube
Outer layer
of ectoderm
• the neural tube will become the
central nervous system (brain and
spinal cord)
Neural
fold
Neural plate
– 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 along the notochord as
a series
– differentiate into sclerotomes
(cartilage and tendons, vertebrae),
myotomes (skeletal muscle) and
dermatomes (dermis)
– other parts differentiate into
migratory mesenchymal cells
• 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
– fuse to become the coelom (body
cavity)
(c) Somites
Archenteron
(digestive
cavity)
• Organogenesis in the chick is very 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
NEXT CLASS
CELL FATE DETERMINATION
& DEVELOPMENT
Mechanisms of Morphogenesis
• morphogenesis = creation of form or shape
• morphogenesis in animals but not plants cell wall of
plants prevents complex processes like gastrulation and
organogenesis
• involves movement of cells
• PLUS movement within the cell
– changes in the cytoskeleton can result in cell shape
changes
• movement of the cell itself = migration
The Cytoskeleton in Morphogenesis
Ectoderm
• reorganization of the cytoskeleton is a
major force in changing cell shape
during development
• e.g. neurulation
• 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
Neural
plate
Microtubules
Actin
filaments
Neural tube
The Cytoskeleton in Organogenesis
•
the cytoskeleton also directs cell migration
during organogenesis
•
cells “crawl” during embryonic development
–
•
•
cytoskeleton produces cellular extensions that
adhere to substrates - retracts to pull the cell
along
the production of an extracellular matrix
substrate 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
http://www.youtube.com/watch?v=NYvgkMUdisU
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
• 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
• 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. Irreversibly  Determination
– 2. Reversibly  Specification or Differentiation
Cytoplasmic determinants and inductive signals contribute to
cell fate specification
• 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
• 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:
– endodermal cells produce FGF  notochord development by anteriorly positioned
cells; mesenchyme (muscle) by posteriorly positioned cells
– transplantation of these FGF producing cells can alter this pattern
Determination and Specification
• 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 & autonomous
– 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
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
individual 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
• cell fate mapping also done in the round worm Caenorhabditis
elegans
• in all animals - complexes of RNA and protein are involved in the
specification of germ cell fate
– germ cells give rise to the gonads
• in C. elegans - these complexes are called P granules
– found in the zygote and persist throughout development
• labelled P granules are distributed
throughout the newly fertilized C.elegans
egg
• move to the posterior end before the first
cleavage division
• with each subsequent cleavage, the P
granules become localized to the
posterior-most cell
20 m
1 Newly fertilized egg
– this cell is now a germ cell
• P granules act as cytoplasmic
determinants – fixing this germ cell fate at
the earliest stage of development
2 Zygote prior to first division
– 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
• additional tracking labelled P granules allows us to see
where germ cells develop and migrate in an embryo
• eventually can be detected in the gonads of the adult
worm
100 m
Axis Formation
• a body plan with bilateral symmetry is found across a range of
animals
– this body plan exhibits asymmetry across the dorsal-ventral and anterior-posterior axes
– the right-left axis is largely symmetrical
• the anterior-posterior axis 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 cytoplasm of the vegetal pole
– changes gene expression in that region through induction by these new
molecules
– 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 dorsalventral 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
Okay – now for some weird stuff &
the biology behind it
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 cytoplasmic
determinants and the pattern of cleavage
• the first two blastomeres of the embryo are totipotent (can
develop into all the possible cell types)
Restricting Developmental Potential
1. fertilized salamander eggs allowed to divide normally  gray crescent in
between the first two blastomeres
2. fertilized eggs constricted by a thread - shifted the gray crescent toward one
blastomere
3. two blastomeres developed into two embryos
RESULTS: blastomere that didn’t receive any material from the gray crescent – loss
of dorsal structures in that embryo
Control egg
(dorsal view)
1a Control
group
Gray
crescent
Experimental egg
(side view)
1b Experimental
Gray
group
crescent
Thread
2
Normal
Belly piece
Normal
The “Organizer” of Spemann and Mangold
• Han Spemann and Hilde Mangold in 1923 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
• called the area of the lip responsible – Spemann’s Organizer
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
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 initially determining
the A-P axes of the embryo through
the formation of a growth factor
gradient from anterior to posterior
– 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
Drosophila Embryogenesis
• 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 A-P 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 embryogenesis
Tetrapod Embryogenesis & Limb Patterning
• 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”
• inductive signal of the ZPA was found to
be Sonic hedgehog
– SHH gradient results in the limb - decreasing
levels of SHH determines anterior
– important roles in humans too
• BUT before that: Hox genes also play
roles in 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
EXPERIMENT
Anterior
New
ZPA
Donor
limb
bud
Host
limb
bud
ZPA
Posterior
RESULTS
4
3
2
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
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