Cell and embryology
Model Systems
Model organisms: vertebrates (frog, mouse, zebrafish)
Model organisms: invertebrates (sea urchin, Drosophila, nematode) Identifying development genes
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Textbook: Wolpert L, Beddington R, Jessell T, Lawrence P, Meyerowitz E, Smith J. (2007) Principles of Development. 3th ed. London: Oxford university press.
Gilbert SF. (2003) Development Biology. 7th ed. Sunderland: Sinaure Associates Inc.
Model organisms in development
A few have been studied extensively; each has advantages and disadvantages.
Xenopus laevis: development is independent (in vitro), easy catch and observation but poor genetics.
Chick: available, surgical manipulation and in vitro culture but poor genetics.
Mouse: surgical manipulation, good genetics, transgenic model, mammalian but development is in utero .
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p
Drosophila: great genetics, great development (recent Nobel Prize to Lewis, Nusslein-Volhard & Wiechaus).
C. elegans: has less than 1000 cells and is transparent.
Sea Urchin: in vitro
Arabidopsisthaliana: flowering plant.
Summary of the main patterns of cleavage
Lecithal
Model organisms: vertebrates
All vertebrate embryos undergo a similar pattern of development.
1) fertilization
2) Cleavage (cell number ↑, but total mass X)
3) blastulation (blastcoel formation and three germ layers) 4) gastrulation (where ectoderm covers embryo, endoderm and
mesoderm are inside), A-P axis (body plan), notochord formation, embryo affected by yolk in egg. In mammalian, yolk to small but have extra-embryonic structure of placenta for nutrition
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nutrition.
5) Phylotypic stage, at which they all more or less resemble each other an show the specific features of notochord, somites and neural tube. Fig. 2.2
Fi 2 1 Fig.2.1
The skeleton of a mouse embryo illustrates the vertebrate body plan
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The phylotypic stage
At the end of gastrulation all embryos appear to be similar (the phylotypic stage).
Structures that are common to the phylotypic stage of the vertebrates are:
1) the notochord (an early mesoderm structure along A/P axis), 2) the somites (blocks of mesoderm on either side of notochord
which form the muscles of the trunk & limbs),
3) the neural tube - ectoderm above notochord forms a tube (brain and spinal cord).
Extra- embryo nic
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Vertebrate embryo to through a phylotypic state, but differences in form before
gastrulation Fig. 2.3
nic tissue
Xenopus laevis: egg (Amphibians)
The egg is composed of an animal and a vegetal
i b th d b it lli b ( l
Advantage: easy observation, fertilized, catch (sperm, egg), low infection
Animal region, both covered by vitelline membrane (gel
coat). Fig.2.4
Meiosis is stopped at 1st division with apparent 1 polar body (the 2nd polar body comes after fertilization).
Box 2A
After fertilization, the cortex (the layer below plasma membrane) rotates to determine future dorsal region at a position opposite to the site of sperm entry
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at a position opposite to the site of sperm entry.
vegetal
Box 2A
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Cleavage of a frog egg.
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Early developmental stages of Xenopus laevis
Blastula morula
2.5 hpf 3.5 hpf 5 hpf 10 hpf
囊胚 囊胚 囊胚
p p p p
hpf: hours post-fertilization
blastocoel-
Xenopus laevis : fertilization and early growth
1. one sperm enters animal region (grow to embryo, plant pore to yolk) 2. completes meiosis
3. egg and sperm nuclei fuse
4 it lli b lift
4. vitelline membrane lifts 5. yolk rotates down (15 minutes) 6. cortical rotation occurs (60 minutes).
7. 1st cleavage occurs (90 mins) Animal / Vegetal (A/V) 8. Every 20 mins, one cleavage
9. 2nd cleavage (110 mins) A/V 90 degrees to 1st
10. 3rd cleavage (130 mins) equatorial (4 small animal and 4 large vegetal= 8 , it is blastomeres).
11. Continued cleavage → blastomeres ↓, cells at vegetal region large than those at the animal region.
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Fig 2.3 Life cycle of the frog Xenopus laevis.
Xenopus laevis: blastulation
The blastula (after 12 divisions) has radial symmetry.
The marginal zone will become The marginal zone will become
mesoderm and endoderm.
Marginal zone, the belt of tissue around the equator , plays a crucial part in future development.
Internalization of the mesoderm and endoderm starts at the
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and endoderm starts at the blastopore.
In blastula stage, it is in the form of a hollow sphere with radial symmetry
Types of cell movement during gastrulation
Invagination Involution
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Involution Ingression Delamination
Eiboly: ectoderm covers embryo
Xenopus laevis: gastrulation Gastrulation step:
1. Mesoderm and endoderm converge and begin to move inwards at dorsal lip of the blastopore.
2. Mesoderm and endoderm extend in along A/P axis.
3. Ectoderm spreads to cover embryo (epiboly).
4. Dorsal endoderm separates mesoderm from the space between the yolk cells, the archenteron (future gut). Do not forget, mesoderm come from ectoderm 5 Lateral mesoderm spread to cover inside of archenteron
5. Lateral mesoderm spread to cover inside of archenteron.
6. dorsal mesoderm is beneath dorsal ectoderm 7. mesoderm spread to cover gut
8. epiboly - ectoderm covers embryo 9. yolk cells are internalized (food source), dorsal mesoderm develops into
a) notochord (rod along dorsal midline) and
b) somites (segmented blocks of mesoderm along notochord).
Blastopore
↓
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↓ Archenteron
↓ Large
↓ Blastocoel
↓ Close
↓ gut
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Xenopus laevis: Neurulation
• Neuralation or neural tube formation:
1) The neural plate is the ectoderm located above notochord and somites.
2) The edge of the neural plate forms neural folds which rise 2) The edge of the neural plate forms neural folds which rise
towards midline.
3) The folds fuse to form neural tube.
4) The neural tube sinks below epidermis.
• The anterior neural tube becomes brain. Mid and posterior neural tube becomes spinal cord.
Gastrulation neurulation neural plate fold tube
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Gastrulation → neurulation → neural plate → fold → tube
notochord Anterior posterior
↓ ↓ Brain spinal cord Neural crest cell
Autonomic nerves
Fig. 2.7 Neurulation in amphibian
Notochord begins to form in the midline
Brain and spinal
Xenopus laevis: Somites The somites formation, after neurulation
The dorsal part of somites have ready begun to differentiate into dermatome (future dermis).
The rest of each somite becomes vertebrae and trunk muscles (and limbs).
Lateral plate mesoderm becomes heart, kidney, gonads and gut muscles.
V l d b bl d f i i
Ventral mesoderm becomes blood-forming tissues.
Also at this stage, the endoderm gives rise to the lining of the gut, liver &
lungs.
Fig. 2.8 A cross-section through a stage 22 Xenopus embryo just after gastrulation and neurlation are completed
The major lineages of the mesoderm
Circulatory body cavity
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Cartilage skeletal dermis
Circulatory body cavity system Scler
Myo tome
Xenopus laevis: tail bud stage
• After gastrulation comes the early tail bud stage In the anterior embryo:
a) the brain is divided, b) eyes and ears form,
c) 3 branchial arches form (anterior arch later becomes the jaw.
Fi 2 9 Th l t ilb d
In the posterior embryo, the tail is formed last from dorsal lip of blastopore by extension of notochord, somites and neural tube.
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Fig. 2.9 The early tailbud stage of Xenopus embryo
Xenopus laevis : neural crest cells
Neural crest cells come from the edges of the neural folds after neural tube fusion. Neural crest cells can form from the dorsal side of the closed neural tube
Neural crest cells detach and migrate as single cells between the mesodermal tissues to become:
1) sensory and autonomic nervous systems 2) skull
3) pigment cells 4) Cartilage → bone
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Only vertebrate
Cell adhesion molecular expressed dependent
Epidermal and neural plate/tube interactions may generate crest cells
Schematic representation of neural crest formation (in chick embryo)
Neural folds meet and adhere
Cells at this junction form neural crest
Closure not simultaneous
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Closed tube detaches – change in adhesion molecule
expression
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Zebrafish
Zebrafish ((DanioDanio reriorerio) ) ---- A Vertebrate ModelA Vertebrate Model
•It is 3 cm long
•Short generation time
•Large clutch size
•External fertilization
•Transparent embryos
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•Rapid development
http://zfin.org/ and
http://www.nih.gov/science/models/zebrafish/
29h Sphere
29h
48h
•Human disease model
•Reverse genetics tool •Transgenics
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Fish (Zebrafish) embryo:
Fig. 2.26
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The development of Zebrafish
Zebrafish
development occurs very rapidly In 24 hr very rapidly. In 24 hr hours of
embryogenesis, shown here, the 1 cell zygote becomes into a vertebrate embryo with a tadpole-like form.
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Characterization of Fish embryo
Telolecithal: most of the egg cell is occupied by yolk Meroblastic: the cell divisions not completely divide the egg Discoidal: since only the blastodisc becomes the embryo, this type of meroblastic cleavage is call discoidal
meroblastic cleavage is call discoidal.
Cleavage can take place only in the blastodisc, a thin region of yolk free cytoplasm at the animal pole of the egg.
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Fig. 2.27 Cleavage of the zebrafish embryo
Three cell populations:
At about the 10th cell division -- the onset of the MBT
1. Yolk syncytial layer (YSL)
2. Deep cells -- forming the embryos proper
3 E l l (EVL) f i th id l
mid-blastula transition
3. Envelope layers (EVL) -- forming the epidermal ANIMAL BODY
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Blastoderm
4 hpf: hours post-fertilization
Fish embryo: blastula stage
About 10 cell division, the onset of mid-blastula transition: gene transcription begins, divisions slow and cell move. And formed three distinct cell populations:
(1)YSL (yolk syncytial layer): location of vegetal edge of the blastoderm and fusion produces a ring of nuclei within the part of the yolk cell cytoplasm that just beneath the blastoderm It is important for directing some of the that just beneath the blastoderm. It is important for directing some of the cell movement of gastrulation.
Internal YSL: the yolk syncytial nuclei move under the blastoderm External YSL: some cell move vegetally, stay ahead of the blastoderm
margin
(2)Enveloping layer (EVL):
Made up of the most superficial cell from the blastoderm, which
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form an epithelial sheet a single cell layer thick.
(3) Deep cells
Both YSL and EVL are the deep cells, that give rise to the embryo proper.
The fate map of the deep cells after mixing has stopped
The fate of the early blastoderm cells are not determined. After much cell mixing during cleavage
Fish embryo: gastrulation
The blastoderm at 30%
completion of epiboly (4.8 hr) Internal
YSL YSL
This stage, no mesoderm, ectoderm
Formation of the hypoblast, either by involution of cells at the margin of the epibolizing
Close-up of the marginal region
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g p g
balstoderm or by delamination and ingression of cells from the epiblast (6hr) The formation of germ layers is started.
Types of cell movement during gastrulation
Invagination Involution
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Involution Ingression Delamination
Eiboly: ectoderm covers embryo
About 90% epiboly (9 hr), mesoderm can be seen surrounding the yolk, between the endoderm and ectoderm
Complete gastrulation (10.3hr)
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Types of cell movement during gastrulation
Invagination I l ti
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Involution Ingression Delamination
Eiboly: ectoderm covers embryo
Fig 2.28 Epiboly and gastrulation in the zebrafish
After fertilization → cell cleavage → spreading out of the layer of cell
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After fertilization → cell cleavage → spreading out of the layer of cell (epiboly) → upper half of the yolk become covered by a cup-shaped blastoderm→ gastrulation by involution of cell → fromed a ring around the edge of the blastoderm → involuting cell converge on the dorsal midline to form the body of the embryo
Fish embryo: gastrulation
Convergence and extension in the gastrula. Mesodermal cell
( d il )
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Convergence and extension in the gastrula.
(A) Dorsal view of convergence and externsion movements during gastrulation. Epiboly spreads the blastoderm over the yolk; involution or ingression generates the hypoblast; convergence and extension bring the hypoblast and epiblast cells to the dorsal side to form the embryonic shield.
(B) Convergent extension of the embryo; it is show by cells expression the gene no tail (a gene is expressed by notochord cells)
(expressed snail gene) flank the notochord
Types of cell movement during gastrulation
Invagination Involution Involution Ingression Delamination
Eiboly: ectoderm covers embryo
Chicken
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Chick (bird) embryo: the blastodisc (blastoderm) The blastodisc arises through cleavage (20 hrs.).
The blastodisc can be divided into two areas:
1) the area pellucida (a light area) surrounded by 2) the area opaca (a dark ring).
犁溝
Fig. 2.10 46
yolk
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The life cycle of the chicken (Fig.2.11)
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The posterior marginal zone forms at the junction of the area pellucida and the area opaca and defines the dorsal side and posterior end of the embryo.
Chick (bird) embryo: the blastodisc (blastoderm)
The hypoblast (the source of extra- embryonic tissues) develops as a layer on top of yolk and develops from cells from the posterior marginal layer and the overlying cells of the blastoderm. It come from two sources: the posterior
opacapellucida opaca Germinal
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from two sources: the posterior marginal zone, which lies at the junction between the opaca and pellucida at the posterior of the embryo. It develop to extra- embryonic structure and related with epiblast.
Fig. 2.12
ectoderm
endoderm
Discoidal meroblastic cleavage in a chick egg
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Formation of two-layered blastoderm of the chick embryo
(A,B) Primary hypoblast cells
Primitive streak
Germinal
delaminate individually to form islands of cell beneath the epiblast
(C) Secondary hypoblast cells from posterior margin → migrate beneath the epiblast and incorporated the poly- invagination islands → move invagination islands → move anterior;
As the hypoblast moves anteriorly → epiblast cell collect at the region anterior to Koller’s sickle to form the primitive streak
Types of cell movement during gastrulation
Invagination Involution Involution Ingression Delamination
Eiboly: ectoderm covers embryo
Chick embryo: the primitive streak
The primitive streak is a slit or line on the disc which lays down the A/P axis. (posterior)
Onset of gastrulation
This structure begins to form from the posterior marginal zone and extends to a point in the central region of the disc
extends to a point in the central region of the disc.
Cells move towards the streak, and mesoderm and endoderm internalize at this site.
Unlike amplibians, cell not only proliferation but also growth in size during
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size, during gastrulation in bird and mammals.
Primitive streak
Chick embryo: the primitive streak
When the primitive streak reaches its greatest length (forward), the anterior end begins to regress back to the posterior end.
Primitive streak form at posterior → forward formation → enough length close and regress → Hensen’s node → backward
The anterior end of the regressing streak is known as Hensen's Node.
length close and regress → Hensen s node → backward regression → formation of head, somites and notochord… (Fig.
2.14)
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Cell movement of the primitive streak of the chick embryo
Head, somite
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The major lineages of the mesoderm
Circulatory body cavity
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Cartilage skeletal dermis Circulatory body cavity
system Scler
Myo tome
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Chick embryo: gastrulation
As Hensen's Node moves toward the posterior, several structures form behind it:
1) The head fold (from ectoderm and endoderm) 2) The notochord and somites (from mesoderm)
3) The neural tube forms above the notochord (from ectoderm) (The anterior structures are formed first while the posterior
structures are completed last.)
4) Neural folds fuse at the dorsal midline and neural crest cells migrate away
5) The head fold separate, gut forms and heart pieces fuse to form heart.
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Chick embryo: neurulation
Neural plate → neural fold → meet midline Intermediate mesoderm→ kidney Splanchnic mesoderm
→ heat Somite star formation
Fig.2.18 Development of the chick embryo
notochord
somites
13 somites
Hensen’s node 20 somites 40 somites
Chick embryo: extra-embryonic structure
Amnion
and amniotic cavity provide mechanical protection Chorion maintain shell
Allantois bridge for oxygen and waste Vitelline vein take nutrient form yolk to embryo
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embryo
Umbilical vein take oxygen to embryo
Mouse embryo
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Egg is small, 100mm very small
Egg surrounded by protective external coat, zona pellucida
Fig.2.20
Mouse embryo: fertilization Fertilization occurs in oviduct. (Fig. 11.26)
Cleavage occurs in oviduct: 1st at 24 hours and every 12 hours after that to form the morula (a ball of cells). (Fig. 2.21)
• Blastomere compaction happens at 8 cell stage.
• Smooth inner membranes and outer membranes are covered with microvilli.
63 Four-cell stage.Remnants of the
mitotic spindle can be seen between the two cells that have just completed the second cleavage division.
(b)
Morula.After further cleavage divisions, the embryo is a multicellular ball that is still surrounded by the fertilization envelope. The blastocoel cavity has begun to form.
(c)
Development of a human embryo form fertilization to implantation
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• Cleavage partitions the cytoplasm of one large cell – Into many smaller cells called blastomeres
65 Fertilized egg.Shown here is the
zygote shortly before the first cleavage division, surrounded by the fertilization envelope.
The nucleus is visible in the center.
(a) Morula.After further cleavage
divisions, the embryo is a multicellular ball that is still surrounded by the fertilization envelope. The blastocoel cavity has begun to form.
(c) Blastula.A single layer of cells
surrounds a large blastocoel cavity. Although not visible here, the fertilization envelope is still present; the embryo will soon hatch from it and begin swimming.
Four-cell stage.Remnants of the (d) mitotic spindle can be seen between the two cells that have just completed the second cleavage division.
(b)
Mouse embryo: In 16 cell morula →
At ~16 cell morula, has two group cells. A small group of internal cell mass (ICM) surrounded by a large group of external (trophectoderm) cells.
Trophectoderm: becomes extra-embryonic tissues (such as placenta).
Inner cell mass (ICM): becomes the embryo plus some extra- embryonic tissues.
The morula (~32 cell stage) has 2 cell fates:
1) inner 8 cells (Inner Cell Mass) 2) outer ~20 cells (trophectoderm).
blastocyst
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Mouse embryo: blastocyst
In the blastocyst (~3½ days), the trophectoderm and ICM are established.
Fluid is pumped in to expand cavity and increase the size of the blastocyst.
blastocyst: preimplantation (3½ - 4½ days)
The surface of ICM will become the primitive endoderm while the remaining becomes primitive ectoderm (= epiblast) remaining becomes primitive ectoderm ( epiblast).
Implantation occurs. The zona pellucida is discarded and blastocyst attaches to uterine wall.
Development of a human embryo form fertilization to implantation
Mouse embryo: post-implantation
Uterine wall
Implantation → trophoblast giant cell invade → trophoectoderm grows → ectoplacental cone & extra-embryonic ectoderm → primitive endoderm cover inner surface of trophectoderm → to visceral endoderm
hypoblast
• In the first two days post-implantation, the mural trophectoderm (cells that are not in contact with the ECM) gives rise to polyploid trophoblast giant cells.
• The rest of trophectoderm becomes the ectoplacental cone and the extra-embryonic ectoderm which give rise to the placenta.
• Primitive mesoderm migrates:
1) to cover inner surface of mural trophectoderm to become the parietal (腔壁)endoderm and 2) to cover egg cylinder and epiblast to become the viseral endoderm
Amnion Chorion Allantois 69
Mouse embryo: gastrulation 6½ days after fertilization:
The primitive streak forms at the start of gastrulation at the future posterior end.
(Inside cup is future dorsal side)
Cells move through the streak and spread forward and laterally between the ectoderm and the visceral endoderm to form the mesoderm.
Later the definitive endoderm (from epiblast) will replace the visceral Later, the definitive endoderm (from epiblast) will replace the visceral
endoderm.
The primitive steak first elongates, then at the anterior tip of the primitive streak, the node forms. (The node formed from anterior → posterior)
Then notochord and somites form anterior to the node (A/P axis).
Cells migrate through mesoderm to form endoderm (gut).
Fig. 2.23 70
Epiblast move through the primitive streak to give rise to the mesoderm and definitive endoderm.
Mouse embryo: late embryogenesis (neurulation)
• By 8½ days after fertilization,
1) the neural folds form at anterior and dorsal, and 2) the embryonic endoderm internalizes to form the gut.
• 9 days after fertilization embryogenesis is complete.
A P
Fig. 2.24
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D
Primitive streak extend→ produce extra-embryonic structure
→chorion, amino, allantois The primitive streak similar to chick (node = Hensen’s node)
Organogenesis in the anterior part Neural folds formation
Amnion Chorion Allantois
Mouse embryo: final stages of gastrulation
1. Complex folding
2. Initially on the ventral surface of embryo 3. Internalize to form the gut
4 Heat and liver move into their positions 4. Heat and liver move into their positions 5. Head becomes distinct
6. Embryo surrounded by extra-embryonic membrane
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Fig. 2.25
Formation of the notochord in the mouse
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Diagram showing the timing of human monozygotic twinning with relation to extra-embryonic membrane
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Amnion Chorion Allantois
Model organism: invertebrate
Life cycle of Drosophila
Drosophila melanogaster: early embryogenesis The Drosophila egg is the shape of a sausage .
Meroblastic (superficial) cleavage and centrolecithal It has a micropyle at the anterior end (site of sperm entry).
With fertilization, the fusion of nuclei is followed by rapid mitotic divisions (9 minutes) and no cytoplasmic cleavage.
A syncytium is formed (many nuclei/common cytoplasm).
After nine divisions, nuclei move to the periphery to form the syncytial blastoderm .
Fig. 2.30 After fertilization, no cell was form, but rapid nuclear
Box 2A
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Drosophila: embryogenesis
By 13 mitoses the membranes sprout to surround the nuclei to form cells (cellular blastoderm).
~15 cells at posterior (= pole cells) are sequestered and become the germline. g
During first ~3 hrs large molecules such as proteins can move between nuclei until the cellularization occurs.
Single layer of cells give rise to all tissues (syncytium ).
Gastrulation starts at ~3 hrs.
Mesoderm forms from ventral tissue, midgut from endoderm at the anterior and posterior ends ectoderm remains on outside
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anterior and posterior ends, ectoderm remains on outside.
During gastrulation, the ventral blastoderm (germ band), comprises extension.
The mesodermal tube forms from ventral tissue then cells separate and move to internal locations under the ectoderm.
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Drosophila melanogaster: gastrulation The mesoderm becomes muscle and connective tissues.
In insects, nerve cord lies ventrally (vertebrates: dorsal).
Neuroblasts form a layer between mesoderm and outer ectoderm.
midgut (anterior & posterior) grow from threads and fuse.
= anterior and posterior midgut ectoderm becomes epidermis.
No cell division occurs during gastrulation.
Afterward, division restarts.
Future mesoderm invaginate ventral region → intrnalized tube → cell leave tube and migrate under the ectoderm
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leave tube and migrate under the ectoderm
The surface of ventral blastoderm → cell leave and form a layer between ventral ectoderm and mesoderm
Anterior and posterior invaginate and fuse → gut Midgut →region endoderm
Foregut and hindgut → ectodermal origin
Future mesoderm invaginate ventral region → internalized tube → cell leave tube and migrate under the ectoderm The surface of ventral blastoderm cell leave
Fig. 2.31 Gastrulation
germline
blastoderm → cell leave and form a layer between ventral ectoderm and mesoderm →nervous system
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Anterior and posterior invaginate and fuse → gut Midgut →region endoderm Foregut and hindgut → ectodermal origin
Ventral view
Dorsal view
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Drosophila melanogaster: segmentation The germ band (ventral blastoderm) is main trunk region.
Germ band extension pushes posterior end over dorsal side.
The first signs of segmentation grooves appear to outline parasegments (early embryo) which give rise to segments (late embryo).
embryo).
Segments are formed from the posterior of one parasegment and the anterior of the next. (formed form posterior to anterior)
There are 14 parasegments: Fig. 2.33 3 mouth, 3 thorax, 8 abdominal.
Fig. 2.32
Drosophila melanogaster: larvae
The larvae hatch at 24 hrs post-fertilization.
Larval structures of note include:
The anterior end is the acron.
The posterior end is the telson.
Along with the head, the larvae has 3 thoractic segments and 8 abdominal segments.
The ventral side of the larvae has denticle belts, alternating patches of denticle hairs and cuticle on each segment used for of denticle hairs and cuticle on each segment, used for locomotion.
Drosophila melanogaster: metamorphosis Three instar stages of larval life are separated by molts.
• 1st instar 2nd instar 3rd instar
molt molt
3rd instar larvae forms pupae (pupa) to undergo metamorphosis.
The adult tissues arise from imaginal discs and histoblasts.
imaginal discs: small sheets of epidermis (~40 cells each of cellular blastoderm) which grow throughout larval life.
Imaginal discs: 6 leg, 2 wing, 2 haltere, 2 eye-antenna, plus genital, head discs
and ~10 histoblasts: nest of cells in the abdomen which give rise to the abdominal segments.
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abdominal segments.
Imaginal discs Formation of adult abdominal segments - gene expression in histoblasts
Larval epidermis degeneration begins prior to imaginal disc eversion Imaginal disc cells and histoblasts will replace the larval epidermis
imaginal discs
histoblasts
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Fig. 2.34 Imaginal discs vs. adult structure
Antenna haltere Genitalia
Caenorhabditis elegans: the model of nematode
After gastrulation
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Fig. 2.35 Life cycle of nematode
THE WORM
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In case of self-fertilization there are ~ 0.1 - 0.3% male worms in the population.
http://www.wormatlas.org/handbook/contents.htm
the model of nematode
Small nematodes that are 1 mm long and 70 µm in diameter.
19,000 gene
Small number of cell (558, first larval stage)
T f b d th id
Transparency of embryo, and growth rapid
The adult hermaphrodite (maless can develop) undergo rapid development.
The egg has a 50 µm diameter which forms a polar body after fertilization, nuclear fusion occurs followed by a set pattern of cleavage.
The normal pattern of cell division has been mapped.
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Many cells undergo programmed cell death.
Hermaphrodite: 959 cells from 1090 somatic nuclei of which 131 undergo programmed cell death; 300 germ cells undergo apoptosis; 116 of the 131 dying cells are cells of the nervous system and ectoderm
Press Release: The 2002 Nobel Prize in Physiology or Medicine 7 October 2002
The Nobel Assembly at Karolinska Institutet has today decided to award The Nobel Prize in Physiology or Medicine for 2002 jointly to
Sydney Brenner, H. Robert Horvitz and John E. Sulston ffor their discoveries concerning
"genetic regulation of organ development and programmed cell death"
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1927 1947 1942
Molecular Regulation of Apoptosis C. elegans
mutagenize
Non- apoptotic apoptotic
wildtype
CED mutants (Cell Death abnormality) wildtype
Fig. 2.36 Cleavage of the nematode embryo
Fertilization →polar bodies formation → asymmetric cleavage → anterior AB cell, smaller posterior P1 cell
DIC image
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Fig.2.37
Cell lineage and cell fate in the early nematode embryo
Fig. 2.38 elegans larva at the L1 stage.
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Anus Pharynx Primordium
Invertebrate: Sea Urchin Radial holoblastic cleavage (isolecithal)
The 4thcleavage, very different from the first three. In animal pole, four cell divide to 8 blastomeres and with the same volume (the 8 cells also called mesomeres). In vegetal pole, undergoes an unequal cleavage to four large cells (macromeres) and four small cells (micromeres).
The animal mesomeres divide equatorially to produced two tiers: an1 and an2.
The vegetal macromeres divide a small cluster beneath the large tier. (not equal)
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equal) 128 cells blastula.
Meridionally
4th cleavage
Sea Urchin: blastula formation
The blastula stage of sea urchin development begins at the 128 cells.
Blastulation: The cells form a hollow sphere surrounding a central cavity (blastocoel). Every cell contact with proteinaceous fluid of the bastoceol (inside) and with the hyaline layer on the outside.
About 9thor 10thcleavage, cells become specified and they end develop cilia.
Ciliated blastula → rotate within fertilization envelop (E→F) → vegetal pole of Ciliated blastula → rotate within fertilization envelop (E→F) → vegetal pole of Bastula become thicken
(forming vegetal plate) → then animal pole synthesis and secret hatching enzyme → digest fertilization envelope → embryo is a free swimming
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embryo is a free swimming hatched blastula.
rotate
Fate maps and the determination of sea urchin blastomeres
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Fate map and cell lineage of the sea urchin.
Fate map of the zygote
Late blastula with ciliary tuft and flattened vegetal plate
blastula
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Pluteus larva Prism-stage larva
Formation of syncytial cables by primary mesenchyme cells of sea urchin
SEM of spicules formed by the fusing of primary mesenchyme cells into syncytial y y cables
C: SEM of primary
mesenchyme cells enmeshed in the extracellular matrix of early gastrula.
Gastrulation star
y g
D: Gastrula-stage mesenchyme cell migration The extracellular matrix fibrils of the bastocoel lie parallel to the animal-vegetal axix
Ingression of primary mesenchyme cells
Fertilization envelope
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Invagination of the vegetal plate
SEM of external surface
f th l t l
CSPG release → into inner lamina → osmotic gradient ↑→ absorb water → swell inner lamina ,but outer lamina attached does not swell → inward
of the early gastrula inward
CSPG: chondroitin sulfate proteoglycan 102
Entire sequence of gastrulation in sea urchin
103
Identification of developmentally important genes
The developmental genetics of Drosophila and mice are best known.
Homologous genes identified in these organisms are found in other species.
Dominant (or semi-dominant) mutations: one copy of mutant gene produces mutant state. These are more easily recoginzed, they don’t cause the eayly death of the embryo in the heterozygous.
Recessive mutations: two copies of a mutant gene gives the mutant state.
104
Allele: The gene is contributed by the male and female Homozygous: both alleles of a pair carry the mutation Heterozygous: just one copy of the mutant gene is present
-/-
105
Recessive mutation vs. Semi-dominant mutation
106
Most mutations are recessive, but usually die in embryo.
heterozygous
Developmental gene can be identified by induced mutation and screening
Genetic screening to produced
homozygous mutant
Heterozygous yg
zerbrafish embryo
Embryos homozygou s the s the induced mutation will be found in the offspring of 25% of the matings
Mutagenesis and genetic screening strategy for identifying developmental mutants in Dorsophila
DTS: dominant temperature- sensitive mutation, up 29oC
→ death
b: a non developmental lethal b: a non-developmental lethal recessive
ethyl methane sulfonate
109
main patterns of cleavage phylotypic stage
Time vs. developmental events
T f ll t d i t l ti
Types of cell movement during gastrulation Primitive streak
gastrulation Neurulation
110
human monozygotic twinning Syncytium
imaginal discs and histoblasts Dominant (or semi-dominant) mutations
