Model organisms in development
Cell and embryology
A few have been studied extensively; each has advantages and
disadvantages.
Model Systems
Xenopus laevis: development is independent (in vitro), easy catch
and observation but poor genetics.
Model organisms: vertebrates (frog, mouse, zebrafish)
Model organisms: invertebrates (sea urchin, Drosophila, nematode)
Chick: available, surgical manipulation and in vitro culture but poor
genetics.
Identifying development genes
Mouse: surgical manipulation, good genetics, transgenic model,
mammalian but development
p
is in utero .
Textbook: Wolpert L, Beddington R, Jessell T, Lawrence P, Meyerowitz E, Smith
J. (2007) Principles of Development. 3th ed. London: Oxford university press.
Drosophila: great genetics, great development (recent Nobel Prize to
Lewis, Nusslein-Volhard & Wiechaus).
C. elegans: has less than 1000 cells and is transparent.
Gilbert SF. (2003) Development Biology. 7th ed. Sunderland: Sinaure
Associates Inc.
Sea Urchin : in vitro
1
Arabidopsis thaliana: flowering plant.
2
Summary of the main patterns of cleavage
Lecithal
3
4
1
Model organisms: vertebrates
All vertebrate embryos undergo a similar pattern of development.
1) fertilization
2) Cleavage (cell number ↑, but total mass X)
Fi 2 1
Fig.2.1
3) blastulation (blastcoel formation and three germ layers)
The skeleton of a mouse
embryo illustrates the
vertebrate body plan
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
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
5
The phylotypic stage
Xenopus laevis: egg
(Amphibians)
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).
Vertebrate embryo
to through a
phylotypic state,
but differences in
form before
gastrulation
6
Advantage: easy observation, fertilized, catch (sperm, egg), low infection
Extraembryo
nic
tissue
The egg is composed of an animal and a vegetal
region,
i
b
both
th covered
db
by vitelline
it lli membrane
b
((gell
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.
entry
Animal
vegetal
Fig. 2.3
7
8
2
Box 2A
Cleavage of a frog egg.
9
10
Xenopus laevis : fertilization and early growth
Early developmental stages of Xenopus laevis
1. one sperm enters animal region (grow to embryo, plant pore to yolk)
morula
Blastula
2. completes meiosis
3. egg and sperm nuclei fuse
4 vitelline
4.
it lli membrane
b
lift
lifts
5. yolk rotates down (15 minutes)
6. cortical rotation occurs (60 minutes).
囊胚
7. 1st cleavage occurs (90 mins) Animal / Vegetal (A/V)
8. Every 20 mins, one cleavage
2.5 hpf
p
3.5 hpf
p
5 hpf
p
9. 2nd cleavage (110 mins) A/V 90 degrees to 1st
10 hpf
p
10. 3rd cleavage (130 mins) equatorial (4 small animal and 4 large
vegetal= 8 , it is blastomeres).
blastocoel -
11. Continued cleavage → blastomeres ↓, cells at vegetal region
large than those at the animal region.
hpf: hours post-fertilization
11
12
3
Xenopus laevis: blastulation
The blastula (after 12 divisions)
has radial symmetry.
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
blastopore.
Fig 2.3 Life cycle of the frog Xenopus laevis.
In blastula stage, it is in the form of a hollow sphere with radial symmetry
13
14
Xenopus laevis: gastrulation
Types of cell movement during gastrulation
Invagination
Involution
Ingression
Delamination
Eiboly: ectoderm covers embryo
15
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
5.
Lateral mesoderm spread to cover inside of archenteron
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
↓
Archenteron
↓
Large
↓
Blastocoel
↓
Close
↓
gut 16
4
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
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
notochord
Neural crest cell
Anterior
posterior
↓
Autonomic nerves ↓
18
Brain
spinal cord
17
Fig. 2.7 Neurulation in amphibian
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
Ventral
l mesoderm
d
b
becomes bl
blood-forming
df
i tissues.
i
Also at this stage, the endoderm gives rise to the lining of the gut, liver &
lungs.
Brain and spinal
Notochord begins to form in the midline
Neural plate develops neural folds
Fig. 2.8 A cross-section
through a stage 22 Xenopus
embryo just after gastrulation
and neurlation are completed
19
20
5
The major lineages of the mesoderm
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.
In the posterior embryo, the tail is formed last from dorsal lip of
blastopore by extension of notochord, somites and neural tube.
Circulatory body cavity
system
Scler
Myo
tome
Cartilage
skeletal
Fig. 2
Fi
2.9
9 Th
The early
l ttailbud
ilb d
stage of Xenopus embryo
dermis
21
22
Schematic representation of neural crest formation
(in chick embryo)
Xenopus laevis : neural crest cells
Neural folds meet and adhere
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
Cells at this junction form neural crest
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
Closure not simultaneous
4) Cartilage → bone
Only vertebrate
Cell adhesion molecular expressed dependent
Epidermal and neural plate/tube interactions may generate crest cells
23
Closed tube detaches – change
in adhesion molecule
expression
24
6
Zebrafish (Danio rerio
rerio)) -- A Vertebrate Model
•It is 3 cm long
•Short generation time
•Large clutch size
•External fertilization
•Transparent embryos
•Rapid development
http://zfin.org/ and
http://www.nih.gov/science/models/zebrafish/
25
26
Sphere
29h
48h
27
28
7
•Human disease model
•Reverse genetics tool
Fish (Zebrafish) embryo:
•Transgenics
Fig. 2.26
29
The development of Zebrafish
30
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
discoidal.
Zebrafish
development occurs
very rapidly
rapidly. In 24 hr
hours of
embryogenesis,
shown here, the 1
cell zygote becomes
into a vertebrate
embryo with a
tadpole-like form.
Cleavage can take place only in the blastodisc, a thin region of yolk free
cytoplasm at the animal pole of the egg.
Fig. 2.27 Cleavage of the zebrafish embryo
31
32
8
Fish embryo: blastula stage
Three cell populations:
At about the 10th cell division -- the onset of the
MBT
mid-blastula transition
1. Yolk syncytial layer (YSL)
2. Deep cells -- forming the embryos proper
3 Envelope
3.
E
l
layers
l
(EVL) -- forming
f
i the
th epidermal
id
l
ANIMAL BODY
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
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
form an epithelial sheet a
single cell layer thick.
(3) Deep cells
Blastoderm
4 hpf: hours post-fertilization
33
Both YSL and EVL are the deep
cells, that give rise to the embryo
proper.
34
Fish embryo: gastrulation
The fate map of the deep cells after mixing has stopped
Internal
YSL
The blastoderm
at 30%
completion of
epiboly (4.8 hr)
This stage, no mesoderm, ectoderm
The fate of the early blastoderm cells are not determined. After much cell
mixing during cleavage
35
36
9
Types of cell movement during gastrulation
Formation of the
hypoblast, either by
involution of cells at the
margin
g of the epibolizing
p
g
balstoderm or by
delamination and
ingression of cells from
the epiblast (6hr)
The formation of germ
layers is started.
Close-up of the
marginal region
Invagination
Involution
Ingression
Delamination
Eiboly: ectoderm covers embryo
37
About 90% epiboly (9 hr), mesoderm
can be seen surrounding the yolk,
between the endoderm and ectoderm
38
Types of cell movement during gastrulation
Complete gastrulation (10.3hr)
Invagination
I
Involution
l ti
Ingression
Delamination
Eiboly: ectoderm covers
embryo
39
40
10
Fish embryo: gastrulation
Fig 2.28 Epiboly and gastrulation in the zebrafish
Convergence and extension in the gastrula.
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
41
Mesodermal cell
(
(expressed
d snailil gene))
flank the notochord
(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
42
(a gene is expressed by notochord cells)
Types of cell movement during gastrulation
Invagination
Involution
Ingression
Delamination
Eiboly: ectoderm covers embryo
43
44
11
Chick (bird) embryo: the blastodisc (blastoderm)
Chicken
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
yolk
45
Fig. 2.10
46
The life cycle of the chicken (Fig.2.11)
47
48
12
Discoidal meroblastic cleavage in a chick egg
Chick (bird) embryo: the blastodisc (blastoderm)
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.
The hypoblast (the source of extraembryonic 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
marginal zone, which lies at the
junction between the opaca and
pellucida at the posterior of the
embryo. It develop to extraembryonic structure and related
with epiblast.
Fig. 2.12
Germinal
opaca pellucida opaca
ectoderm
endoderm
49
Primitive streak
Formation of two-layered
blastoderm of the chick
embryo
50
Types of cell movement during gastrulation
Germinal
(A,B) Primary hypoblast cells
delaminate individually to form
islands of cell beneath the
epiblast
(C) Secondary hypoblast cells
from posterior margin →
migrate beneath the epiblast
and incorporated the polyinvagination islands → move
anterior;
As the hypoblast moves
anteriorly → epiblast cell
collect at the region anterior to
Koller’s sickle to form the
primitive streak
Invagination
Involution
Ingression
Delamination
Eiboly: ectoderm covers embryo
51
52
13
Chick embryo: the primitive streak
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
disc.
Cells move towards the streak, and mesoderm and endoderm
internalize at this site.
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
Hensen s node → backward
regression → formation of head, somites and notochord… (Fig.
2.14)
The anterior end of the regressing streak is known as Hensen's
Node.
Unlike amplibians, cell
not only proliferation
but also growth in
size during
size,
gastrulation in bird
and mammals.
Primitive streak
53
54
The major lineages of the mesoderm
Cell movement of the primitive streak of the
chick embryo
Head, somite
Circulatory body cavity
system
Scler
Myo
tome
55
Cartilage
skeletal
dermis
56
14
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.
57
58
Fig.2.18 Development of the chick embryo
Chick embryo: neurulation
notochord
Neural plate → neural
fold → meet midline
Intermediate
mesoderm→ kidney
Splanchnic mesoderm
→ heat
somites
Somite star formation
13 somites
Hensen’s node
59
20 somites
40 somites
60
15
Mouse embryo
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
Umbilical vein take
oxygen to embryo
Fig.2.20
Egg is small, 100mm very small
Egg surrounded by protective external coat, zona
pellucida
61
62
Development of a human embryo form fertilization to implantation
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.
(b) Four-cell stage. Remnants of the
mitotic spindle can be seen
between the two cells that have
just completed the second
cleavage division.
(c) 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.
63
64
16
Mouse embryo: In 16 cell morula →
• Cleavage partitions the cytoplasm of one large cell
– Into many smaller cells called blastomeres
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 extraembryonic tissues.
The morula (~32 cell stage) has 2 cell fates:
1) inner 8 cells (Inner Cell Mass)
2) outer ~20 cells (trophectoderm).
blastocyst
(a) Fertilized egg. Shown here is the (b) Four-cell stage. Remnants of the (c) Morula. After further cleavage
divisions, the embryo is a
zygote shortly before the first
mitotic spindle can be seen
multicellular ball that is still
cleavage division, surrounded
between the two cells that have
surrounded by the fertilization
by the fertilization envelope.
just completed the second
envelope. The blastocoel cavity
The nucleus is visible in the
cleavage division.
has begun to form.
center.
(d) 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.
65
66
Mouse embryo: post-implantation
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).
epiblast)
Implantation occurs. The zona pellucida is discarded and blastocyst
attaches to uterine wall.
Uterine wall
hypoblast
Development of a human embryo form fertilization to implantation
Implantation → trophoblast giant cell invade → trophoectoderm grows →
ectoplacental cone & extra-embryonic ectoderm → primitive endoderm cover
inner surface of trophectoderm → to visceral endoderm
•
67
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
• Six days after fertilization, the epiblast is cup-shaped.
68
17
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,
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).
Epiblast move through
the primitive streak to
give rise to the
mesoderm and
definitive endoderm.
69
Amnion Chorion Allantois
Fig. 2.23
Mouse embryo: late embryogenesis (neurulation)
Mouse embryo: final stages of gastrulation
• 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.
1.
2.
3.
4
4.
5.
6.
Fig. 2.24
A
70
Complex folding
Initially on the ventral surface of embryo
Internalize to form the gut
Heat and liver move into their positions
Head becomes distinct
Embryo surrounded by extra-embryonic membrane
P
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
Fig. 2.25
71
72
18
Diagram showing the timing of human monozygotic twinning with relation to
extra-embryonic membrane
Formation of the notochord in the mouse
Amnion Chorion Allantois
73
74
Model organism: invertebrate
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. 2.29
Life cycle of Drosophila
Fig. 2.30
75
After fertilization, no cell was form, but rapid nuclear
division in a cytoplasm
76
19
Box 2A
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
ends, ectoderm remains on outside
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.
77
78
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
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
79
80
20
Future mesoderm
invaginate ventral region →
internalized tube → cell
leave tube and migrate
under the ectoderm
Ventral view
Fig. 2.31 Gastrulation
Dorsal view
germline
The surface of ventral
blastoderm → cell leave
and form a layer between
ventral ectoderm and
mesoderm →nervous
system
Anterior and posterior
invaginate and fuse → gut
Midgut →region endoderm
Foregut and hindgut →
ectodermal origin
81
82
Drosophila melanogaster: segmentation
Drosophila melanogaster: larvae
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).
Segments are formed from the posterior of one parasegment and the
anterior of the next. (formed form posterior to anterior)
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,
segment used for
locomotion.
Fig. 2.32
There are 14 parasegments: Fig. 2.33
3 mouth, 3 thorax, 8 abdominal.
83
84
21
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.
imaginal discs
histoblasts
Larval epidermis degeneration begins prior to imaginal disc eversion
Imaginal disc cells and histoblasts will replace the larval epidermis
Formation of adult abdominal segments - gene expression in
histoblasts
Imaginal discs
Fig. 2.34 Imaginal discs vs. adult structure
Antenna
haltere
Genitalia
85
86
Caenorhabditis elegans: the model of nematode
THE WORM
After
gastrulation
In case of self-fertilization
there are ~ 0.1 - 0.3% male
worms in the population.
Fig. 2.35 Life cycle of nematode
http://www.wormatlas.org/handbook/contents.htm
87
88
22
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
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
Transparency
off embryo,
b
and
d growth
th rapid
id
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.
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
Sydney Brenner, H. Robert Horvitz and John E. Sulston
for their discoveries concerning
"genetic regulation of organ development and programmed cell
death"
1927
89
1947
1942
90
Molecular Regulation of Apoptosis
C. elegans
mutagenize
Non- apoptotic
apoptotic
wildtype
CED mutants
(Cell Death
abnormality)
91
92
23
Fig. 2.36
Cleavage of
the nematode
embryo
Fertilization →polar bodies formation → asymmetric cleavage → anterior AB
cell, smaller posterior P1 cell
DIC image
Fig. 2.38 elegans larva at the L1 stage.
Fig.2.37
Cell lineage and cell fate
in the early nematode
embryo
Anus
Pharynx
Primordium
93
Sea Urchin: blastula formation
Invertebrate: Sea Urchin
Radial holoblastic cleavage (isolecithal)
The 4th cleavage, 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)
128 cells blastula.
94
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 9th or 10th cleavage, cells become specified and they end develop cilia.
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
hatched blastula.
4th
cleavage
Meridionally
rotate
95
96
24
Fate maps and the determination of sea urchin blastomeres
Fate map of the zygote
Late blastula with ciliary tuft
and flattened vegetal plate
blastula
Fate map and cell lineage of the sea urchin.
97
98
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
Prism-stage larva
Gastrulation star
C: SEM of primary
mesenchyme cells enmeshed
in the extracellular matrix of
early
y gastrula.
g
D: Gastrula-stage
mesenchyme cell migration
Pluteus larva
The extracellular matrix fibrils
of the bastocoel lie parallel to
the animal-vegetal axix
99
100
25
Ingression of primary mesenchyme cells
Invagination of the vegetal plate
SEM of external surface
off the
th early
l gastrula
t l
CSPG release → into inner lamina → osmotic
gradient ↑→ absorb water → swell inner lamina
,but outer lamina attached does not swell →
inward
Fertilization envelope
101
CSPG: chondroitin sulfate proteoglycan
102
Identification of developmentally important genes
Entire sequence of
gastrulation in sea urchin
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.
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
103
104
26
Recessive mutation vs. Semi-dominant mutation
-/-
Most mutations are recessive, but
usually die in embryo.
105
106
Developmental gene can be identified by induced mutation and
screening
Genetic screening to
produced
homozygous
yg
mutant
zerbrafish embryo
Heterozygous
Embryos
homozygou
s the
induced
mutation will
be found in
the offspring
of 25% of
the matings
heterozygous
107
108
27
Mutagenesis and genetic screening strategy for identifying
developmental mutants in Dorsophila
main patterns of cleavage
phylotypic stage
DTS: dominant temperaturesensitive mutation, up 29oC
→ death
b: a non
non-developmental
developmental lethal
recessive
Time vs. developmental events
T
Types
off cell
ll movementt during
d i gastrulation
t l ti
Primitive streak
gastrulation
Neurulation
ethyl methane sulfonate
human monozygotic twinning
Syncytium
imaginal discs and histoblasts
109
Dominant (or semi-dominant) mutations
110
28