Summary of the main patterns of cleavage

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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

Arabidopsis thaliana: flowering plant.

Summary of the main patterns of cleavage

Lecithal

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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 1Fig.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-embryonic

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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

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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 5 hpf 10 hpf3.5 hpf

囊胚囊胚囊胚

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p p pp

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 lift4. vitelline membrane lifts5. 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 cleavage9. 2nd cleavage (110 mins) A/V 90 degrees to 1st

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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 becomeThe 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

InvaginationInvolution

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InvolutionIngressionDelaminationEiboly: ectoderm covers embryo

Xenopus laevis: gastrulationGastrulation 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 ectoderm5 Lateral mesoderm spread to cover inside of archenteron5. Lateral mesoderm spread to cover inside of archenteron.6. dorsal mesoderm is beneath dorsal ectoderm7. mesoderm spread to cover gut8. epiboly - ectoderm covers embryo9. 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 rise2) 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

19Notochord begins to form in the midlineNeural plate develops neural folds

Brain and spinal

Xenopus laevis: Somites The somites formation, after neurulationThe 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 iVentral mesoderm becomes blood-forming tissues. Also at this stage, the endoderm gives rise to the lining of the gut, liver &

lungs.

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Fig. 2.8 A cross-section through a stage 22 Xenopus embryo just after gastrulation and neurlation are completed

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The major lineages of the mesoderm

Circulatory body cavity

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Cartilage skeletal dermis

Circulatory body cavitysystem

SclerMyotome

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 vertebrateCell 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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ZebrafishZebrafish ((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/

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29h

Sphere

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29h

48h

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•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 hrvery 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 yolkMeroblastic: the cell divisions not completely divide the eggDiscoidal: since only the blastodisc becomes the embryo, this type of meroblastic cleavage is call discoidalmeroblastic 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

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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 proper3 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 thethat 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 blastodermExternal 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

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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

YSLYSL

This stage, no mesoderm, ectoderm

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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 gbalstoderm or by delamination and ingression of cells from the epiblast (6hr)The formation of germ layers is started.



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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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InvaginationI l ti

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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



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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

46Fig. 2.10

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

opaca opacapellucida

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

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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



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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 gastrulationThis structure begins to form from the posterior marginal zone and

extends to a point in the central region of the discextends 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 cavitysystem

SclerMyotome

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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→ kidneySplanchnic mesoderm → heat Somite star formation

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Fig.2.18 Development of the chick embryo

notochord

somites

13 somites

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Hensen’s node20 somites 40 somites

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Chick embryo: extra-embryonic structure

Amnion and amniotic cavity provide mechanical protectionChorion maintain shellAllantois bridge for oxygen and waste Vitelline vein take nutrient form yolk to embryo

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embryoUmbilical 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.

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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 stillsurrounded by the fertilization envelope. The blastocoel cavityhas 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

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Fertilized egg. Shown here is thezygote 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 stillsurrounded by the fertilization envelope. The blastocoel cavityhas 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.

(d)Four-cell stage. Remnants of the 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

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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

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• 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.

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69Amnion Chorion Allantois

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 visceralLater, 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).

70Fig. 2.23

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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DPrimitive streak extend→ produce extra-embryonic structure →chorion, amino, allantois The primitive streak similar to chick (node = Hensen’s node)

Organogenesis in the anterior partNeural folds formation

Amnion Chorion Allantois

Mouse embryo: final stages of gastrulation

1. Complex folding2. Initially on the ventral surface of embryo3. Internalize to form the gut4 Heat and liver move into their positions4. Heat and liver move into their positions5. Head becomes distinct6. Embryo surrounded by extra-embryonic membrane

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Fig. 2.25

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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

75Fig. 2.29 Life cycle of Drosophila

Drosophila melanogaster: early embryogenesis

The Drosophila egg is the shape of a sausage .Meroblastic (superficial) cleavage and centrolecithalIt 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 .

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Fig. 2.30 After fertilization, no cell was form, but rapid nuclear division in a cytoplasm

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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 ectodermThe surface of ventral blastoderm → cell leave and form a layer between ventral ectoderm and mesoderm

Anterior and posterior invaginate and fuse → gutMidgut →region endodermForegut and hindgut → ectodermal origin

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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 → gutMidgut →region endodermForegut and hindgut → ectodermal origin

Ventral viewDorsal 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)

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There are 14 parasegments: Fig. 2.333 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

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of denticle hairs and cuticle on each segment, used for locomotion.

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Drosophila melanogaster: metamorphosis

Three instar stages of larval life are separated by molts. • 1st instar 2nd instar 3rd instar

molt molt3rd 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 haltereGenitalia

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-fertilizationthere are ~ 0.1 - 0.3% maleworms in the population.

http://www.wormatlas.org/handbook/contents.htm

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the model of nematode

Small nematodes that are 1 mm long and 70 µm in diameter. 19,000 geneSmall number of cell (558, first larval stage)T f b d th idTransparency of embryo, and growth rapidThe 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 Medicine7 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"

901927 19421947

Molecular Regulation of ApoptosisC. elegans

mutagenizeNon- apoptoticapoptotic

wildtype

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CED mutants(Cell Death

abnormality)

wildtype

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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.2.37Cell 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 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)

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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 9th or 10th cleavage, cells become specified and they end develop cilia.Ciliated blastula → rotate within fertilization envelop (E→F) → vegetal pole ofCiliated 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

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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 zygoteLate blastula with ciliary tuft and flattened vegetal plate

blastula

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Pluteus larvaPrism-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 ycables

C: SEM of primary mesenchyme cells enmeshed in the extracellular matrix of early gastrula.

Gastrulation star

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y gD: Gastrula-stage mesenchyme cell migration

The extracellular matrix fibrils of the bastocoel lie parallel to the animal-vegetal axix

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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 → inwardof the early gastrula inward

102CSPG: chondroitin sulfate proteoglycan

Entire sequence of gastrulation in sea urchin

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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.

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Allele: The gene is contributed by the male and femaleHomozygous: both alleles of a pair carry the mutationHeterozygous: just one copy of the mutant gene is present

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-/-

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Recessive mutation vs. Semi-dominant mutation

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Most mutations are recessive, but usually die in embryo.

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heterozygous

Developmental gene can be identified by induced mutation and screening

Genetic screening to produced homozygous mutant

Heterozygous

ygzerbrafish embryo

Embryos homozygous the

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s the induced mutation will be found in the offspring of 25% of the matings

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Mutagenesis and genetic screening strategy for identifying developmental mutants in Dorsophila

DTS: dominant temperature-sensitive mutation, up 29oC → deathb: a non developmental lethalb: a non-developmental lethal recessive

ethyl methane sulfonate

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main patterns of cleavage

phylotypic stage

Time vs. developmental events

T f ll t d i t l tiTypes of cell movement during gastrulation

Primitive streak

gastrulation

Neurulation

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human monozygotic twinning

Syncytium

imaginal discs and histoblasts

Dominant (or semi-dominant) mutations

Summary of the main patterns of cleavage

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