1

Animal Development

Lecture 8

Development of Birds

2The reproductive tract of birds. (Wilt and Hake, Ch 5, 2004)

3

The bodycavity of a hen.

4

The bodycavity of a hen.

Ovary

Oviduct

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The bodycavity of a hen.

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Chick blastoderm: the axes dependon gravity

•The blastoderm is a disc of cells which sits on top ofthe yolk.

•The D/V axis depends upon the dorsal side formingaway from the yolk while the ventral side is next toit.

•Initial, radial symmetry is broken after the egg islaid, denser cells form at the posterior marginalzone (PMZ).

•The primitive streak develops from the PMZ.

•The egg rotates through the hen's uterus (onceevery 6 minutes) as the embryo develops.

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The dorso-ventral axis ofthe chick blastoderm isspecified in relation to theyolk and the antero-posterior axis is set bygravity.

A = anterior

P = posterior marginal zone

(Wolpert, L. Ch 3, 2002)

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The posterior marginal zone of the chick specifiesthe posterior end of the antero-poterior axis

Not always occurs. The more advanced of the streaks is theonly one to develop because it inhibits the development ofthe other

(Wolpert, L. Ch 3, 2002)

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Chick blastoderm: primitive streakformation

•As the egg rotates, the embryo developsdepending upon the influence of gravity.

•The egg is directed point down, the PMZ is at thetop.

•The rotation causes the blastoderm to 'tip' in thedirection of rotation (somewhat like an 'off-balanced' top).

•The primitive streak acts like a NieuwkoopCentre in that it can induce a new axis iftransplanted to another embryo.

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Chick blastoderm: primitive streakformation

•However, normally only one centre takes overwith the more advanced primitive streakinhibiting the other one.

•Chick Vg-1 is expressed in the primitive streak.••Cells that express Vg-1 can induce a new

primitive streak.

11(Wilt and Hake, Ch 4, 2004)

The formation of the hypoblast.

12(Wilt and Hake, Ch 4, 2004)

The formation of the hypoblast.The hypoblast forms from cells delaminatedfrom the epiblast.

13(Wilt and Hake, Ch 4, 2004)

The formation of the hypoblast.The hypoblast forms from cells delaminatedfrom the epiblast and migrating forward fromKöller’s sickle.

14(Wilt and Hake, Ch 4, 2004)

Localization of the ability to form an axisin the early chick blastoderm.

15(Wilt and Hake, Ch 4, 2004)

16(Wilt and Hake, Ch 4, 2004)

The influence of the hypoblast on placement ofthe axis.

17(Wilt and Hake, Ch 4, 2004)

The influence of the hypoblast on placement ofthe axis.

18(Wilt and Hake, Ch 4, 2004)

The influence of the hypoblast on placement ofthe axis.

19(Wilt and Hake, Ch 4, 2004)

The influence of the hypoblast on placement ofthe axis.

20(Wilt and Hake, Ch 4, 2004)

Primitive streak formation.

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Fate map of a chick embryo when the primitivestreak has fully formed

(Wolpert, L. Ch 3, 2002)

22(Wilt and Hake, Ch 4, 2004)

Hensen’s node and its regression.

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Regression of Hensen’s node

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Somite formation and patterning

•Somites are formed in a well-defined orderalong the antero-posterior axis

1. In all vertebrate embryosomite formation : anterior posterior

2. Somite formation in unsegmented region (pre-somiticmesoderm)

unaffected by transverse cut in the plate of pre-somiticmesoderm (an autonomous process)

3. Before somite formation signals specify the time of formation of each somite has

already been lain down in pre-somitic mesoderm prospective identity of each somite is due to the

temporal order in which they leave the pre-somiticmesoderm

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The temporal order of somite formation isspecified early in embryonic development

Somiteformation

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The somites are generated successively from pre-somitic mesodermderived from somitic stem cells. As pre-somitic cells are released intothe posterior pre-somitic mesoderm, a new pair of somites buds fromanterior end every 90 min. SI, the most recently formed somite; SII, thelast but one somite formed; S0, somite in the process of formation, whoseboundaries are not yet set; S-1, S-II, blocks of pre-somitic cells that willform somites

Somite formation in the chick.

c-hairy1

pre-somitic cell

Internal clockFGF-8

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The pre-somitic mesoderm has apositional identity before somite formation

Spcification by position has occurred beforesomite formation begins during gastrulation

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The fate map of a somite in the chickembryo

Sclerotome, to form the cartilage of the vertebrae;

Dermomyotome, to form the dermatome and myotome, whichgive rise to the dermis and abdominal and limbmuscle

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A signal from the notochord induces sclerotome formation

A graft of an additional notochord to the dorsal region of a somite ina 10-somite embryo suppresses the formation of the dermomytomefrom the dorsal portion of the somite and induces the formation ofsclerotome, which develops into cartilage. The graft also affects theshape of the neural tube.

An inducer ofcartilage

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Sonic hedgehog

A model forpatterning of somite

differentiation

•Sclerotome is specified by adiffusible signal (sonichedgehog protein) from thenotochord and floor plate(blue arrows).

•Signals from the dorsal neuraltube, ectoderm (red arrows)and lateral mesoderm (green)specify the dermomyotome

Wnt family

TGF-family :BMP-4

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•The fate of somite cells is determined by signalsfrom the adjacent tissues

1. Cells in dorsal and lateral regions of newly formedsomite dermomyotome (expresses Pax3 homeobox gene)

a. myotome muscle cells b. dermatome (epithelial-sheet) dermis

2. Cells form medial region of somite (comes from cells inthe primitive streak close to Hensen’s node) axial and back muscle, expresses muscle-specific

TF,MyoD and related proteins

3. Cells form lateral somite (comes from cells more posterior) abdominal and limb muscles

4. Ventral part of medial somite contains sclertome surround notochord, expresses Pax1 homeobox gene

develop into vertebrae and ribs

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•The fate of somite cells is determined bysignals from the adjacent tissues (cont.)

5. The dorso-ventral orientation of newly formed somitesis inverted and they still develop normally.

signals from tissues adjacent to the somite a. myotome determined within hours of somite

formation b. sclerotome determined later

6. The neural tube and notochord produce signals topattern the somite and its future

removal of neural tube and notochord the somites undergo apoptosis neither vertebrae nor axial muscle develop limb musculature still develops

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7. Neural tube cartilage-inducing effect on somite mediated by the most ventral region of the tube, the floor

plate

8. The lateral plate of mesoderm specifying the lateral part of the dermomyotome

9. The overlying ectoderm specifying the dermomyotome

•The fate of somite cells is determined bysignals from the adjacent tissues (cont.)

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Positional identity of somites along the antero-posterior axis is specified by Hox geneexpression

1. All homeobox genes encode TFs which contain a similar DNA-binding region

≒ 60 amino acids homeodomain helix-turn-helixDNA-binding motif 180 bp homeobox

2. Homeobox genes specifying positional identity alongthe antero-posterior axis originally identified in the fruit fly Drosophila

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Positional identity of somites along the antero-posterior axis is specified by Hox geneexpression

3. Homeotic transformation one structure replace another

4. Vertebrates - 4 separate clusters of Hox genes(Zebrafish has 6 clusters)

arisen by duplications of the genes within a cluster

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The nervous systemin a 3-day chick

embryo

rhombomeres –r1~r8cranial nerves III~XIIB1 jawsB2 bony parts of the earS = somite

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•Signals that pattern the neural plate may travelwithin the neural plate itself

1. Two routes by which the mesoderm can induce thenervous system a. traditional vertical or transverse route from the

mesoderm to the overlying ectoderm b. planar, signal being generated within the neural plate

itself and traveling within the ectodermal sheet

•The hindbrain is segmented into rhombomeresby boundaries of cell lineage restriction

1. Chick embryo, three segmented systems can be seenin the posterior head region by 3 day development

a. mesoderm somiteb. hindbrain 8 rhombomeresc. lateral mesoderm branchial arches

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2. Neural tube give rise to: a. cranial nerves face + neck

b. neural crest cells peripheral nerves skeletal elements

3. Lineage restriction: once the boundaries form, cells and their descendants are

confined within a rhombomere, not cross from one side of aboundary to the other

4. Cells of a rhombomere share some adhesive propertythat prevents them mixing with those of a adjacentrhombomere

cells in each rhombomere may be under control of thesame gene a rhombomere is a developmental unit

5. Odd- and even-No. rhombomere new boundary forms

Odd-No. rhombomere no boundary forms

cells have similar surface properties

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Lineage restriction in rhombomeres of theembryonic chick hindbrain

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Signaling by ephrins and their receptors

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•Neural crest cells have positional values

1. Branchial arches 1, 2, 3 from rhombomere 2, 4, 6

2. In chick, rhombomere 3, 5

eliminated by programmed cell death

•Hox genes provide positional identity in thehindbrain region

1. Hox gene expression provides a possible molecularbasis for the positional identity of both rhombomeresand the neural crest

2. Transplantation of rhombomeres from an anterior to amore posterior position alter the expression pattern (signal from neural tube, not

surrounding tissue)

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Expression of Hox genes in the branchialregion of the head

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•The embryo is patterned by the neural stageinto organ-forming regions that can stillregulate

1. Neurula stage body plan established regions will form limbs, eyes, heart, other

organs have been determined

2. Blastula stage

determination not occur basic vertebrate phylotypic body plan is

established during gastrulation

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Patterning of the vertebrate axial body plan

Gastrulation and Spemann organizer activity

Somite develops into sclerotomeand dermamyotome

Hox gene expression establishes positional identity formesoderm, endoderm, and ectoderm

Somite receive signals fromnotochord, neural tube, and ectoderm

Mesoderm develops into notochord,somites and lateral plate ectoderm

The Hox gene complexes are expressed along theanterior-posterior axis

Rhombomeres and neural crest in thehindbrain are characterized by regional

patterns of Hox gene expression

Mesoderm and planar ectodermal signalsgive regional identity to neural tube

Mesoderm includes neural platefrom ectoderm

45(Wilt and Hake, Ch 4, 2004)

An avian embryo in cross section.

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Amniote

47(Wilt and Hake, Ch 4, 2004)

Extraembryonic membrances of avianembryos.

48(Wilt and Hake, Ch 4, 2004)

Extraembryonic membrances of avianembryos.

49(Wilt and Hake, Ch 4, 2004)

Extraembryonic membrances of avianembryos.

50(Wilt and Hake, Ch 4, 2004)

Extraembryonic membrances of avianembryos.

51(Wilt and Hake, Ch 4, 2004)

Extraembryonic membrances of avianembryos.